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Bibliography - Thomas R Knutson
Winton, Michael, Alistair Adcroft, John P Dunne, Isaac M Held, Elena Shevliakova, Ming Zhao, Huan Guo, William J Hurlin, John P Krasting, Thomas R Knutson, David J Paynter, Levi G Silvers, and Rong Zhang, January 2020: Climate Sensitivity of GFDL's CM4.0. Journal of Advances in Modeling Earth Systems, 12(1), DOI:10.1029/2019MS001838.
GFDL's new CM4.0 climate model has high transient and equilibrium climate sensitivities near the middle of the upper half of CMIP5 models. The CMIP5 models have been criticized for excessive sensitivity based on observations of present‐day warming and heat uptake and estimates of radiative forcing. An ensemble of historical simulations with CM4.0 produces warming and heat uptake that are consistent with these observations under forcing that is at the middle of the assessed distribution. Energy budget‐based methods for estimating sensitivities based on these quantities underestimate CM4.0's sensitivities when applied to its historical simulations. However, we argue using a simple attribution procedure that CM4.0's warming evolution indicates excessive transient sensitivity to greenhouse gases. This excessive sensitivity is offset prior to recent decades by excessive response to aerosol and land use changes.
Bhatia, Kieran, Gabriel A Vecchi, Thomas R Knutson, Hiroyuki Murakami, J Kossin, Keith W Dixon, and Carolyn E Whitlock, February 2019: Recent increases in tropical cyclone intensification rates. Nature Communications, 10, 635, DOI:10.1038/s41467-019-08471-z.
Tropical cyclones that rapidly intensify are typically associated with the highest forecast errors and cause a disproportionate amount of human and financial losses. Therefore, it is crucial to understand if, and why, there are observed upward trends in tropical cyclone intensification rates. Here, we utilize two observational datasets to calculate 24-hour wind speed changes over the period 1982–2009. We compare the observed trends to natural variability in bias-corrected, high-resolution, global coupled model experiments that accurately simulate the climatological distribution of tropical cyclone intensification. Both observed datasets show significant increases in tropical cyclone intensification rates in the Atlantic basin that are highly unusual compared to model-based estimates of internal climate variations. Our results suggest a detectable increase of Atlantic intensification rates with a positive contribution from anthropogenic forcing and reveal a need for more reliable data before detecting a robust trend at the global scale.
Knutson, Thomas R., et al., October 2019: Tropical Cyclones and Climate Change Assessment: Part I. Detection and Attribution. Bulletin of the American Meteorological Society, 100(10), DOI:10.1175/BAMS-D-18-0189.1.
We assess whether detectable changes in tropical cyclone activity have been identified in observations and whether any changes can be attributed to anthropogenic climate change. An assessment was made of whether detectable changes in tropical cyclone (TC) activity are identifiable in observations and whether any changes can be attributed to anthropogenic climate change. Overall, historical data suggest detectable TC activity changes in some regions associated with TC track changes, while data quality and quantity issues create greater challenges for analyses based on TC intensity and frequency. A number of specific published conclusions (case studies) about possible detectable anthropogenic influence on TCs were assessed using the conventional approach of preferentially avoiding Type I errors (i.e., overstating anthropogenic influence or detection). We conclude there is at least low-to-medium confidence that the observed poleward migration of the latitude of maximum intensity in the western North Pacific is detectable, or highly unusual compared to expected natural variability. Opinion on the author team was divided on whether any observed TC changes demonstrate discernible anthropogenic influence, or whether any other observed changes represent detectable changes. The issue was then reframed by assessing evidence for detectable anthropogenic influence while seeking to reduce the chance of Type II errors (i.e., missing or understating anthropogenic influence or detection). For this purpose, we used a much weaker "balance of evidence" criterion for assessment. This leads to a number of more speculative TC detection and/or attribution statements, which we recognize have substantial potential for being false alarms (i.e., overstating anthropogenic influence or detection) but which may be useful for risk assessment. Several examples of these alternative statements, derived using this approach, are presented in the report.
Knutson, Thomas R., et al., in press: Tropical Cyclones and Climate Change Assessment: Part II. Projected Response to Anthropogenic Warming. Bulletin of the American Meteorological Society. DOI:10.1175/BAMS-D-18-0194.1. August 2019.
We assess model-projected changes in tropical cyclone activity for a 2°C anthropogenic warming. Medium-to-high confidence projections include increased tropical cyclone rainfall rates, intensity, and proportion of storms that reach Category 4-5 intensity globally. Model projections of tropical cyclone (TC) activity response to anthropogenic warming in climate models are assessed. Observations, theory, and models, with increasing robustness, indicate rising global TC risk for some metrics -- that are projected to impact multiple regions. A 2°C anthropogenic global warming is projected to impact TC activity as follows: i) The most confident TC-related projection is that sea level rise accompanying the warming will lead to higher storm inundation levels, assuming all other factors are unchanged. ii) For TC precipitation rates, there is at least medium-to-high confidence in an increase globally, with a median projected increase of 14%, or close to the rate of tropical water vapor increase with warming, at constant relative humidity. iii) For TC intensity, ten of 11 authors had at least medium-to-high confidence that the global average will increase. The median projected increase in lifetime maximum surface wind speeds is about 5% (range 1–10%) in available higher resolution studies. iv) For the global proportion (as opposed to frequency) of TCs that reach very intense (Category 4–5) levels, there is at least medium-to-high confidence in an increase, with a median projected change of +13%. Author opinion was more mixed and confidence levels lower for the following projections: v) a further poleward expansion of the latitude of maximum TC intensity in the western North Pacific; vi) a decrease of global TC frequency, as projected in most studies; vii) an increase in global very intense TC frequency (Category 4–5), seen most prominently in higher resolution models; and viii) a slowdown in TC translation speed.
Liu, Maofeng, Gabriel A Vecchi, J A Smith, and Thomas R Knutson, October 2019: Causes of large projected increases in hurricane precipitation rates with global warming. npj Climate and Atmospheric Science, 2, 38, DOI:10.1038/s41612-019-0095-3.
Recent climate modeling studies point to an increase in tropical cyclone rainfall rates in response to climate warming. These studies indicate that the percentage increase in tropical cyclone rainfall rates often outpaces the increase in saturation specific humidity expected from the Clausius-Clapeyron relation (~7% °C−1). We explore the change in tropical cyclone rainfall rates over all oceans under global warming using a high-resolution climate model with the ability to simulate the entire intensity spectrum of tropical cyclones. Consistent with previous results, we find a robust increase of tropical cyclone rainfall rates. The percentage increase for inner-core tropical cyclone rainfall rates in our model is markedly larger than the Clausius-Clapeyron rate. However, when the impact of storm intensity is excluded, the rainfall rate increase shows a much better match with the Clausius-Clapeyron rate, suggesting that the "super Clausius-Clapeyron" scaling of rainfall rates with temperature increase is due to the warming-induced increase of tropical cyclone intensity. The increase of tropical cyclone intensity and environmental water vapor, in combination, explain the tropical cyclone rainfall rate increase under global warming.
Vecchi, Gabriel A., Thomas L Delworth, Hiroyuki Murakami, Seth D Underwood, Andrew T Wittenberg, Fanrong Zeng, Wei Zhang, Jane Baldwin, Kieran Bhatia, William F Cooke, Jie He, Sarah B Kapnick, Thomas R Knutson, G Villarini, Karin van der Wiel, Whit G Anderson, V Balaji, Jan-Huey Chen, Keith W Dixon, Richard G Gudgel, Lucas Harris, Liwei Jia, Nathaniel C Johnson, Shian-Jiann Lin, Maofeng Liu, Chin Ho Justin Ng, Anthony Rosati, J A Smith, and Xiaosong Yang, November 2019: Tropical cyclone sensitivities to CO2 doubling: roles of atmospheric resolution, synoptic variability and background climate changes. Climate Dynamics, 53(9-10), DOI:10.1007/s00382-019-04913-y.
Responses of tropical cyclones (TCs) to CO2 doubling are explored using coupled global climate models (GCMs) with increasingly refined atmospheric/land horizontal grids (~ 200 km, ~ 50 km and ~ 25 km). The three models exhibit similar changes in background climate fields thought to regulate TC activity, such as relative sea surface temperature (SST), potential intensity, and wind shear. However, global TC frequency decreases substantially in the 50 km model, while the 25 km model shows no significant change. The ~ 25 km model also has a substantial and spatially-ubiquitous increase of Category 3–4–5 hurricanes. Idealized perturbation experiments are performed to understand the TC response. Each model's transient fully-coupled 2 × CO2 TC activity response is largely recovered by "time-slice" experiments using time-invariant SST perturbations added to each model's own SST climatology. The TC response to SST forcing depends on each model's background climatological SST biases: removing these biases leads to a global TC intensity increase in the ~ 50 km model, and a global TC frequency increase in the ~ 25 km model, in response to CO2-induced warming patterns and CO2 doubling. Isolated CO2 doubling leads to a significant TC frequency decrease, while isolated uniform SST warming leads to a significant global TC frequency increase; the ~ 25 km model has a greater tendency for frequency increase. Global TC frequency responds to both (1) changes in TC "seeds", which increase due to warming (more so in the ~ 25 km model) and decrease due to higher CO2 concentrations, and (2) less efficient development of these"seeds" into TCs, largely due to the nonlinear relation between temperature and saturation specific humidity.
Yan, Xiaoqin, Rong Zhang, and Thomas R Knutson, April 2019: A multivariate AMV index and associated discrepancies between observed and CMIP5 externally forced AMV. Geophysical Research Letters, 46(8), DOI:10.1029/2019GL082787.
Atlantic Multidecadal Variability (AMV) is a multivariate phenomenon. Here for the first time we obtain a multivariate AMV index (MAI) and associated patterns using Multivariate Empirical Orthogonal Function (MEOF) analysis to explore the multivariate nature of AMV. Coherent multidecadal variability that is unique to the Atlantic is found in the observed MEOF‐extracted AMV, various AMV‐related indices, and an Atlantic Meridional Overturning Circulation (AMOC) fingerprint. For comparison, the signal associated with global mean sea surface temperature (SST) is removed from both observations and Coupled Model Intercomparison Project Phase 5 (CMIP5) simulations. The residual CMIP5 forced basin‐wide SST‐based AMV index disagrees strongly with the observed residual, which retains a strong AMV signal. The MEOF approach still extracts a residual CMIP5 forced AMV signal that is unique to the Atlantic, although very different from observations. Our findings suggest that the observed AMV is not dominated by external forcing.
Zhang, Gan, Thomas R Knutson, and Stephen T Garner, December 2019: Impacts of Extratropical Weather Perturbations on Tropical Cyclone Activity: Idealized Sensitivity Experiments with a Regional Atmospheric Model. Geophysical Research Letters, 46(23), DOI:10.1029/2019GL085398.
Extratropical weather perturbations have been linked to Atlantic tropical cyclones (TC) activity in observations. However, modeling studies of the extratropical impact are scarce and disagree about its importance and climate implications. Using a non‐hydrostatic regional atmospheric model, we explore the extratropical impact by artificially suppressing extratropical weather perturbations at the tropical–extratropical interface. Our 22‐year simulations of August–October suggest that the extratropical suppression adds ~3.7 Atlantic TCs per season on average, although the response varies among individual years. The TC response mainly appears within 30°N–40°N, where tropical cyclogenesis frequency quadruples compared to control simulations. This increased cyclogenesis, accompanied by a strong increase of mid‐tropospheric relative humidity, arises as the perturbation suppression reduces the extratropical interference of TC development. The suppression of extratropical perturbations is highly idealized but may suggest mechanisms by which extratropical atmospheric variability potentially influences TC activity in past or future altered climate states.
Kam, Jonghun, Thomas R Knutson, Fanrong Zeng, and Andrew T Wittenberg, January 2018: CMIP5 Model-Based Assessment of Anthropogenic Influence on Highly Anomalous Arctic Warmth During November-December 2016, [in "Explaining Extreme Events of 2016 from a Climate Perspective"]. Bulletin of the American Meteorological Society, 99(1), DOI:10.1175/BAMS-D-17-0116.1S34-S38.
Kam, Jonghun, Thomas R Knutson, and P C D Milly, July 2018: Climate model assessment of changes in winter-spring streamflow timing over North America. Journal of Climate, 31(14), DOI:10.1175/JCLI-D-17-0813.1.
Over regions where snow-melt runoff substantially contributes to winter-spring streamflows, warming can accelerate snow melt and reduce dry-season streamflows. However, conclusive detection of changes and attribution to anthropogenic forcing is hindered by brevity of observational records, model uncertainty, and uncertainty concerning internal variability. In this study, a detection/attribution of changes in mid-latitude North American winter-spring streamflow timing is examined using nine global climate models under multiple forcing scenarios. In this study, robustness across models, start/end dates for trends, and assumptions about internal variability is evaluated. Marginal evidence for an emerging detectable anthropogenic influence (according to four or five of nine models) is found in the north-central U.S., where winter-spring streamflows have been coming earlier. Weaker indications of detectable anthropogenic influence (three of nine models) are found in the mountainous western U.S./southwestern Canada and in extreme northeastern U.S./Canadian Maritimes. In the former region, a recent shift toward later streamflows has rendered the full-record trend toward earlier streamflows only marginally significant, with possible implications for previously published climate change detection findings for streamflow timing in this region. In the latter region, no forced model shows as large a shift toward earlier streamflow timing as the detectable observed shift. In other (including warm, snow-free) regions, observed trends are typically not detectable, although in the U.S. central plains we find detectable delays in streamflow, which are inconsistent with forced model experiments.
Knutson, Thomas R., Jonghun Kam, Fanrong Zeng, and Andrew T Wittenberg, January 2018: CMIP5 Model-Based Assessment of Anthropogenic Influence on Record Global Warmth During 2016, [in "Explaining Extreme Events of 2016 from a Climate Perspective"]. Bulletin of the American Meteorological Society, 99(1), DOI:10.1175/BAMS-D-17-0104.1S11-S15.
Knutson, Thomas R., and Fanrong Zeng, June 2018: Model Assessment of Observed Precipitation Trends Over Land Regions: Detectable Human Influences and Possible Low Bias in Model Trends. Journal of Climate, 31(12), DOI:10.1175/JCLI-D-17-0672.1.
Precipitation trends for 1901-2010, 1951-2010 and 1981- 2010 over relatively well-observed global land regions are assessed for detectable anthropogenic influences and for consistency with Coupled Model Intercomparison Project 5 (CMIP5) historical simulations. The CMIP5 historical All-Forcing runs are broadly consistent with the observed trend pattern (1901-2010), but with an apparent low trend bias tendency in the simulations. Despite this bias, observed and modeled trends are statistically consistent over 59% of the analyzed area. Over 20% (9%) of the analyzed area, increased (decreased) precipitation is partly attributable to anthropogenic forcing. These inferred human-induced changes include: increases over regions of the north-central U.S., southern Canada, Europe, and southern South America; and decreases over parts of the Mediterranean region and northern tropical Africa. Trends for the shorter periods (1951-2010 and 1981-2010) do not indicate a prominent low trend bias in the models, as found for the 1901-2010 trends. An atmosphere-only model, forced with observed sea surface temperatures and other climate forcing agents, also under-predicts the observed precipitation increase in the northern hemisphere extratropics since 1901. The CMIP5 All-Forcing ensemble's low bias in simulated trends since 1901 is a tentative finding which, if borne out in further studies, suggests that precipitation projections using these regions/models could overestimate future drought risk, and underestimate future flooding risk.
Li, Dawei, Rong Zhang, and Thomas R Knutson, February 2018: Comparison of Mechanisms for Low-Frequency Variability of Summer Arctic Sea Ice in Three Coupled Climate Models. Journal of Climate, 31(3), DOI:10.1175/JCLI-D-16-0617.1.
In this study, the mechanisms for low-frequency variability of summer Arctic sea ice are analyzed using long control simulations from three coupled climate models (GFDL CM2.1, GFDL CM3, and NCAR CESM). Despite different Arctic sea ice mean states, there are many robust features in the response of low-frequency summer Arctic sea ice variability to the three key predictors (Atlantic/Pacific oceanic heat transport into the Arctic and the Arctic Dipole) across all three models. In all three models, an enhanced Atlantic (Pacific) heat transport into the Arctic induces summer Arctic sea ice decline and surface warming, especially over the Atlantic (Pacific) sector of the Arctic. A positive phase of the Arctic Dipole induces summer Arctic sea ice decline and surface warming on the Pacific side, and opposite changes on the Atlantic side. There is robust Bjerknes Compensation at low frequency, so that the northward atmospheric heat transport provides a negative feedback to summer Arctic sea ice variations. The influence of the Arctic Dipole on summer Arctic sea ice extent is more (less) effective in simulations with less (excessive) climatological summer sea ice in the Atlantic sector. The response of Arctic sea ice thickness (SIT) to the three key predictors is stronger in models that have thicker climatological Arctic sea ice.
Yan, Xiaoqin, Rong Zhang, and Thomas R Knutson, May 2018: Underestimated AMOC variability and implications for AMV and predictability in CMIP models. Geophysical Research Letters, 45(9), DOI:10.1029/2018GL077378.
The Atlantic Meridional Overturning Circulation (AMOC) has profound impacts on various climate phenomena. Using both observations and simulations from the Coupled Model Intercomparison Project Phase 3 and 5 (CMIP3 and CMIP5), here we show that most models underestimate the amplitude of low‐frequency (decadal) AMOC variability. We further show that stronger low‐frequency AMOC variability leads to stronger linkages between the AMOC and key variables associated with the Atlantic multidecadal variability (AMV), and between the subpolar AMV signal and northern hemisphere surface air temperature (NHSAT). Low‐frequency extra‐tropical NHSAT variability might increase with the amplitude of low‐frequency AMOC variability. Atlantic decadal predictability is much higher in models with stronger low‐frequency AMOC variability, and much lower in models with weaker or without AMOC variability. Our results suggest that simulating realistic low‐frequency AMOC variability is very important, both for simulating realistic linkages between AMOC and AMV‐related variables and for achieving substantially higher Atlantic decadal predictability.
Barcikowska, Monika, Thomas R Knutson, and Rong Zhang, January 2017: Observed and simulated fingerprints of multidecadal climate variability, and their contributions to periods of global SST stagnation. Journal of Climate, 30(2), DOI:10.1175/JCLI-D-16-0443.1.
This study investigates spatio-temporal features of multidecadal climate variability, using observations and climate model simulation. Aside from a long-term warming trend, observational SST and atmospheric circulation records are dominated by a ~65yr variability component. Though its center of action is over the North Atlantic, but it manifests also over the Pacific and Indian Oceans, suggesting a tropical inter-basin teleconnection maintained through an atmospheric bridge. Our analysis shows that simulated internal climate variability in a coupled climate model (CSIRO-Mk3.6.0) reproduces the main spatio-temporal features of the observed component. Model-based multidecadal variability comprises a coupled ocean-atmosphere teleconnection, established through a zonally oriented atmospheric overturning circulation between the tropical North Atlantic and eastern tropical Pacific. During the warm SST phase in the North Atlantic, increasing SSTs over the tropical North Atlantic strengthen locally ascending air motion and intensify subsidence and low-level divergence in the eastern tropical Pacific. This corresponds with a strengthening of trade winds and cooling in the tropical central Pacific. The model's derived component substantially shapes its global climate variability and is tightly linked to multidecadal variability of the Atlantic Meridional Overturning Circulation (AMOC). This suggests potential predictive utility and underscores the importance of correctly representing North Atlantic variability in simulations of global and regional climate. If the observations-based component of variability originates from internal climate processes, as found in the model, the recently observed (1970s-2000s) North Atlantic warming and eastern tropical Pacific cooling might presage an ongoing transition to a cold North Atlantic phase with possible implications for near-term global temperature evolution.
Easterling, D R., K E Kunkel, J R Arnold, and Thomas R Knutson, et al., November 2017: Precipitation change in the United States In Climate Science Special Report: Fourth National Climate Assessment, Volume I, Washington, DC, Wuebbles, D.J., D.W. Fahey, K.A. Hibbard, D.J. Dokken, B.C. Stewart, and T.K. Maycock (eds.), U.S. Global Change Research Program, Washington, DC, DOI:10.7930/J0H993CC207-230.
Knutson, Thomas R., J Kossin, C Mears, J Perlwitz, and M F Wehner, November 2017: Detection and attribution of climate change In Climate Science Special Report: Fourth National Climate Assessment, Volume I, Wuebbles, D.J., D.W. Fahey, K.A. Hibbard, D.J. Dokken, B.C. Stewart, and T.K. Maycock (eds.), Washin, U.S. Global Change Research Program, DOI:10.7930/J01834ND114-132.
Kossin, J, T Hall, and Thomas R Knutson, et al., November 2017: Extreme storms In Climate Science Special Report: Fourth National Climate Assessment, Volume I, Washington, DC, Wuebbles, D.J., D.W. Fahey, K.A. Hibbard, D.J. Dokken, B.C. Stewart, and T.K. Maycock (eds.), U.S. Global Change Research Program, Washington, DC, DOI:10.7930/J07S7KXX257-276.
Li, Dawei, Rong Zhang, and Thomas R Knutson, April 2017: On the discrepancy between observed and CMIP5 multi-model simulated Barents Sea winter sea ice decline. Nature Communications, 8, 14991, DOI:10.1038/ncomms14991.
This study aims to understand the relative roles of external forcing versus internal climate variability in causing the observed Barents Sea winter sea ice extent (SIE) decline since 1979. We identify major discrepancies in the spatial patterns of winter Northern Hemisphere sea ice concentration trends over the satellite period between observations and CMIP5 multi-model mean externally forced response. The CMIP5 externally forced decline in Barents Sea winter SIE is much weaker than that observed. Across CMIP5 ensemble members, March Barents Sea SIE trends have little correlation with global mean surface air temperature trends, but are strongly anti-correlated with trends in Atlantic heat transport across the Barents Sea Opening (BSO). Further comparison with control simulations from coupled climate models suggests that enhanced Atlantic heat transport across the BSO associated with regional internal variability may have played a leading role in the observed decline in winter Barents Sea SIE since 1979.
Perlwitz, J, and Thomas R Knutson, et al., November 2017: Large-scale circulation and climate variability In Climate Science Special Report: Fourth National Climate Assessment, Volume I, Washington, DC, Wuebbles, D.J., D.W. Fahey, K.A. Hibbard, D.J. Dokken, B.C. Stewart, and T.K. Maycock (eds.), U.S. Global Change Research Program, Washington, DC, DOI:10.7930/J0RV0KVQ161-184.
Wehner, M F., J R Arnold, and Thomas R Knutson, et al., November 2017: Droughts, floods, and wildfires In Climate Science Special Report: Fourth National Climate Assessment, Volume I, Washington, DC, Wuebbles, D.J., D.W. Fahey, K.A. Hibbard, D.J. Dokken, B.C. Stewart, and T.K. Maycock (eds.), U.S. Global Change Research Program, Washington, DC, DOI:10.7930/J0CJ8BNN231-256.
Wuebbles, D J., D R Easterling, K Hayhoe, and Thomas R Knutson, et al., November 2017: Our globally changing climate In Climate Science Special Report: Fourth National Climate Assessment, Volume I, Washington, DC, Wuebbles, D.J., D.W. Fahey, K.A. Hibbard, D.J. Dokken, B.C. Stewart, and T.K. Maycock (eds.), U.S. Global Change Research Program, Washington, DC, DOI:10.7930/J08S4N3535-72.
Yan, Xiaoqin, Rong Zhang, and Thomas R Knutson, November 2017: The role of Atlantic overturning circulation in the recent decline of Atlantic major hurricane frequency. Nature Communications, 8, 1695, DOI:10.1038/s41467-017-01377-8.
Observed Atlantic major hurricane frequency has exhibited pronounced multidecadal variability since the 1940s. However, the cause of this variability is debated. Using observations and a coupled earth system model (GFDL-ESM2G), here we show that the decline of the Atlantic major hurricane frequency during 2005–2015 is associated with a weakening of the Atlantic Meridional Overturning Circulation (AMOC) inferred from ocean observations. Directly observed North Atlantic sulfate aerosol optical depth has not increased (but shows a modest decline) over this period, suggesting the decline of the Atlantic major hurricane frequency during 2005–2015 is not likely due to recent changes in anthropogenic sulfate aerosols. Instead, we find coherent multidecadal variations involving the inferred AMOC and Atlantic major hurricane frequency, along with indices of Atlantic Multidecadal Variability and inverted vertical wind shear. Our results provide evidence for an important role of the AMOC in the recent decline of Atlantic major hurricane frequency.
Kam, Jonghun, Thomas R Knutson, Fanrong Zeng, and Andrew T Wittenberg, December 2016: Multimodel Assessment of Anthropogenic Influence on Record Global and Regional Warmth During 2015, Section 2 of "[Explaining extreme events of 2015 from a climate perspective]". Bulletin of the American Meteorological Society, 97(12), DOI:10.1175/BAMS-D-16-0138.1S4-S8.
Knutson, Thomas R., and Jeff J Ploshay, September 2016: Detection of anthropogenic influence on a summertime heat stress index. Climatic Change, 138(1-2), DOI:10.1007/s10584-016-1708-z.
One of the most consequential impacts of anthropogenic warming on humans may be increased heat stress, combining temperature and humidity effects. Here we examine whether there are now detectable changes in summertime heat stress over land regions. As a heat stress metric we use a simplified wet bulb globe temperature (WBGT) index. Observed trends in WBGT (1973–2012) are compared to trends from CMIP5 historical simulations (eight-model ensemble) using either anthropogenic and natural forcing agents combined or natural forcings alone. Our analysis suggests that there has been a detectable anthropogenic increase in mean summertime heat stress since 1973, both globally and in most land regions analyzed. A detectable increase is found over a larger fraction of land for WBGT than for temperature, as WBGT summertime means have lower interannual variability than surface temperature at gridbox scales. Notably, summertime WBGT over land has continued increasing in recent years--consistent with climate models--despite the apparent 'hiatus' in global warming and despite a decreasing tendency in observed relative humidity over land since the late 1990s.
Knutson, Thomas R., Rong Zhang, and Larry W Horowitz, November 2016: Prospects for a prolonged slowdown in global warming in the early 21st century. Nature Communications, 7, 13676, DOI:DOI:10.1038/ncomms13676.
Global mean temperature over 1998 to 2015 increased at a slower rate (0.1 K decade−1) compared with the ensemble mean (forced) warming rate projected by Coupled Model Intercomparison Project 5 (CMIP5) models (0.2 K decade−1). Here we investigate the prospects for this slower rate to persist for a decade or more. The slower rate could persist if the transient climate response is overestimated by CMIP5 models by a factor of two, as suggested by recent low-end estimates. Alternatively, using CMIP5 models' warming rate, the slower rate could still persist due to strong multidecadal internal variability cooling. Combining the CMIP5 ensemble warming rate with internal variability episodes from a single climate model—having the strongest multidecadal variability among CMIP5 models—we estimate that the warming slowdown (<0.1 K decade−1 trend beginning in 1998) could persist, due to internal variability cooling, through 2020, 2025 or 2030 with probabilities 16%, 11% and 6%, respectively.
Tuleya, Robert E., Morris A Bender, Thomas R Knutson, Joseph J Sirutis, B Thomas, and I Ginis, October 2016: Impact of upper tropospheric temperature anomalies and vertical wind shear on tropical cyclone evolution using an idealized version of the operational GFDL hurricane model. Journal of the Atmospheric Sciences, 73(10), DOI:10.1175/JAS-D-16-0045.1.
The GFDL hurricane modelling system, initiated in the 1970s, has progressed from a research tool to an operational system over four decades. This system is still in use today in research and operations, and its evolution will be briefly described. This study used an idealized version of the 2014 GFDL model to test its sensitivity across a wide range of three environmental factors that are often identified as key factors in tropical cyclone (TC) evolution: SST, atmospheric stability (upper air thermal anomalies), and vertical wind shear (westerly through easterly). A wide range of minimum central pressure intensities resulted (905 to 980hPa). The results confirm that a scenario (e.g., global warming) in which the upper troposphere warms relative to the surface will have less TC intensification than one with a uniform warming with height. TC rainfall is also investigated for the SST-stability parameter space. Rainfall increases for combinations of SST increase and increasing stability similar to global warming scenarios, consistent with climate change TC downscaling studies with the GFDL model. The forecast system's sensitivity to vertical shear was also investigated. The idealized model simulations showed weak disturbances dissipating under strong easterly and westerly shear of 10 m s-1. A small bias for greater intensity under easterly sheared versus westerly sheared environments was found at lower values of SST. The impact of vertical shear on intensity was different when a strong vortex was used in the simulations. In this case none of the initial disturbances weakened, and most intensified to some extent.
Walsh, K J., J McBride, P J Klotzbach, S Balachandran, S J Camargo, G Holland, Thomas R Knutson, J Kossin, T-C Lee, A Sobel, and M Sugi, January 2016: Tropical cyclones and climate change. Wiley Interdisciplinary Reviews: Climate Change, 7(1), DOI:10.1002/wcc.371.
Recent research has strengthened the understanding of the links between climate and tropical cyclones (TCs) on various timescales. Geological records of past climates have shown century-long variations in TC numbers. While no significant trends have been identified in the Atlantic since the late 19th century, significant observed trends in TC numbers and intensities have occurred in this basin over the past few decades, and trends in other basins are increasingly being identified. However, understanding of the causes of these trends is incomplete, and confidence in these trends continues to be hampered by a lack of consistent observations in some basins. A theoretical basis for maximum TC intensity appears now to be well established, but a climate theory of TC formation remains elusive. Climate models mostly continue to predict future decreases in global TC numbers, projected increases in the intensities of the strongest storms and increased rainfall rates. Sea level rise will likely contribute toward increased storm surge risk. Against the background of global climate change and sea level rise, it is important to carry out quantitative assessments on the potential risk of TC-induced storm surge and flooding to densely populated cities and river deltas. Several climate models are now able to generate a good distribution of both TC numbers and intensities in the current climate. Inconsistent TC projection results emerge from modeling studies due to different downscaling methodologies and warming scenarios, inconsistencies in projected changes of large-scale conditions, and differences in model physics and tracking algorithms.
Kam, Jonghun, Thomas R Knutson, Fanrong Zeng, and Andrew T Wittenberg, December 2015: Record annual mean warmth over Europe, the northeast Pacific, and the northwest Atlantic during 2014: Assessment of anthropogenic influence. Bulletin of the American Meteorological Society, 96(12), DOI:10.1175/BAMS-EEE_2014_ch13.1.
Knutson, Thomas R., March 2015: Tropical Cyclones and Climate Change In Encyclopedia of Atmospheric Sciences 2nd edition, Vol 6, Gerald R. North (editor-in-chief), John Pyle and Fuqing Zhang (editors), Oxford, Academic Press, 111-122.
Knutson, Thomas R., Joseph J Sirutis, Ming Zhao, Robert E Tuleya, Morris A Bender, Gabriel A Vecchi, G Villarini, and D Chavas, September 2015: Global Projections of Intense Tropical Cyclone Activity for the Late Twenty-First Century from Dynamical Downscaling of CMIP5/RCP4.5 Scenarios. Journal of Climate, 28(18), DOI:10.1175/JCLI-D-15-0129.1.
Global projections of intense tropical cyclone activity are derived from the Geophysical Fluid Dynamics Laboratory (GFDL) HiRAM (50 km grid) atmospheric model and the GFDL Hurricane Model using a two-stage downscaling procedure. First, tropical cyclone genesis is simulated globally using the HiRAM atmospheric model. Each storm is then downscaled into the GFDL Hurricane Model, with horizontal grid-spacing near the storm of 6 km, and including ocean coupling (e.g., 'cold wake' generation). Simulations are performed using observed sea surface temperatures (SSTs) (1980-2008); for a "control run" with 20 repeating seasonal cycles; and for a late 21st century projection using an altered SST seasonal cycle obtained from a CMIP5/RCP4.5 multi-model ensemble. In general agreement with most previous studies, projections with this framework indicate fewer tropical cyclones globally in a warmer late-21st-century climate, but also an increase in average cyclone intensity, precipitation rates, and in the number and occurrence-days of very intense category 4-5 storms. While these changes are apparent in the globally averaged tropical cyclone statistics, they are not necessarily present in each individual basin. The inter-basin variation of changes in most of the tropical cyclone metrics we examined is directly correlated to the variation in magnitude of SST increases between the basins. Finally, the framework is shown capable of reproducing both the observed global distribution of outer storm size--albeit with a slight high bias--and its inter-basin variability. Projected median size is found to remain nearly constant globally, with increases in most basins offset by decreases in the Northwest Pacific.
Kossin, J, Thomas R Knutson, K A Emanuel, T R Karl, K E Kunkel, and J O'Brien, July 2015: Reply to 'Comment on "Monitoring and Understanding Trends in Extreme Storms - State of Knowledge"'. Bulletin of the American Meteorological Society, 96(7), DOI:10.1175/BAMS-D-14-00261.1.
Msadek, Rym, Gabriel A Vecchi, and Thomas R Knutson, October 2015: North Atlantic Hurricane Activity: Past, Present and Future In Climate Change: Multidecadal and Beyond, Singapore, World Scientific, DOI:10.1142/9789814579933_0018.
We review past, present and future North Atlantic hurricane activity based on analysis of observational records and models projections. When adjusted for likely missed tropical cyclones, the observational record does not show any significant increase or decrease of North Atlantic hurricane frequency. Downscaling results for most available CMIP5 models show a decrease or little change in overall frequency of tropical storms and hurricanes, although in the Atlantic basin, previous studies by other investigators report a wider range of change (+/−60%). Some model projections of late 21st century hurricane activity indicate an increase in frequency of the strongest storms (category 4–5 hurricanes). The projected increase is substantial (+100% per century) in the CMIP3 ensemble model downscaling, but much smaller (+40%) and only marginally significant in the CMIP5 ensemble model downscaling. Rainfall rates in the inner core of the hurricanes are projected to increase with potentially a substantial damage impact. The largest source of uncertainty in predicting changes in Atlantic tropical storms activity over the first half of the 21st century arises from the internal variability of the climate system. Nonetheless, some of these natural fluctuations appear to be predictable beyond seasonal time scale. We review recent predictability assessment results based on two CMIP5 models. Initializing these models with observational estimates leads to encouraging results in predicting multi-year variations in North Atlantic hurricane frequency. However the short record and the persistent character of the time series limits the ability to confidently predict North Atlantic hurricane activity for now. Remaining model biases, despite the tremendous improvement over the recent decades, and the changing observational system make it an ongoing challenge to simulate past hurricane activity and project or predict its future behavior.
Wright, D, Thomas R Knutson, and J A Smith, December 2015: Regional climate model projections of rainfall from U.S. landfalling tropical cyclones. Climate Dynamics, 45(11-12), DOI:10.1007/s00382-015-2544-y.
The eastern United States is vulnerable to flooding from tropical cyclone rainfall. Understanding how both the frequency and intensity of this rainfall will change in the future climate is a major challenge. One promising approach is the dynamical downscaling of relatively coarse general circulation model results using higher-resolution regional climate models (RCMs). In this paper, we examine the frequency of landfalling tropical cyclones and associated rainfall properties over the eastern United States using Zetac, an 18-km resolution RCM designed for modeling Atlantic tropical cyclone activity. Simulations of 1980–2006 tropical cyclone frequency and rainfall intensity for the months of August–October are compared against results from previous studies and observation-based datasets. The 1980–2006 control simulations are then compared against results from three future climate scenarios: CMIP3/A1B (late twenty-first century) and CMIP5/RCP4.5 (early and late twenty-first century). In CMIP5 early and late twenty-first century projections, the frequency of occurrence of post-landfall tropical cyclones shows little net change over much of the eastern U.S. despite a decrease in frequency over the ocean. This reflects a greater landfalling fraction in CMIP5 projections, which is not seen in CMIP3-based projections. Average tropical cyclone rain rates over land within 500 km of the storm center increase by 8–17 % in the future climate projections relative to control. This is at least as much as expected from the Clausius–Clapeyron relation, which links a warmer atmosphere to greater atmospheric water vapor content. Over land, the percent enhancement of area-averaged rain rates from a given tropical cyclone in the warmer climate is greater for larger averaging radius (300–500 km) than near the storm, particularly for the CMIP3 projections. Although this study does not focus on attribution, the findings are broadly consistent with historical tropical cyclone rainfall changes documented in a recent observational study. The results may have important implications for future flood risks from tropical cyclones.
Christensen, J H., K K Kanikicharla, Thomas R Knutson, Hiroyuki Murakami, Mary Jo Nath, and Andrew T Wittenberg, et al., March 2014: Climate Phenomena and their Relevance for Future Regional Climate Change In Climate Change 2013: The Physical Science Basis, Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change, DOI:10.1017/CBO9781107415324.0281217-1308.
Holbrook, N J., J Li, M Collins, E Di Lorenzo, F-F Jin, and Thomas R Knutson, et al., August 2014: Decadal Climate Variability and Cross-Scale Interactions: ICCL 2013 Expert Assessment Workshop. Bulletin of the American Meteorological Society, 95(8), DOI:10.1175/BAMS-D-13-00201.1.
Irish, J L., A Sleath, M A Cialone, Thomas R Knutson, and R E Jensen, March 2014: Simulations of Hurricane Katrina (2005) under sea level and climate conditions for 1900. Climatic Change, 122, DOI:10.1007/s10584-013-1011-1.
Global warming may result in substantial sea level rise and more intense hurricanes over the next century, leading to more severe coastal flooding. Here, observed climate and sea level trends over the last century (c. 1900s to 2000s) are used to provide insight regarding future coastal inundation trends. The actual impacts of Hurricane Katrina (2005) in New Orleans are compared with the impacts of a similar hypothetical hurricane occurring c. 1900. Estimated regional sea level rise since 1900 of 0.75 m, which contains a dominant land subsidence contribution (0.57 m), serves as a 'prototype' for future climate-change induced sea level rise in other regions. Landform conditions c. 1900 were estimated by changing frictional resistance based on expected additional wetlands at lower sea levels. Surge simulations suggest that flood elevations would have been 15 to 60 % lower c. 1900 than the conditions observed in 2005. This drastic change suggests that significantly more flood damage occurred in 2005 than would have occurred if sea level and climate conditions had been like those c. 1900. We further show that, in New Orleans, sea level rise dominates surge-induced flooding changes, not only by increasing mean sea level, but also by leading to decreased wetland area. Together, these effects enable larger surges. Projecting forward, future global sea level changes of the magnitude examined here are expected to lead to increased flooding in coastal regions, even if the storm climate is unchanged. Such flooding increases in densely populated areas would presumably lead to more widespread destruction.
Kim, Hyeong-Seog, Gabriel A Vecchi, Thomas R Knutson, Whit G Anderson, Thomas L Delworth, Anthony Rosati, Fanrong Zeng, and Ming Zhao, November 2014: Tropical Cyclone Simulation and Response to CO2 Doubling in the GFDL CM2.5 High-Resolution Coupled Climate Model. Journal of Climate, 27(21), DOI:10.1175/JCLI-D-13-00475.1.
Global tropical cyclone (TC) activity is simulated by the Geophysical Fluid Dynamics Laboratory (GFDL) CM2.5, which is a fully coupled global climate model with horizontal resolution of about 50km for atmosphere and 25 km for ocean. The present climate simulation shows fairly realistic global TC frequency, seasonal cycle, and geographical distribution. The model has some notable biases in regional TC activity, including simulating too few TCs in the North Atlantic. The regional biases in TC activity are associated with simulation biases in the large-scale environment such as sea surface temperature, vertical wind shear, and vertical velocity. Despite these biases, the model simulates the large-scale variations of TC activity induced by El Nino/Southern Oscillation fairly realistically. The response of TC activity in the model to global warming is investigated by comparing the present climate with a CO2 doubling experiment. Globally, TC frequency decreases (-19%) while the intensity increases (+2.7%) in response to CO2 doubling, consistent with previous studies. The average TC lifetime decreases by -4.6%, while the TC size and rainfall increase by about 3% and 12%, respectively. These changes are generally reproduced across the different basins in terms of the sign of the change, although the percent changes vary from basin to basin and within individual basins. For the Atlantic basin, although there is an overall reduction in frequency from CO2 doubling, the warmed climate exhibits increased interannual hurricane frequency variability so that the simulated Atlantic TC activity is enhanced more during unusually warm years in the CO2-warmed climate relative to that in unusually warm years in the control climate.
Knutson, Thomas R., Fanrong Zeng, Andrew T Wittenberg, Hyeong-Seog Kim, Joseph J Sirutis, Morris A Bender, Ming Zhao, and Robert E Tuleya, January 2014: Recent Research at GFDL on Surface Temperature Trends and Simulations of Tropical Cyclone Activity in the Indian Ocean Region In Monitoring and Prediction of Tropical Cyclones in the Indian Ocean and Climate Change, New Delhi, India, Springer, DOI:10.1007/978-94-007-7720-0_5.
In this extended abstract, we report on progress in two areas of research at GFDL relating to Indian Ocean regional climate and climate change. The first topic is an assessment of regional surface temperature trends in the Indian Ocean and surrounding region. Here we illustrate the use of a multi-model approach (CMIP3 or CMIP5 model ensembles) to assess whether an anthropogenic warming signal has emerged in the historical data, including identification of where the observed trends are consistent or not with current climate models. Trends that are consistent with All Forcing runs but inconsistent with Natural Forcing Only runs are ones which we can attribute, at least in part, to anthropogenic forcing.
Knutson, Thomas R., Fanrong Zeng, and Andrew T Wittenberg, September 2014: Seasonal and Annual Mean Precipitation Extremes Occurring During 2013: A U.S. Focused Analysis [in "Explaining Extremes of 2013 from a Climate Perspective"]. Bulletin of the American Meteorological Society, 95(9), S19-S23.
Explaining Extreme Events of 2013 from a Climate Perspective. Bull. Amer. Meteor. Soc., 95, S1–S104. doi: http://dx.doi.org/10.1175/1520-0477-95.9.S1.1
Knutson, Thomas R., Fanrong Zeng, and Andrew T Wittenberg, September 2014: Multimodel Assessment of Extreme Annual-Mean Warm Anomalies During 2013 over Regions of Australia and the Western Tropical Pacific [in "Explaining Extremes of 2013 from a Climate Perspective"]. Bulletin of the American Meteorological Society, 95(9), S26-S30.
Villarini, G, D A Lavers, E Scoccimarro, Ming Zhao, M F Wehner, Gabriel A Vecchi, Thomas R Knutson, and K A Reed, June 2014: Sensitivity of Tropical Cyclone Rainfall to Idealized Global Scale Forcings. Journal of Climate, 27(12), DOI:10.1175/JCLI-D-13-00780.1.
Heavy rainfall and flooding associated with tropical cyclones (TCs) are responsible for a large number of fatalities and economic damage worldwide. Despite their large socio-economic impacts, research into heavy rainfall and flooding associated with TCs has received limited attention to date, and still represents a major challenge. Our capability to adapt to future changes in heavy rainfall and flooding associated with TCs is inextricably linked to and informed by our understanding of the sensitivity of TC rainfall to likely future forcing mechanisms. Here we use a set of idealized high-resolution atmospheric model experiments produced as part of the U.S. CLIVAR Hurricane Working Group activity to examine TC response to idealized global-scale perturbations: the doubling of CO2, uniform 2K increases in global sea surface temperature (SST), and their combined impact. As a preliminary but key step, daily rainfall patterns of composite TCs within climate model outputs are first compared and contrasted to the observational records. To assess similarities and differences across different regions in response to the warming scenarios, analyses are performed at the global and hemispheric scales and in six global TC ocean basins. The results indicate a reduction in TC daily precipitation rates in the doubling CO2 scenario (on the order of 5% globally), and an increase in TC rainfall rates associated with a uniform increase of 2K in SST (both alone and in combination with CO2 doubling; on the order of 10-20% globally).
Wu, Liang, C Chou, C-T Chen, Ronghui Huang, Thomas R Knutson, Joseph J Sirutis, Stephen T Garner, and Christopher Kerr, et al., May 2014: Simulations of the present and late 21st century western North Pacific tropical cyclone activity using a regional model. Journal of Climate, 27(9), DOI:10.1175/JCLI-D-12-00830.1.
A high-resolution regional atmospheric model is used to simulate present-day western North Pacific (WNP) tropical cyclone (TC) activity and investigate the projected changes for the late 21st century. Compared to observations, the model can realistically simulate many basic features of the WNP TC activity climatology, such as the TC genesis location, track, and lifetime. A number of spatial and temporal features of observed TC interannual variability are captured, although observed variations in basin-wide TC number are not. A relatively well-simulated feature is the contrast of years when the Asian summer monsoon trough extends eastward (retreats westward), more (fewer) TCs form within the southeastern quadrant of the WNP, and the corresponding TC activity is above (below) normal over most parts of the WNP east of 125°E. Future projections with the Coupled Model Intercomparison Project 3 (CMIP3) A1B scenario show a weak tendency for decreases in the number of WNP TCs, and of increases in the more intense TCs; these simulated changes are significant at the 80% level. The present-day simulation of intensity is limited to storms of intensity less than about 55 m s-1. There is also a weak (80% significance level) tendency for projected WNP TC activity to shift poleward under global warming. A regional-scale feature is a projected increase of the TC activity north of Taiwan, which would imply an increase in TCs making landfall in North China, the Korean Peninsula and parts of Japan. However, given the weak statistical significance found for the simulated changes, an assessment of the robustness of such regional-scale projections will require further study.
Knutson, Thomas R., Fanrong Zeng, and Andrew T Wittenberg, November 2013: Multimodel assessment of regional surface temperature trends: CMIP3 and CMIP5 Twentieth Century simulations. Journal of Climate, 26(22), DOI:10.1175/JCLI-D-12-00567.1.
Regional surface temperature trends from the CMIP3 and CMIP5 20th century runs are compared with observations -- at spatial scales ranging from global averages to individual grid points -- using simulated intrinsic climate variability from pre-industrial control runs to assess whether observed trends are detectable and/or consistent with the models' historical run trends. The CMIP5 models are also used to detect anthropogenic components of the observed trends, by assessing alternative hypotheses based on scenarios driven with either anthropogenic plus natural forcings combined, or with natural forcings only. Modeled variability is assessed via inspection of control run time series, standard deviation maps, spectral analyses, and low-frequency variance consistency tests. The models are found to provide plausible representations of internal climate variability, though there is room for improvement. The influence of observational uncertainty on the trends is assessed, and found to be generally small compared to intrinsic climate variability. Observed temperature trends over 1901-2010 are found to contain detectable anthropogenic warming components over a large fraction (about 80%) of the analyzed global area. In several regions, the observed warming is significantly underestimated by the models, including parts of the southern Ocean, south Atlantic, far eastern Atlantic, and far west Pacific. Regions without detectable warming signals include the high latitude North Atlantic, the eastern U.S., and parts of the eastern Pacific. For 1981-2010, the observed warming trends over about 45% of the globe are found to contain a detectable anthropogenic warming; this includes much of the globe within about 40-45 degrees of the equator, except for the eastern Pacific.
Knutson, Thomas R., Joseph J Sirutis, Gabriel A Vecchi, Stephen T Garner, Ming Zhao, Hyeong-Seog Kim, Morris A Bender, Robert E Tuleya, Isaac M Held, and G Villarini, September 2013: Dynamical downscaling projections of 21st century Atlantic hurricane activity: CMIP3 and CMIP5 model-based scenario. Journal of Climate, 26(17), DOI:10.1175/JCLI-D-12-00539.1.
Twenty-first-century projections of Atlantic climate change are downscaled to explore the robustness of potential changes in hurricane activity. Multimodel ensembles using the phase 3 of the Coupled Model Intercomparison Project (CMIP3)/Special Report on Emissions Scenarios A1B (SRES A1B; late-twenty-first century) and phase 5 of the Coupled Model Intercomparison Project (CMIP5)/representative concentration pathway 4.5 (RCP4.5; early- and late-twenty-first century) scenarios are examined. Ten individual CMIP3 models are downscaled to assess the spread of results among the CMIP3 (but not the CMIP5) models. Downscaling simulations are compared for 18-km grid regional and 50-km grid global models. Storm cases from the regional model are further downscaled into the Geophysical Fluid Dynamics Laboratory (GFDL) hurricane model (9-km inner grid spacing, with ocean coupling) to simulate intense hurricanes at a finer resolution. A significant reduction in tropical storm frequency is projected for the CMIP3 (−27%), CMIP5-early (−20%) and CMIP5-late (−23%) ensembles and for 5 of the 10 individual CMIP3 models. Lifetime maximum hurricane intensity increases significantly in the high-resolution experiments—by 4%–6% for CMIP3 and CMIP5 ensembles. A significant increase (+87%) in the frequency of very intense (categories 4 and 5) hurricanes (winds ≥ 59 m s−1) is projected using CMIP3, but smaller, only marginally significant increases are projected (+45% and +39%) for the CMIP5-early and CMIP5-late scenarios. Hurricane rainfall rates increase robustly for the CMIP3 and CMIP5 scenarios. For the late-twenty-first century, this increase amounts to +20% to +30% in the model hurricane's inner core, with a smaller increase (~10%) for averaging radii of 200 km or larger. The fractional increase in precipitation at large radii (200–400 km) approximates that expected from environmental water vapor content scaling, while increases for the inner core exceed this level.
Knutson, Thomas R., Fanrong Zeng, and Andrew T Wittenberg, September 2013: The Extreme March-2012 Warm Anomaly over the Eastern United States: Global Context and Multimodel Trend Analysis [in "Explaining Extreme Events of 2012 from a Climate Perspective"]. Bulletin of the American Meteorological Society, 94(9), S13-S17.
Kunkel, K E., and Thomas R Knutson, et al., April 2013: Monitoring and Understanding Trends in Extreme Storms: State of Knowledge. Bulletin of the American Meteorological Society, 94(4), DOI:10.1175/BAMS-D-11-00262.1.
The state of knowledge regarding trends and an understanding of their causes is presented for a specific subset of extreme weather and climate types. For severe convective storms (tornadoes, hailstorms, and severe thunderstorms), differences in time and space of practices of collecting reports of events make using the reporting database to detect trends extremely difficult. Overall, changes in the frequency of environments favorable for severe thunderstorms have not been statistically significant. For extreme precipitation, there is strong evidence for a nationally averaged upward trend in the frequency and intensity of events. The causes of the observed trends have not been determined with certainty, although there is evidence that increasing atmospheric water vapor may be one factor. For hurricanes and typhoons, robust detection of trends in Atlantic and western North Pacific tropical cyclone (TC) activity is significantly constrained by data heterogeneity and deficient quantification of internal variability. Attribution of past TC changes is further challenged by a lack of consensus on the physical link- ages between climate forcing and TC activity. As a result, attribution of trends to anthropogenic forcing remains controversial. For severe snowstorms and ice storms, the number of severe regional snowstorms that occurred since 1960 was more than twice that of the preceding 60 years. There are no significant multidecadal trends in the areal percentage of the contiguous United States impacted by extreme seasonal snowfall amounts since 1900. There is no distinguishable trend in the frequency of ice storms for the United States as a whole since 1950.
Vecchi, Gabriel A., Stephan Fueglistaler, Isaac M Held, Thomas R Knutson, and Ming Zhao, June 2013: Impacts of atmospheric temperature trends on tropical cyclone activity. Journal of Climate, 26(11), DOI:10.1175/JCLI-D-12-00503.1.
Impacts of tropical temperature changes in the upper troposphere (UT) and the tropical tropopause layer (TTL) on tropical cyclone (TC) activity are explored. UT and lower TTL cooling both lead to an overall increase in potential intensity (PI), while temperatures 70hPa and higher have negligible effect. Idealized experiments with a high-resolution global model show that lower temperatures in the UT are associated with increases in global and North Atlantic TC frequency, but modeled TC frequency changes are not significantly affected by TTL temperature changes nor do they scale directly with PI. Future projections of hurricane activity have been made with models that simulate the recent upward Atlantic TC trends while assuming or simulating very different tropical temperature trends. Recent Atlantic TC trends have been simulated by: i) high-resolution global models with nearly moist-adiabatic warming profiles, and ii) regional TC downscaling systems that impose the very strong UT and TTL trends of the NCEP Reanalysis, an outlier among observational estimates. Impact of these differences in temperature trends on TC activity is comparable to observed TC changes, affecting assessments of the connection between hurricanes and climate. Therefore, understanding the character of and mechanisms behind changes in UT and TTL temperature is important to understanding past and projecting future TC activity changes. We conclude that the UT and TTL temperature trends in NCEP are unlikely to be accurate, and likely drive spuriously positive TC and PI trends, and an inflated connection between absolute surface temperature warming and TC activity increases.
Zhang, Rong, and Thomas R Knutson, September 2013: The role of global climate change in the extreme low summer Arctic sea ice extent in 2012 [in "Explaining Extreme Events of 2012 from a Climate Perspective"]. Bulletin of the American Meteorological Society, 94(9), S23-S26.
Zwiers, F, L V Alexander, G Hegerl, and Thomas R Knutson, et al., 2013: Climate Extremes: Challenges in Estimating and Understanding Recent Changes in the Frequency and Intensity of Extreme Climate and Weather Events In Climate Science for Serving Society: Research, Modeling and Prediction Priorities, Springer, 339-389.
This paper focuses primarily on extremes in the historical instrumental period. We consider a range of phenomena, including temperature and precipitation extremes, tropical and extra-tropical storms, hydrological extremes, and transient extreme sea-level events. We also discuss the extent to which detection and attribution research has been able to link observed changes to external forcing of the climate system. Robust results are available that detect and often attribute changes in frequency and intensity of temperature extremes to external forcing. There is also some evidence that on a global scale, precipitation extremes have intensified due to forcing. However, robustly detecting and attributing forced changes in other important extremes, such as tropical and extratropical storms or drought remains challenging. In our review we find that there are multiple challenges that constrain advances in research on extremes. These include the state of the historical observational record, limitations in the statistical and other tools that are used for analyzing observed changes in extremes, limitations in the understanding of the processes that are involved in the production of extreme events, and in the ability to describe the natural variability of extremes with models and other tools. Despite these challenges, it is clear that enormous progress is being made in the quest to improve the understanding of extreme events, and ultimately, to produce predictive products that will help society to manage the associated risks.
Lee, T-C, and Thomas R Knutson, et al., May 2012: Impacts of climate change on tropical cyclones in the western North Pacific basin, Part I: Past observations. Tropical Cyclone Research and Review, 1(2), 213-230.
This paper reviews the current state of the science on the relationship between climate change and historical tropical cyclone (TC) activity in the western North Pacific (WNP) basin, which is the region of the ESCAP/WMO Typhoon Committee members. Existing studies of observed changes of TC activity in this basin, such as frequency, intensity, precipitation, genesis location and track pattern are summarized. Results from a survey on impacts of past TC activity on various members of Typhoon Committee are reported, along with a review of studies of past WNP landfalling TCs. With considerable interannual and interdecadal variations in the TC activity in this basin, it remains uncertain whether there has been any detectable human influence on tropical cyclone frequency, intensity, precipitation, track, or related aggregated storm activity metrics. Also, the issues on of homogeneity and consistency of best track data sets in the WNP further add uncertainty to relevant research studies. Observations indicate some regional shifts in TC activity in the basin, such as a decreasing trend in TC occurrence in part of the South China Sea and an increasing trend along the east coast of China during the past 40 years. This change is apparently related to local circulation changes in the eastern Asia and WNP, though the cause of the circulation changes remains unknown. http://tcrr.typhoon.gov.cn/EN/10.6057/2012TCRR02.08
Williams, S J., and Thomas R Knutson, et al., 2012: Physical climate forces In Coastal Impacts, Adaptation, and Vulnerability: A Technical Input to the 2012 National Climate Assessment, Cooperative Report to the 2013 National Climate Assessment, Washington, DC, Island Press, 10-51.
Ying, M, and Thomas R Knutson, et al., May 2012: Impacts of Climate Change on Tropical Cyclones in the Western North Pacific Basin, Part II: Late Twenty-First Century Projections. Tropical Cyclone Research and Review, 1(2), 231-241.
This paper reviews the latest studies on the relationship between projected late 21st century climate changes and tropical cyclone (TC) activity in the western North Pacific (WNP) basin, which is the region of the United Nations Economic and Social Commission for Asia and the Pacific (ESCAP)/ World Meteorological Organization (WMO) Typhoon Committee members. Existing studies of projected changes of TC activity in this basin, such as frequency, intensity, precipitation, genesis location and track pattern are summarized, based on an assumed A1B future climate change scenario. A review of available studies on projected future changes in WNP landfalling TC activity is also included. While it remains uncertain whether there has been any detectable human influence on tropical cyclone frequency, intensity, precipitation, track, or related aggregated storm activity metrics in the basin, modeling studies suggest changes in future tropical cyclone activity for the WNP basin. More models project decreases than increases in tropical storm frequency (range −70% to +60%); most studies project an increase in the TC intensity (range −3% to +18%); and all six available studies that include the WNP basin project increases in TC precipitation rates (range +5 to +30%). http://tcrr.typhoon.gov.cn/EN/10.6057/2012TCRR02.09
Donner, Leo J., Bruce Wyman, Richard S Hemler, Larry W Horowitz, Yi Ming, Ming Zhao, J-C Golaz, Paul Ginoux, Shian-Jiann Lin, M Daniel Schwarzkopf, John Austin, G Alaka, William F Cooke, Thomas L Delworth, Stuart Freidenreich, C Tony Gordon, Stephen M Griffies, Isaac M Held, William J Hurlin, Stephen A Klein, Thomas R Knutson, Amy R Langenhorst, Hyun-Chul Lee, Yanluan Lin, B I Magi, Sergey Malyshev, P C D Milly, Vaishali Naik, Mary Jo Nath, R Pincus, Jeff J Ploshay, V Ramaswamy, Charles J Seman, Elena Shevliakova, Joseph J Sirutis, William F Stern, Ronald J Stouffer, R John Wilson, Michael Winton, Andrew T Wittenberg, and Fanrong Zeng, July 2011: The dynamical core, physical parameterizations, and basic simulation characteristics of the atmospheric component AM3 of the GFDL Global Coupled Model CM3. Journal of Climate, 24(13), DOI:10.1175/2011JCLI3955.1.
The Geophysical Fluid Dynamics Laboratory (GFDL) has developed a coupled general circulation model (CM3) for atmosphere, oceans, land, and sea ice. The goal of CM3 is to address emerging issues in climate change, including aerosol-cloud interactions, chemistry-climate interactions, and coupling between the troposphere and stratosphere. The model is also designed to serve as the physical-system component of earth-system models and models for decadal prediction in the near-term future, for example, through improved simulations in tropical land precipitation relative to earlier-generation GFDL models. This paper describes the dynamical core, physical parameterizations, and basic simulation characteristics of the atmospheric component (AM3) of this model. Relative to GFDL AM2, AM3 includes new treatments of deep and shallow cumulus convection, cloud-droplet activation by aerosols, sub-grid variability of stratiform vertical velocities for droplet activation, and atmospheric chemistry driven by emissions with advective, convective, and turbulent transport. AM3 employs a cubed-sphere implementation of a finite-volume dynamical core and is coupled to LM3, a new land model with eco-system dynamics and hydrology. Most basic circulation features in AM3 are simulated as realistically, or more so, than in AM2. In particular, dry biases have been reduced over South America. In coupled mode, the simulation of Arctic sea ice concentration has improved. AM3 aerosol optical depths, scattering properties, and surface clear-sky downward shortwave radiation are more realistic than in AM2. The simulation of marine stratocumulus decks and the intensity distributions of precipitation remain problematic, as in AM2. The last two decades of the 20th century warm in CM3 by .32°C relative to 1881-1920. The Climate Research Unit (CRU) and Goddard Institute for Space Studies analyses of observations show warming of .56°C and .52°C, respectively, over this period. CM3 includes anthropogenic cooling by aerosol cloud interactions, and its warming by late 20th century is somewhat less realistic than in CM2.1, which warmed .66°C but did not include aerosol cloud interactions. The improved simulation of the direct aerosol effect (apparent in surface clear-sky downward radiation) in CM3 evidently acts in concert with its simulation of cloud-aerosol interactions to limit greenhouse gas warming in a way that is consistent with observed global temperature changes.
Vecchi, Gabriel A., and Thomas R Knutson, March 2011: Estimating annual numbers of Atlantic hurricanes missing from the HURDAT database (1878-1965) using ship track density. Journal of Climate, 24(6), DOI:10.1175/2010JCLI3810.1.
In this study we assess the impact of imperfect sampling in the pre-satellite era (between 1878 and 1965) on North Atlantic hurricane activity measures, and on the long-term trends in those measures. Our results suggest that a substantial upward adjustment of hurricane counts is needed prior to 1965 to account for likely 'missed' hurricanes due to sparse density of reporting ship traffic. After adjusting for our estimate of 'missed' hurricanes in the basin, the long-term (1878-2008) trend in hurricane counts changes from significantly positive to no significant change (with a nominally negative trend). The adjusted hurricane count record is more strongly connected to the difference between main development region (MDR) sea surface temperature (SST) and tropical-mean SST, than with MDR SST. Our results do not support the notion that the warming of the tropical North Atlantic due to anthropogenic greenhouse gas emissions has caused Atlantic hurricane frequency to increase.
Villarini, G, Gabriel A Vecchi, Thomas R Knutson, Ming Zhao, and J A Smith, July 2011: North Atlantic tropical storm frequency response to anthropogenic forcing: Projections and sources of uncertainty. Journal of Climate, 24(13), DOI:10.1175/2011JCLI3853.1.
The impact of future anthropogenic forcing on the frequency of tropical storms in the North Atlantic basin has been the subject of intensive investigation. However, whether the number of North Atlantic tropical storms will increase or decrease in a warmer climate is still heavily debated and a consensus has yet to be reached. To shed light on this issue, the authors use a recently developed statistical model, in which the frequency of North Atlantic tropical storms is modeled by a conditional Poisson distribution with rate of occurrence parameter that is a function of tropical Atlantic and mean tropical sea surface temperatures (SSTs). It is shown how the disagreement among dynamical modeling projections of late-twenty-first-century tropical storm frequency can be largely explained by differences in large-scale SST patterns from the different climate model projections used in these studies. The results do not support the notion of large (~200%) increases in tropical storm frequency in the North Atlantic basin over the twenty-first century in response to increasing greenhouse gases (GHGs). Because the statistical model is computationally inexpensive, it is used to examine the impact of different climate models and climate change scenarios on the frequency of North Atlantic tropical storms. The authors estimate that the dominant drivers of uncertainty in projections of tropical storm frequency over the twenty-first century are internal climate variations and systematic intermodel differences in the response of SST patterns to increasing GHGs. Relative to them, uncertainties in total GHG emissions or other climate forcings, within the scenarios explored here, represent a minor source of uncertainty in tropical storm frequency projections. These results suggest that reducing uncertainty in future projections of North Atlantic tropical storm frequency may depend as critically on reducing the uncertainty in the sensitivity of tropical Atlantic warming relative to the tropical mean, in response to GHG increase, as on improving dynamical or statistical downscaling techniques. Moreover, the large uncertainties on century-scale trends that are due to internal climate variability are likely to remain irreducible for the foreseeable future. As a further illustration of the statistical model's utility, the authors model projected changes in U.S. landfalling tropical storm activity under a variety of different climate change scenarios and climate models. These results are similar to those for the overall number of North Atlantic tropical storms, and do not point to a large increase in U.S. landfalling tropical storms over the twenty-first century in response to increasing GHGs.
Villarini, G, Gabriel A Vecchi, Thomas R Knutson, and J A Smith, May 2011: Is the recorded increase in short duration North Atlantic tropical storms spurious? Journal of Geophysical Research: Atmospheres, 116, D10114, DOI:10.1029/2010JD015493.
The number of North Atlantic tropical storms lasting two days or less exhibits a very large increase starting from the middle of the twentieth century. Still lacking are quantitative analyses to assess whether this behavior is more likely associated with climate variability or with changes in the observational system. By using statisti- cal methods combined with the current understanding of the physical processes, we provide further supporting evidence that the trend in North Atlantic tropical storms lasting two days or less is likely to be spurious. These results imply that studies ex- amining trends in the frequency of North Atlantic tropical storms from the nineteenth century should focus on storms of duration greater than about two days.
Bender, Morris A., Thomas R Knutson, Robert E Tuleya, Joseph J Sirutis, Gabriel A Vecchi, Stephen T Garner, and Isaac M Held, January 2010: Modeled impact of anthropogenic warming on the frequency of intense Atlantic hurricanes. Science, 327(5964), DOI:10.1126/science.1180568.
Several recent models suggest that the frequency of Atlantic tropical cyclones could decrease as the climate warms. However, these models are unable to reproduce storms of category 3 or higher intensity. We explored the influence of future global warming on Atlantic hurricanes with a downscaling strategy by using an operational hurricane-prediction model that produces a realistic distribution of intense hurricane activity for present-day conditions. The model projects nearly a doubling of the frequency of category 4 and 5 storms by the end of the 21st century, despite a decrease in the overall frequency of tropical cyclones, when the downscaling is based on the ensemble mean of 18 global climate-change projections. The largest increase is projected to occur in the Western Atlantic, north of 20°N.
Knutson, Thomas R., C Landsea, and K A Emanuel, May 2010: Tropical cyclones and climate change: A review In Global Perspectives on Tropical Cyclones: From Science to Mitigation, Singapore, World Scientific Publishing Company, 243-284.
A review of the science on the relationship between climate change and tropical cyclones (TCs) is presented. Topics include changes in aspects of tropical climate that are relevant to TC activity; observed trends and low-frequency variability of TC activity; paleoclimate proxy studies; theoretical and modeling studies; future projections; roadblocks to resolution of key issues; and recommendations for making future progress.
Knutson, Thomas R., J McBride, J Chan, K A Emanuel, G Holland, C Landsea, Isaac M Held, J Kossin, A K Srivastava, and M Sugi, March 2010: Tropical cyclones and climate change. Nature Geoscience, 3, DOI:doi:10.1038/ngeo779.
Whether the characteristics of tropical cyclones have changed or will change in a warming climate — and if so, how — has been the subject of considerable investigation, often with conflicting results. Large amplitude fluctuations in the frequency and intensity of tropical cyclones greatly complicate both the detection of long-term trends and their attribution to rising levels of atmospheric greenhouse gases. Trend detection is further impeded by substantial limitations in the availability and quality of global historical records of tropical cyclones. Therefore, it remains uncertain whether past changes in tropical cyclone activity have exceeded the variability expected from natural causes. However, future projections based on theory and high-resolution dynamical models consistently indicate that greenhouse warming will cause the globally averaged intensity of tropical cyclones to shift towards stronger storms, with intensity increases of 2–11% by 2100. Existing modelling studies also consistently project decreases in the globally averaged frequency of tropical cyclones, by 6–34%. Balanced against this, higher resolution modelling studies typically project substantial increases in the frequency of the most intense cyclones, and increases of the order of 20% in the precipitation rate within 100 km of the storm centre. For all cyclone parameters, projected changes for individual basins show large variations between different modelling studies.
Landsea, C, Gabriel A Vecchi, L Bengtsson, and Thomas R Knutson, May 2010: Impact of duration thresholds on Atlantic tropical cyclone counts. Journal of Climate, 23(10), DOI:10.1175/2009JCLI3034.1.
Records of Atlantic basin tropical cyclones (TCs) since the late nineteenth century indicate a very large upward trend in storm frequency. This increase in documented TCs has been previously interpreted as resulting from anthropogenic climate change. However, improvements in observing and recording practices provide an alternative interpretation for these changes: recent studies suggest that the number of potentially missed TCs is sufficient to explain a large part of the recorded increase in TC counts. This study explores the influence of another factor—TC duration—on observed changes in TC frequency, using a widely used Atlantic hurricane database (HURDAT). It is found that the occurrence of short-lived storms (duration of 2 days or less) in the database has increased dramatically, from less than one per year in the late nineteenth–early twentieth century to about five per year since about 2000, while medium- to long-lived storms have increased little, if at all. Thus, the previously documented increase in total TC frequency since the late nineteenth century in the database is primarily due to an increase in very short-lived TCs. The authors also undertake a sampling study based upon the distribution of ship observations, which provides quantitative estimates of the frequency of missed TCs, focusing just on the moderate to long-lived systems with durations exceeding 2 days in the raw HURDAT. Upon adding the estimated numbers of missed TCs, the time series of moderate to long-lived Atlantic TCs show substantial multidecadal variability, but neither time series exhibits a significant trend since the late nineteenth century, with a nominal decrease in the adjusted time series. Thus, to understand the source of the century-scale increase in Atlantic TC counts in HURDAT, one must explain the relatively monotonic increase in very short-duration storms since the late nineteenth century. While it is possible that the recorded increase in short-duration TCs represents a real climate signal, the authors consider that it is more plausible that the increase arises primarily from improvements in the quantity and quality of observations, along with enhanced interpretation techniques. These have allowed National Hurricane Center forecasters to better monitor and detect initial TC formation, and thus incorporate increasing numbers of very short-lived systems into the TC database.
Lowe, J A., and Thomas R Knutson, et al., 2010: Past and future changes in extreme sea levels and waves In Understanding Sea-Level Rise and Variability, Oxford, UK, Wiley-Blackwell, 326-375.
Garner, Stephen T., Isaac M Held, Thomas R Knutson, and Joseph J Sirutis, September 2009: The roles of wind shear and thermal stratification in past and projected changes of Atlantic tropical cyclone activity. Journal of Climate, 22(17), DOI:10.1175/2009JCLI2930.1.
Atlantic tropical cyclone activity has trended upward in recent decades. The increase coincides with favorable changes in local sea surface temperature and other environmental indices, principally associated with vertical shear and the thermodynamic profile. The relative importance of these environmental factors has not been firmly established. A recent study using a high-resolution dynamical downscaling model has captured both the trend and interannual variations in Atlantic storm frequency with considerable fidelity. In the present work, this downscaling framework is used to assess the importance of the large-scale thermodynamic environment relative to other factors influencing Atlantic tropical storms. Separate assessments are done for the recent multidecadal trend (1980–2006) and a model-projected global warming environment for the late 21st century. For the multidecadal trend, changes in the seasonal-mean thermodynamic environment (sea surface temperature and atmospheric temperature profile at fixed relative humidity) account for more than half of the observed increase in tropical cyclone frequency, with other seasonal-mean changes (including vertical shear) having a somewhat smaller combined effect. In contrast, the model's projected reduction in Atlantic tropical cyclone activity in the warm climate scenario appears to be driven mostly by increased seasonal-mean vertical shear in the western Atlantic and Caribbean rather than by changes in the SST and thermodynamic profile.
Chen, C-T, and Thomas R Knutson, 2008: On the verification and comparison of extreme rainfall indices from climate models. Journal of Climate, 21(7), DOI:10.1175/2007JCLI1494.1.
The interpretation of model precipitation output (e.g., as a gridpoint estimate versus as an areal mean) has a large impact on the evaluation and comparison of simulated daily extreme rainfall indices from climate models. It is first argued that interpretation as a gridpoint estimate (i.e., corresponding to station data) is incorrect. The impacts of this interpretation versus the areal mean interpretation in the context of rainfall extremes are then illustrated. A high-resolution (0.25° × 0.25° grid) daily observed precipitation dataset for the United States [from Climate Prediction Center (CPC)] is used as idealized perfect model gridded data. Both 30-yr return levels of daily precipitation (P30) and a simple daily intensity index are substantially reduced in these data when estimated at coarser resolution compared to the estimation at finer resolution. The reduction of P30 averaged over the conterminous United States is about 9%, 15%, 28%, 33%, and 43% when the data were first interpolated to 0.5° × 0.5°, 1° × 1°, 2° × 2°, 3° × 3°, and 4° × 4° grid boxes, respectively, before the calculation of extremes. The differences resulting from the point estimate versus areal mean interpretation are sensitive to both the data grid size and to the particular extreme rainfall index analyzed. The differences are not as sensitive to the magnitude and regional distribution of the indices. Almost all Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4) models underestimate U.S. mean P30 if it is compared directly with P30 estimated from the high-resolution CPC daily rainfall observation. On the other hand, if CPC daily data are first interpolated to various model resolutions before calculating the P30 (a more correct procedure in our view), about half of the models show good agreement with observations while most of the remaining models tend to overestimate the mean intensity of heavy rainfall events. A further implication of interpreting model precipitation output as an areal mean is that use of either simple multimodel ensemble averages of extreme rainfall or of intermodel variability measures of extreme rainfall to assess the common characteristics and range of uncertainties in current climate models is not appropriate if simulated extreme rainfall is analyzed at a model's native resolution. Owing to the large sensitivity to the assumption used, the authors recommend that for analysis of precipitation extremes, investigators interpret model precipitation output as an area average as opposed to a point estimate and then ensure that various analysis steps remain consistent with that interpretation.
Gutowski, W J., Thomas R Knutson, and Ronald J Stouffer, et al., 2008: Causes of observed changes in extremes and projections of future changes In Weather and Climate Extremes in a Changing Climate. Regions of Focus: North America, Hawaii, Caribbean, and U.S. Pacific Islands. T.R. Karl, G.A. Meehl, C.D. Miller, S.J. Hassol, A.M. Waple, and W.L. Murray (eds.), Washington, DC, Department of Commerce/NCDC, 81-116.
Knutson, Thomas R., and Robert E Tuleya, May 2008: Tropical cyclones and climate change: Revisiting recent studies at GFDL In Climate Extremes and Society, Diaz, H.F. and R.J. Murnane, Eds., New York, NY, Cambridge University Press, 120-144.
In this chapter, we revisit two recent studies performed at the Geophysical Fluid Dynamics Laboratory (GFDL), with a focus on issues relevant to tropical cyclones and climate change. The first study was a model-based assessment of twentieth-century regional surface temperature trends. The tropical Atlantic Main Development Region (MDR) for hurricane activity was found to have warmed by several tenths of a degree Celsius over the twentieth century. Coupled model historical simulations using current best estimates of radiative forcing suggest that the century-scale warming trend in the MDR may contain a significant contribution from anthropogenic forcing, including increases in atmospheric greenhouse gas concentrations. The results further suggest that the low-frequency variability in the MDR, apart from the trend, may contain substantial contributions from both radiative forcing (natural and anthropogenic) and internally generated climate variability. The second study used the GFDL huyrricane model, in an idealized setting, to simulate the impact of a pronounced CO2-induced warming on hurricane intensities and precipitation. A 1.75°C warming increases the intensities of hurricanes in the model by 5.8% in terms of surface wind speeds, 14% in terms of central pressure fall, or about one half category on the Saffir-Simpson Hurricane Scale. A revised storm-core accumulated (six-hour) rainfall measure shows a 21.6% increase under high CO2 conditions. Our simulated storm intensities are substantially less sensitive to sea surface temperature (SST) changes than recently reported historical observational trends are - a difference we are not able to completely reconcile at this time.
Knutson, Thomas R., Joseph J Sirutis, Stephen T Garner, Gabriel A Vecchi, and Isaac M Held, 2008: Simulated reduction in Atlantic hurricane frequency under twenty-first-century warming conditions. Nature Geoscience, 1(6), DOI:10.1038/ngeo202.
Increasing sea surface temperatures in the tropical Atlantic Ocean and measures of Atlantic hurricane activity have been reported to be strongly correlated since at least 1950 (refs 1, 2, 3, 4, 5), raising concerns that future greenhouse-gas-induced warming6 could lead to pronounced increases in hurricane activity. Models that explicitly simulate hurricanes are needed to study the influence of warming ocean temperatures on Atlantic hurricane activity, complementing empirical approaches. Our regional climate model of the Atlantic basin reproduces the observed rise in hurricane counts between 1980 and 2006, along with much of the interannual variability, when forced with observed sea surface temperatures and atmospheric conditions7. Here we assess, in our model system7, the changes in large-scale climate that are projected to occur by the end of the twenty-first century by an ensemble of global climate models8, and find that Atlantic hurricane and tropical storm frequencies are reduced. At the same time, near-storm rainfall rates increase substantially. Our results do not support the notion of large increasing trends in either tropical storm or hurricane frequency driven by increases in atmospheric greenhouse-gas concentrations.
Kunkel, K E., and Thomas R Knutson, et al., 2008: Observed changes in weather and climate extremes In Weather and Climate Extremes in a Changing Climate. Regions of Focus: North America, Hawaii, Caribbean, and U.S. Pacific Islands. T.R. Karl, G.A. Meehl, C.D. Miller, S.J. Hassol, A.M. Waple, and W.L. Murray (eds.), Washington, DC, Department of Commerce/NCDC, 35-80.
Vecchi, Gabriel A., and Thomas R Knutson, January 2008: On estimates of historical North Atlantic tropical cyclone activity. Journal of Climate, 21(14), DOI:10.1175/2008JCLI2178.1.
In this study, an estimate of the expected number of Atlantic tropical cyclones (TCs) that were missed by the observing system in the presatellite era (between 1878 and 1965) is developed. The significance of trends in both number and duration since 1878 is assessed and these results are related to estimated changes in sea surface temperature (SST) over the "main development region" ("MDR"). The sensitivity of the estimate of missed TCs to underlying assumptions is examined. According to the base case adjustment used in this study, the annual number of TCs has exhibited multidecadal variability that has strongly covaried with multidecadal variations in MDR SST, as has been noted previously. However, the linear trend in TC counts (1878–2006) is notably smaller than the linear trend in MDR SST, when both time series are normalized to have the same variance in their 5-yr running mean series. Using the base case adjustment for missed TCs leads to an 1878–2006 trend in the number of TCs that is weakly positive, though not statistically significant, with p ~ 0.2. The estimated trend for 1900–2006 is highly significant (+~ 4.2 storms century−1) according to the results of this study. The 1900–2006 trend is strongly influenced by a minimum in 1910–30, perhaps artificially enhancing significance, whereas the 1878–2006 trend depends critically on high values in the late 1800s, where uncertainties are larger than during the 1900s. The trend in average TC duration (1878–2006) is negative and highly significant. Thus, the evidence for a significant increase in Atlantic storm activity over the most recent 125 yr is mixed, even though MDR SST has warmed significantly. The decreasing duration result is unexpected and merits additional exploration; duration statistics are more uncertain than those of storm counts. As TC formation, development, and track depend on a number of environmental factors, of which regional SST is only one, much work remains to be done to clarify the relationship between anthropogenic climate warming, the large-scale tropical environment, and Atlantic TC activity.
Donner, S D., Thomas R Knutson, and M Oppenheimer, March 2007: Model-based assessment of the role of human-induced climate change in the 2005 Caribbean coral bleaching event. Proceedings of the National Academy of Sciences, 104(13), DOI:10.1073/pnas.0610122104.
Episodes of mass coral bleaching around the world in recent decades have been attributed to periods of anomalously warm ocean temperatures. In 2005, the sea surface temperature (SST) anomaly in the tropical North Atlantic that may have contributed to the strong hurricane season caused widespread coral bleaching in the Eastern Caribbean. Here, we use two global climate models to evaluate the contribution of natural climate variability and anthropogenic forcing to the thermal stress that caused the 2005 coral bleaching event. Historical temperature data and simulations for the 1870–2000 period show that the observed warming in the region is unlikely to be due to unforced climate variability alone. Simulation of background climate variability suggests that anthropogenic warming may have increased the probability of occurrence of significant thermal stress events for corals in this region by an order of magnitude. Under scenarios of future greenhouse gas emissions, mass coral bleaching in the Eastern Caribbean may become a biannual event in 20–30 years. However, if corals and their symbionts can adapt by 1–1.5°C, such mass bleaching events may not begin to recur at potentially harmful intervals until the latter half of the century. The delay could enable more time to alter the path of greenhouse gas emissions, although long-term "committed warming" even after stabilization of atmospheric CO2 levels may still represent an additional long-term threat to corals.
Frappier, A, Thomas R Knutson, K-B Liu, and K A Emanuel, 2007: Perspective: coordinating paleoclimate research on tropical cyclones with hurricane-climate theory and modelling. Tellus A, 59(4), 529-537.
Extending the meteorological record back in time can offer critical data for assessing tropical cyclone-climate links. While paleotempestology, the study of ancient storms, can provide a more realistic view of past 'worst case scenarios', future environmental conditions may have no analogues in the paleoclimate record. The primary value in paleotempestology proxy records arises from their ability to quantify climate–tropical cyclone interactions by sampling tropical cyclone activity during pre-historic periods with a wider range of different climates. New paleotempestology proxies are just beginning to be applied, encouraging new collaboration between the paleo and tropical cyclone dynamics communities. The aim of this paper is to point out some paths toward closer coordination by outlining target needs of the tropical cyclone theory and modelling community and potential contributions of the paleotempestology community. We review recent advances in paleotempestology, summarize the range of types and quality of paleodata generation, and identify future research opportunities for paleotempestology, tropical cyclone dynamics and climate change impacts and attribution communities.
Knutson, Thomas R., Joseph J Sirutis, Stephen T Garner, Isaac M Held, and Robert E Tuleya, 2007: Simulation of the Recent Multidecadal Increase of Atlantic Hurricane Activity Using an 18-km-Grid Regional Model. Bulletin of the American Meteorological Society, 88(10), DOI:10.1175/BAMS-88-10-1549.
In this study, a new modeling framework for simulating Atlantic hurricane activity is introduced. The model is an 18-km-grid nonhydrostatic regional model, run over observed specified SSTs and nudged toward observed time-varying large-scale atmospheric conditions (Atlantic domain wavenumbers 0–2) derived from the National Centers for Environmental Prediction (NCEP) reanalyses. Using this "perfect large-scale model" approach for 27 recent August–October seasons (1980–2006), it is found that the model successfully reproduces the observed multidecadal increase in numbers of Atlantic hurricanes and several other tropical cyclone (TC) indices over this period. The correlation of simulated versus observed hurricane activity by year varies from 0.87 for basin-wide hurricane counts to 0.41 for U.S. landfalling hurricanes. For tropical storm count, accumulated cyclone energy, and TC power dissipation indices the correlation is 0.75, for major hurricanes the correlation is 0.69, and for U.S. landfalling tropical storms, the correlation is 0.57. The model occasionally simulates hurricanes intensities of up to category 4 (942 mb) in terms of central pressure, although the surface winds (< 47 m s-1 ) do not exceed category-2 intensity. On interannual time scales, the model reproduces the observed ENSO-Atlantic hurricane covariation reasonably well. Some notable aspects of the highly contrasting 2005 and 2006 seasons are well reproduced, although the simulated activity during the 2006 core season was excessive. The authors conclude that the model appears to be a useful tool for exploring mechanisms of hurricane variability in the Atlantic (e.g., shear versus potential intensity contributions). The model may be capable of making useful simulations/projections of pre-1980 or twentieth-century Atlantic hurricane activity. However, the reliability of these projections will depend on obtaining reliable large-scale atmospheric and SST conditions from sources external to the model.
Shepherd, J M., and Thomas R Knutson, 2007: The current debate on the linkage between global warming and hurricanes. Geography Compass, 1(1), DOI:10.1111/j.1749-8198.2006.00002.x.
Following Hurricane Katrina and the parade of storms that affected the conterminous United States in 2004–2005, the apparent recent increase in intense hurricane activity in the Atlantic basin, and the reported increases in recent decades in some hurricane intensity and duration measures in several basins have received considerable attention. An important ongoing avenue of investigation in the climate and meteorology research communities is to determine the relative roles of anthropogenic forcing (i.e., global warming) and natural variability in producing the observed recent increases in hurricane frequency in the Atlantic, as well as the reported increases of tropical cyclone activity measures in several other ocean basins. A survey of the existing literature shows that many types of data have been used to describe hurricane intensity, and not all records are of sufficient length to reliably identify historical trends. Additionally, there are concerns among researchers about possible effects of data inhomogeneities on the reported trends. Much of the current debate has focused on the relative roles of sea-surface temperatures or large-scale potential intensity versus the role of other environmental factors such as vertical wind shear in causing observed changes in hurricane statistics. Significantly more research – from observations, theory, and modeling – is needed to resolve the current debate around global warming and hurricanes.
Delworth, Thomas L., Anthony J Broccoli, Anthony Rosati, Ronald J Stouffer, V Balaji, J A Beesley, William F Cooke, Keith W Dixon, John P Dunne, Krista A Dunne, J W Durachta, Kirsten L Findell, Paul Ginoux, Anand Gnanadesikan, C Tony Gordon, Stephen M Griffies, Richard G Gudgel, Matthew J Harrison, Isaac M Held, Richard S Hemler, Larry W Horowitz, Stephen A Klein, Thomas R Knutson, P J Kushner, Amy R Langenhorst, Hyun-Chul Lee, Shian-Jiann Lin, Jian Lu, Sergey Malyshev, P C D Milly, V Ramaswamy, J L Russell, M Daniel Schwarzkopf, Elena Shevliakova, Joseph J Sirutis, Michael J Spelman, William F Stern, Michael Winton, Andrew T Wittenberg, Bruce Wyman, Fanrong Zeng, and Rong Zhang, 2006: GFDL's CM2 Global Coupled Climate Models. Part I: Formulation and Simulation Characteristics. Journal of Climate, 19(5), DOI:10.1175/JCLI3629.1.
The formulation and simulation characteristics of two new global coupled climate models developed at NOAA's Geophysical Fluid Dynamics Laboratory (GFDL) are described. The models were designed to simulate atmospheric and oceanic climate and variability from the diurnal time scale through multicentury climate change, given our computational constraints. In particular, an important goal was to use the same model for both experimental seasonal to interannual forecasting and the study of multicentury global climate change, and this goal has been achieved. Two versions of the coupled model are described, called CM2.0 and CM2.1. The versions differ primarily in the dynamical core used in the atmospheric component, along with the cloud tuning and some details of the land and ocean components. For both coupled models, the resolution of the land and atmospheric components is 2° latitude × 2.5° longitude; the atmospheric model has 24 vertical levels. The ocean resolution is 1° in latitude and longitude, with meridional resolution equatorward of 30° becoming progressively finer, such that the meridional resolution is 1/3° at the equator. There are 50 vertical levels in the ocean, with 22 evenly spaced levels within the top 220 m. The ocean component has poles over North America and Eurasia to avoid polar filtering. Neither coupled model employs flux adjustments. The control simulations have stable, realistic climates when integrated over multiple centuries. Both models have simulations of ENSO that are substantially improved relative to previous GFDL coupled models. The CM2.0 model has been further evaluated as an ENSO forecast model and has good skill (CM2.1 has not been evaluated as an ENSO forecast model). Generally reduced temperature and salinity biases exist in CM2.1 relative to CM2.0. These reductions are associated with 1) improved simulations of surface wind stress in CM2.1 and associated changes in oceanic gyre circulations; 2) changes in cloud tuning and the land model, both of which act to increase the net surface shortwave radiation in CM2.1, thereby reducing an overall cold bias present in CM2.0; and 3) a reduction of ocean lateral viscosity in the extratropics in CM2.1, which reduces sea ice biases in the North Atlantic. Both models have been used to conduct a suite of climate change simulations for the 2007 Intergovernmental Panel on Climate Change (IPCC) assessment report and are able to simulate the main features of the observed warming of the twentieth century. The climate sensitivities of the CM2.0 and CM2.1 models are 2.9 and 3.4 K, respectively. These sensitivities are defined by coupling the atmospheric components of CM2.0 and CM2.1 to a slab ocean model and allowing the model to come into equilibrium with a doubling of atmospheric CO2. The output from a suite of integrations conducted with these models is freely available online (see http://nomads.gfdl.noaa.gov/). Manuscript received 8 December 2004, in final form 18 March 2005
Findell, Kirsten L., Thomas R Knutson, and P C D Milly, 2006: Weak simulated extratropical responses to complete tropical deforestation. Journal of Climate, 19(12), 2835-2850.
The Geophysical Fluid Dynamics Laboratory atmosphere–land model version 2 (AM2/LM2) coupled to a 50-m-thick slab ocean model has been used to investigate remote responses to tropical deforestation. Magnitudes and significance of differences between a control run and a deforested run are assessed through comparisons of 50-yr time series, accounting for autocorrelation and field significance. Complete conversion of the broadleaf evergreen forests of South America, central Africa, and the islands of Oceania to grasslands leads to highly significant local responses. In addition, a broad but mild warming is seen throughout the tropical troposphere (<0.2°C between 700 and 150 mb), significant in northern spring and summer. However, the simulation results show very little statistically significant response beyond the Tropics. There are no significant differences in any hydroclimatic variables (e.g., precipitation, soil moisture, evaporation) in either the northern or the southern extratropics. Small but statistically significant local differences in some geopotential height and wind fields are present in the southeastern Pacific Ocean. Use of the same statistical tests on two 50-yr segments of the control run show that the small but significant extratropical differences between the deforested run and the control run are similar in magnitude and area to the differences between nonoverlapping segments of the control run. These simulations suggest that extratropical responses to complete tropical deforestation are unlikely to be distinguishable from natural climate variability.
Knutson, Thomas R., Thomas L Delworth, Keith W Dixon, Isaac M Held, Jian Lu, V Ramaswamy, M Daniel Schwarzkopf, G Stenchikov, and Ronald J Stouffer, 2006: Assessment of Twentieth-Century regional surface temperature trends using the GFDL CM2 coupled models. Journal of Climate, 19(9), DOI:10.1175/JCLI3709.1.
Historical climate simulations of the period 1861–2000 using two new Geophysical Fluid Dynamics Laboratory (GFDL) global climate models (CM2.0 and CM2.1) are compared with observed surface temperatures. All-forcing runs include the effects of changes in well-mixed greenhouse gases, ozone, sulfates, black and organic carbon, volcanic aerosols, solar flux, and land cover. Indirect effects of tropospheric aerosols on clouds and precipitation processes are not included. Ensembles of size 3 (CM2.0) and 5 (CM2.1) with all forcings are analyzed, along with smaller ensembles of natural-only and anthropogenic-only forcing, and multicentury control runs with no external forcing. Observed warming trends on the global scale and in many regions are simulated more realistically in the all-forcing and anthropogenic-only forcing runs than in experiments using natural-only forcing or no external forcing. In the all-forcing and anthropogenic-only forcing runs, the model shows some tendency for too much twentieth-century warming in lower latitudes and too little warming in higher latitudes. Differences in Arctic Oscillation behavior between models and observations contribute substantially to an underprediction of the observed warming over northern Asia. In the all-forcing and natural-only forcing runs, a temporary global cooling in the models during the 1880s not evident in the observed temperature records is volcanically forced. El Niño interactions complicate comparisons of observed and simulated temperature records for the El Chichón and Mt. Pinatubo eruptions during the early 1980s and early 1990s. The simulations support previous findings that twentieth-century global warming has resulted from a combination of natural and anthropogenic forcing, with anthropogenic forcing being the dominant cause of the pronounced late-twentieth-century warming. The regional results provide evidence for an emergent anthropogenic warming signal over many, if not most, regions of the globe. The warming signal has emerged rather monotonically in the Indian Ocean/western Pacific warm pool during the past half-century. The tropical and subtropical North Atlantic and the tropical eastern Pacific are examples of regions where the anthropogenic warming signal now appears to be emerging from a background of more substantial multidecadal variability.
Stouffer, Ronald J., Thomas L Delworth, Keith W Dixon, Richard G Gudgel, Isaac M Held, Richard S Hemler, Thomas R Knutson, M Daniel Schwarzkopf, Michael J Spelman, Michael Winton, Anthony J Broccoli, Hyun-Chul Lee, Fanrong Zeng, and Brian J Soden, 2006: GFDL's CM2 Global Coupled Climate Models. Part IV: Idealized Climate Response. Journal of Climate, 19(5), DOI:10.1175/JCLI3632.1.
The climate response to idealized changes in the atmospheric CO2 concentration by the new GFDL climate model (CM2) is documented. This new model is very different from earlier GFDL models in its parameterizations of subgrid-scale physical processes, numerical algorithms, and resolution. The model was constructed to be useful for both seasonal-to-interannual predictions and climate change research. Unlike previous versions of the global coupled GFDL climate models, CM2 does not use flux adjustments to maintain a stable control climate. Results from two model versions, Climate Model versions 2.0 (CM2.0) and 2.1 (CM2.1), are presented. Two atmosphere–mixed layer ocean or slab models, Slab Model versions 2.0 (SM2.0) and 2.1 (SM2.1), are constructed corresponding to CM2.0 and CM2.1. Using the SM2 models to estimate the climate sensitivity, it is found that the equilibrium globally averaged surface air temperature increases 2.9 (SM2.0) and 3.4 K (SM2.1) for a doubling of the atmospheric CO2 concentration. When forced by a 1% per year CO2 increase, the surface air temperature difference around the time of CO2 doubling [transient climate response (TCR)] is about 1.6 K for both coupled model versions (CM2.0 and CM2.1). The simulated warming is near the median of the responses documented for the climate models used in the 2001 Intergovernmental Panel on Climate Change (IPCC) Working Group I Third Assessment Report (TAR). The thermohaline circulation (THC) weakened in response to increasing atmospheric CO2. By the time of CO2 doubling, the weakening in CM2.1 is larger than that found in CM2.0: 7 and 4 Sv (1 Sv 106 m3 s−1), respectively. However, the THC in the control integration of CM2.1 is stronger than in CM2.0, so that the percentage change in the THC between the two versions is more similar. The average THC change for the models presented in the TAR is about 3 or 4 Sv; however, the range across the model results is very large, varying from a slight increase (+2 Sv) to a large decrease (−10 Sv).
Webb, M J., C A Senior, D M H Sexton, W J Ingram, K D Williams, M A Ringer, B McAveney, R Colman, Brian J Soden, Richard G Gudgel, Thomas R Knutson, S Emori, T Ogura, Y Tsushima, N Andronova, B Li, I Musat, S Bony, and K E Taylor, 2006: On the contribution of local feedback mechanisms to the range of climate sensitivity in two GCM ensembles. Climate Dynamics, 27(1), DOI:10.1007/s00382-006-0111-2.
Global and local feedback analysis techniques have been applied to two ensembles of mixed layer equilibrium CO2 doubling climate change experiments, from the CFMIP (Cloud Feedback Model Intercomparison Project) and QUMP (Quantifying Uncertainty in Model Predictions) projects. Neither of these new ensembles shows evidence of a statistically significant change in the ensemble mean or variance in global mean climate sensitivity when compared with the results from the mixed layer models quoted in the Third Assessment Report of the IPCC. Global mean feedback analysis of these two ensembles confirms the large contribution made by inter-model differences in cloud feedbacks to those in climate sensitivity in earlier studies; net cloud feedbacks are responsible for 66% of the inter-model variance in the total feedback in the CFMIP ensemble and 85% in the QUMP ensemble. The ensemble mean global feedback components are all statistically indistinguishable between the two ensembles, except for the clear-sky shortwave feedback which is stronger in the CFMIP ensemble. While ensemble variances of the shortwave cloud feedback and both clear-sky feedback terms are larger in CFMIP, there is considerable overlap in the cloud feedback ranges; QUMP spans 80% or more of the CFMIP ranges in longwave and shortwave cloud feedback. We introduce a local cloud feedback classification system which distinguishes different types of cloud feedbacks on the basis of the relative strengths of their longwave and shortwave components, and interpret these in terms of responses of different cloud types diagnosed by the International Satellite Cloud Climatology Project simulator. In the CFMIP ensemble, areas where low-top cloud changes constitute the largest cloud response are responsible for 59% of the contribution from cloud feedback to the variance in the total feedback. A similar figure is found for the QUMP ensemble. Areas of positive low cloud feedback (associated with reductions in low level cloud amount) contribute most to this figure in the CFMIP ensemble, while areas of negative cloud feedback (associated with increases in low level cloud amount and optical thickness) contribute most in QUMP. Classes associated with high-top cloud feedbacks are responsible for 33 and 20% of the cloud feedback contribution in CFMIP and QUMP, respectively, while classes where no particular cloud type stands out are responsible for 8 and 21%.
Williams, K D., M A Ringer, C A Senior, M J Webb, B McAveney, N Andronova, S Bony, J-L Dufresne, S Emori, Richard G Gudgel, Thomas R Knutson, B Li, K Lo, I Musat, J Wegner, A Slingo, and J F B Mitchell, 2006: Evaluation of a component of the cloud response to climate change in an intercomparison of climate models. Climate Dynamics, 26(2-3), DOI:10.1007/s00382-005-0067-7.
Most of the uncertainty in the climate sensitivity of contemporary general circulation models (GCMs) is believed to be connected with differences in the simulated radiative feedback from clouds. Traditional methods of evaluating clouds in GCMs compare time–mean geographical cloud fields or aspects of present-day cloud variability, with observational data. In both cases a hypothetical assumption is made that the quantity evaluated is relevant for the mean climate change response. Nine GCMs (atmosphere models coupled to mixed-layer ocean models) from the CFMIP and CMIP model comparison projects are used in this study to demonstrate a common relationship between the mean cloud response to climate change and present-day variability. Although atmosphere–mixed-layer ocean models are used here, the results are found to be equally applicable to transient coupled model simulations. When changes in cloud radiative forcing (CRF) are composited by changes in vertical velocity and saturated lower tropospheric stability, a component of the local mean climate change response can be related to present-day variability in all of the GCMs. This suggests that the relationship is not model specific and might be relevant in the real world. In this case, evaluation within the proposed compositing framework is a direct evaluation of a component of the cloud response to climate change. None of the models studied are found to be clearly superior or deficient when evaluated, but a couple appear to perform well on several relevant metrics. Whilst some broad similarities can be identified between the 60°N–60°S mean change in CRF to increased CO2 and that predicted from present-day variability, the two cannot be quantitatively constrained based on changes in vertical velocity and stability alone. Hence other processes also contribute to the global mean cloud response to climate change.
Held, Isaac M., Thomas L Delworth, Jian Lu, Kirsten L Findell, and Thomas R Knutson, 2005: Simulation of Sahel drought in the 20th and 21st centuries. Proceedings of the National Academy of Sciences, 102(50), DOI:10.1073/pnas.0509057102.
The Sahel, the transition zone between the Saharan desert and the rainforests of Central Africa and the Guinean Coast, experienced a severe drying trend from the 1950s to the 1980s, from which there has been partial recovery. Continuation of either the drying trend or the more recent ameliorating trend would have far-ranging implications for the economy and ecology of the region. Coupled atmosphere/ocean climate models being used to simulate the future climate have had difficulty simulating Sahel rainfall variations comparable to those observed, thus calling into question their ability to predict future climate change in this region. We describe simulations using a new global climate model that capture several aspects of the 20th century rainfall record in the Sahel. An ensemble mean over eight realizations shows a drying trend in the second half of the century of nearly half of the observed amplitude. Individual realizations can be found that display striking similarity to the observed time series and drying pattern, consistent with the hypothesis that the observations are a superposition of an externally forced trend and internal variability. The drying trend in the ensemble mean of the model simulations is attributable to anthropogenic forcing, partly to an increase in aerosol loading and partly to an increase in greenhouse gases. The model projects a drier Sahel in the future, due primarily to increasing greenhouse gases.
Knutson, Thomas R., and Robert E Tuleya, 2005: Reply. Journal of Climate, 18(23), DOI:10.1175/JCLI3593.1.
A response is made to the comments of Michaels et al. concerning a recent study by the authors. Even after considering Michaels et al.'s comments, the authors stand behind the conclusions of the original study. In contrast to Michaels et al., who exclusively emphasize uncertainties that lead to smaller future changes, uncertainties are noted that could lead to either smaller or larger changes in future intensities of hurricanes than those summarized in the original study, with accompanying smaller or larger societal impacts.
Knutson, Thomas R., and Robert E Tuleya, 2004: Impact of CO2-induced warming on simulated hurricane intensity and precipitation: Sensitivity to the choice of climate model and convective parameterization. Journal of Climate, 17(18), 3477-3495.
Previous studies have found that idealized hurricanes, simulated under warmer, high-CO2 conditions, are more intense and have higher precipitation rates than under present-day conditions. The present study explores the sensitivity of this result to the choice of climate model used to define the CO2-warmed environment and to the choice of convective parameterization used in the nested regional model that simulates the hurricanes. Approximately 1300 five-day idealized simulations are performed using a higher-resolution version of the GFDL hurricane prediction system (grid spacing as fine as 9 km, with 42 levels). All storms were embedded in a uniform 5 m s−1 easterly background flow. The large-scale thermodynamic boundary conditions for the experiments— atmospheric temperature and moisture profiles and SSTs—are derived from nine different Coupled Model Intercomparison Project (CMIP2+) climate models. The CO2-induced SST changes from the global climate models, based on 80-yr linear trends from +1% yr−1 CO2 increase experiments, range from about +0.8° to +2.4°C in the three tropical storm basins studied. Four different moist convection parameterizations are tested in the hurricane model, including the use of no convective parameterization in the highest resolution inner grid. Nearly all combinations of climate model boundary conditions and hurricane model convection schemes show a CO2-induced increase in both storm intensity and near-storm precipitation rates. The aggregate results, averaged across all experiments, indicate a 14% increase in central pressure fall, a 6% increase in maximum surface wind speed, and an 18% increase in average precipitation rate within 100 km of the storm center. The fractional change in precipitation is more sensitive to the choice of convective parameterization than is the fractional change of intensity. Current hurricane potential intensity theories, applied to the climate model environments, yield an average increase of intensity (pressure fall) of 8% (Emanuel) to 16% (Holland) for the high-CO2 environments. Convective available potential energy (CAPE) is 21% higher on average in the high-CO2 environments. One implication of the results is that if the frequency of tropical cyclones remains the same over the coming century, a greenhouse gas–induced warming may lead to a gradually increasing risk in the occurrence of highly destructive category-5 storms.
Broccoli, Anthony J., Keith W Dixon, Thomas L Delworth, Thomas R Knutson, Ronald J Stouffer, and Fanrong Zeng, 2003: Twentieth-century temperature and precipitation trends in ensemble climate simulations including natural and anthropogenic forcing. Journal of Geophysical Research, 108(D24), 4798, DOI:10.1029/2003JD003812.
We present results from a series of ensemble integrations of a global coupled atmosphere-ocean model for the period 1865-1997. Each ensemble consists of three integrations initialized from different points in a long-running GFDL R30 coupled model control simulation. The first ensemble includes time-varying forcing from greenhouse gases only. In the remaining three ensembles, forcings from anthropogenic sulfate aerosols, solar variability, and volcanic aerosols in the stratosphere are added progressively, such that the fourth ensemble uses all four of these forcings. The effects of anthropogenic sulfate aerosols are represented by changes in surface albedo, and the effects of volcanic aerosols are represented by latitude-dependent perturbations in incident solar radiation. Comparisons with observations reveal that the addition of the natural forcings (solar and volcanic) improves the simulation of global multidecadal trends in temperature, precipitation, and ocean heat content. Solar and volcanic forcings are important contributors to early twentieth century warming. Volcanic forcing reduces the warming simulated for the late twentieth century. Interdecadal variations in global mean surface air temperature from the ensemble of experiments with all four forcings are very similar to observed variations during most of the twentieth century. The improved agreement of simulated and observed temperature trends when natural climate forcings are included supports the climatic importance of variations in radiative forcing during the twentieth century.
Dixon, Keith W., Thomas L Delworth, Thomas R Knutson, Michael J Spelman, and Ronald J Stouffer, 2003: A comparison of climate change simulations produced by two GFDL coupled climate models. Global & Planetary Change, 37(1-2), 81-102.
The transient responses of two versions of the Geophysical Fluid Dynamics Laboratory (GFDL) coupled climate model to a climate change forcing scenario are examined. The same computer codes were used to construct the atmosphere, ocean, sea ice and land surface components of the two models, and they employ the same types of sub-grid-scale parameterization schemes. The two model versions differ primarily, but not solely, in their spatial resolution. Comparisons are made of results from six coarse-resolution R15 climate change experiments and three medium-resolution R30 experiments in which levels of greenhouse gases (GHGs) and sulfate aerosols are specified to change over time. The two model versions yield similar global mean surface air temperature responses until the second half of the 21st century, after which the R15 model exhibits a somewhat larger response. Polar amplification of the Northern Hemisphere's warming signal is more pronounced in the R15 model, in part due to the R15's cooler control climate, which allows for larger snow and ice albedo positive feedbacks. Both models project a substantial weakening of the North Atlantic overturning circulation and a large reduction in the volume of Arctic sea ice to occur in the 21st century. Relative to their respective control integrations, there is a greater reduction of Arctic sea ice in the R15 experiments than in the R30 simulations as the climate system warms. The globally averaged annual mean precipitation rate is simulated to increase over time, with both model versions projecting an increase of about 8% to occur by the decade of the 2080s. While the global mean precipitation response is quite similar in the two models, regional differences exist, with the R30 model displaying larger increases in equatorial regions.
Davey, M K., M Huddleston, K R Sperber, P Braconnot, F O Bryan, D Chen, R Colman, C Cooper, U Cubasch, P Delecluse, D G DeWitt, L Fairhead, G M Flato, C Tony Gordon, T Hogan, M Ji, , A Kitoh, Thomas R Knutson, M Latif, H Le Treut, T Li, Syukuro Manabe, C R Mechoso, G A Meehl, S B Power, E Roeckner, L Terray, A Vintzileos, R Voss, B Wang, W M Washington, I Yoshikawa, J-Y Yu, S Yukimoto, and S E Zebiak, 2002: STOIC: A study of coupled model climatology and variability in tropical ocean regions. Climate Dynamics, 18(5), 403-420.
We describe the behavior of 23 dynamical ocean-atmosphere models, in the context of comparison with observations in a common framework. Fields of tropical sea surface temperature (SST), surface wind stress and upper ocean vertically averaged temperature (VAT) are assessed with regard to annual mean, seasonal cycle, and interannual variability characteristics. Of the participating models, 21 are coupled GCMs, of which 13 use no form of flux adjustment in the tropics. The models vary widely in design, components and purpose; nevertheless several common features are apparent. In most models without flux adjustment, the annual mean equatorial SST in the central Pacific is too cool and the Atlantic zonal SST gradient has the wrong sign. Annual mean wind stress is often too weak in the central Pacific and in the Atlantic, but too strong in the west Pacific. Few models have an upper ocean VAT seasonal cycle like that observed in the equatorial Pacific. Interannual variability is commonly too weak in the models: in particular, wind stress variability is low in the equatorial Pacific. Most models have difficulty in reproducing the observed Pacific 'horseshoe' pattern of negative SST correlations with interannual Niño 3 SST anomalies, or the observed Indian-Pacific lag correlations. The results for the fields examined indicate that several substantial model improvements are needed, particularly with regard to surface wind stress.
Delworth, Thomas L., Ronald J Stouffer, Keith W Dixon, Michael J Spelman, Thomas R Knutson, Anthony J Broccoli, P J Kushner, and Richard T Wetherald, 2002: Review of simulations of climate variability and change with the GFDL R30 coupled climate model. Climate Dynamics, 19(7), 555-574.
A review is presented of the development and simulation characteristics of the most recent version of a global coupled model for climate variability and change studies at the Geophysical Fluid Dynamics Laboratory, as well as a review of the climate change experiments performed with the model. The atmospheric portion of the coupled model uses a spectral technique with rhomboidal 30 truncation, which corresponds to a transform grid with a resolution of approximately 3.75° longitude by 2.25° latitude. The ocean component has a resolution of approximately 1.875° longitude by 2.25° latitude. Relatively simple formulations of river routing, sea ice, and land surface processes are included. Two primary versions of the coupled model are described, differing in their initialization techniques and in the specification of sub-grid scale oceanic mixing of heat and salt. For each model a stable control integration of near milennial scale duration has been conducted, and the characteristics of both the time-mean and variability are described and compared to observations. A review is presented of a suite of climate change experiments conducted with these models using both idealized and realistic estimates of time-varying radiative forcing. Some experiments include estimates of forcing from past changes in volcanic aerosols and solar irradiance. The experiments performed are described, and some of the central findings are highlighted. In particular, the observed increase in global mean surface temperature is largely contained within the spread of simulated global mean temperatures from an ensemble of experiments using observationally-derived estimates of the changes in radiative forcing from increasing greenhouse gases and sulfate aerosols.
Tuleya, Robert E., and Thomas R Knutson, 2002: Impact of climate change on tropical cyclones In Atmosphere-Ocean Interactions, Vol. 1, Southampton, UK, WIT Press, 293-312.
One of the possible impacts of global warming is on tropical cyclones, on their formation, track, intensity and decay rates. One of the consequences of global warming appears to be not only an increase in sea surface temperature, but more importantly a potential increase in the overall energy flux at the tropical ocean surface. Theoretical considerations imply that this increased surface disequilibrium may lead to more intense tropical storms. Three-dimensional numerical modeling is another approach to evaluating these potential consequences. Since global models are still rather limited in simulating mesoscale storm structure, this paper describes a regional modeling approach utilizing a multiple nested technique which has already been shown to be practical in operational forecasts. These 3-D model results confirm theoretical methods that indicate an increase of 3 to 10% in maximum wind speeds for a CO2 tropical SST warming of ~2.5°C. Perhaps more importantly, model results indicate a 20 to 30% increase in hurricane-related precipitation. Furthermore, the resulting increases in intensity and precipitation appear to be qualitatively insensitive to changes in convective parameterization. This paper emphasizes the impact of global warming on storm intensity and precipitation. The question of the possible impact on tropical storm frequency and track is still problematic.
Cubasch, U, G A Meehl, G J Boer, Ronald J Stouffer, M R Dix, A Noda, C A Senior, S C B Raper, K S Yap, A Abe-Ouchi, S Brinkop, M Claussen, M Collins, J Evans, I Fischer-Bruns, J C Fyfe, A Ganopolski, J M Gregory, Z-Z Hu, F Joos, Thomas R Knutson, R Knutti, C Landsea, L Mearns, P C D Milly, J F B Mitchell, T Nozawa, H Paeth, J Räisänen, R Sausen, S Smith, T F Stocker, A Timmermann, U Ulbrich, A J Weaver, J Wegner, P Whetton, T M L Wigley, Michael Winton, and F Zwiers, 2001: Projections of future climate change In Climate Change 2001: The Scientific Basis. Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change, Cambridge, UK, Cambridge University Press, 526-582.
Knutson, Thomas R., Robert E Tuleya, W Shen, and I Ginis, 2001: Impact of CO2-induced warming on hurricane intensities simulated in a hurricane model with ocean coupling. Journal of Climate, 14(11), 2458-2468.
This study explores how a carbon dioxide (CO2) warming-induced enhancement of hurricane intensity could be altered by the inclusion of hurricane-ocean coupling. Simulations are performed using a coupled version of the Geophysical Fluid Dynamics Laboratory hurricane prediction system in an idealized setting with highly simplified background flow fields. The large-scale atmospheric boundary conditions for these high-resolution experiments (atmospheric temperature and moisture profiles and moisture profiles and SSTs) are derived from control and high-CO2 climatologies obtained from a low-resolution (R30) global coupled ocean-atmosphere climate model. The high-CO2 conditions are obtained from years 71-120 of a transient +1% yr -1 CO2-increase experiment with the global model. The CO2-induced SST changes from the global climate model range from +2.2° to +2.7°C in the six tropical storm basins studied. In the storm simulations, ocean coupling significantly reduces the intensity of simulated tropical cyclones, in accord with previous studies. However, the net impact of ocean coupling on the simulated CO2 warming-induced intensification of tropical cyclones is relatively minor. For both coupled and uncoupled simulations, the percentage increase in maximum surface wind speeds averages about 5%-6% over the six basins and varies from about 3% to 10% across the different basins. Both coupled and uncoupled simulations also show strong increases of near-storm precipitation under high-CO2 climate conditions, relative to control (present day) conditions.
Latif, M, Thomas R Knutson, and Syukuro Manabe, et al., 2001: ENSIP: The El Niño simulation intercomparison project. Climate Dynamics, 18(3-4), 255-276.
Abstract An ensemble of twenty four coupled ocean-atmosphere models has been compared with respect to their performance in the tropical Pacific. The coupled models span a large portion of the parameter space and differ in many respects. The intercomparison includes TOGA (Tropical Ocean Global Atmosphere)-type models consisting of high-resolution tropical ocean models and coarse-resolution global atmosphere models, coarse-resolution global coupled models, and a few global coupled models with high resolution in the equatorial region in their ocean components. The performance of the annual mean state, the seasonal cycle and the interannual variability are investigated. The primary quantity analysed is sea surface temperature (SST). Additionally, the evolution of interannual heat content variations in the tropical Pacific and the relationship between the interannual SST variations in the equatorial Pacific to fluctuations in the strength of the Indian summer monsoon are investigated. The results can be summarised as follows: almost all models (even those employing flux corrections) still have problems in simulating the SST climatology, although some improvements are found relative to earlier intercomparison studies. Only a few of the coupled models simulate the El Niño/Southern Oscillation (ENSO) in terms of gross equatorial SST anomalies realistically. In particular, many models overestimate the variability in the western equatorial Pacific and underestimate the SST variability in the east. The evolution of interannual heat content variations is similar to that observed in almost all models. Finally, the majority of the models show a strong connection between ENSO and the strength of the Indian summer monsoon.
Manabe, Syukuro, Thomas R Knutson, Ronald J Stouffer, and Thomas L Delworth, 2001: Exploring natural and anthropogenic variation of climate. Quarterly Journal of the Royal Meteorological Society, 127(571), 1-24.
This lecture discusses the low-frequency variability of surface temperature using a coupled ocean-atmosphere-land-surface model developed at the Geophysical Fluid Dynamics Laboratory/NOAA. Despite the highly idealized parametrization of various physical processes, the coupled model simulates reasonably well the variability of local and global mean surface temperature. The first half of the lecture explores the basic physical mechanisms responsible for the variability. The second half examines the trends of local surface temperature during the last half century in the context of decadal variability simulated by the coupled model.
Barnett, T P., G Hegerl, Thomas R Knutson, and S F B Tett, 2000: Uncertainty levels in predicted patterns of anthropogenic climate change. Journal of Geophysical Research, 105(D12), 15525-15542.
This paper investigates the uncertainties in different model estimates of an expected anthropogenic signal in the near-surface air temperature field. We first consider nine coupled global climate models (CGCMs) forced by CO2 increasing at the rate of 1%/yr. Averaged over years 71-80 of their integrations, the approximate time of CO2 doubling, the models produce a global mean temperature change that agrees to within about 25% of the nine model average. However, the spatial patterns of change can be rather different. This is likely to be due to different representations of various physical processes in the respective models, especially those associated with land and sea ice processes. We next analyzed 11 different runs from three different CGCMs, each forced by observed/projected greenhouse gases (GHG) and estimated direct sulfate aerosol effects. Concentrating on the patterns of trend of near-surface air temperature change over the period 1945–1995, we found that the raw individual model simulations often bore little resemblance to each other or to the observations. This was due partially to large magnitude, small-scale spatial noise that characterized all the model runs, a feature resulting mainly from internal model variability. Heavy spatial smoothing and ensemble averaging improved the intermodel agreement. The existence of substantial differences between different realizations of an ensemble produced by identical forcing almost requires that detection and attribution work be done with ensembles of scenario runs, as single runs can be misleading. Application of recent detection and attribution methods, coupled with ensemble averaging, produced a reasonably consistent match between model predictions of expected patterns of temperature trends due to a combination of GHG and direct sulfate aerosols and those observed. This statement is provisional since the runs studied here did not include other anthropogenic pollutants thought to be important (e.g., indirect sulfate aerosol effects, tropospheric ozone) nor do they include natural forcing mechanisms (volcanoes, solar variability). Our results demonstrate the need to use different estimates of the anthropogenic fingerprint in detection studies. Different models give different estimates of these fingerprints, and we do not currently know which is most correct. Further, the intramodel uncertainty in both the fingerprints and, particularly, the scenario runs can be relatively large. In short, simulation, detection, and attribution of an anthropogenic signal is a job requiring multiple inputs from a diverse set of climate models.
Delworth, Thomas L., and Thomas R Knutson, 2000: Simulation of early 20th Century global warming. Science, 287(5461), 2246-2250.
The observed global warming of the past century occurred primarily in two distinct 20-year periods, from 1925 to 1944 and from 1978 to the present. Although the latter warming is often attributed to a human-induced increase of greenhouse gases, causes of the earlier warming are less clear because this period precedes the time of strongest increases in human-induced greenhouse gas (radiative) forcing. Results from a set of six integrations of a coupled ocean-atmosphere climate model suggest that the warming of the early 20th century could have resulted from a combination of human-induced radiative forcing and an unusually large realization of internal multidecadal variability of the coupled ocean-atmosphere system. This conclusion is dependent on the model's climate sensitivity, internal variability, and the specification of the time-varying human-induced radiative forcing.
Delworth, Thomas L., Anthony J Broccoli, Keith W Dixon, Isaac M Held, Thomas R Knutson, P J Kushner, Michael J Spelman, Ronald J Stouffer, K Y Vinnikov, and Richard T Wetherald, 1999: Coupled climate modelling at GFDL: Recent accomplishments and future plans. Clivar Exchanges, 4(4), 15-20.
Delworth, Thomas L., Jerry D Mahlman, and Thomas R Knutson, 1999: Changes in heat index associated with CO2 -induced global warming. Climatic Change, 43(2), 369-386.
Changes in Heat Index (a combined measure of temperature and humidity) associated with global warming are evaluated based on the output from four extended integrations of the GFDL coupled ocean-atmosphere climate model. The four integrations are: a control with constant levels of atmospheric carbon dioxide (CO2), a second integration in which an estimate of the combined radiative forcing of greenhouse gases and sulfate aerosols over the period 1765-2065 is used to force the model, and a third (fourth) integration in which atmospheric CO2 increases at the rate of 1% per year to double (quadruple) its initial value, and is held constant thereafter. While the spatial patterns of the changes in Heat Index are largely determined by the changes in surface air temperature, increases in atmospheric moisture can substantially amplify the changes in Heat Index over regions which are warm and humid in the Control integration. The regions most prone to this effect include humid regions of the Tropics and summer hemisphere extra-tropics, including the southeastern United States, India, southeast Asia and northern Australia.
Knutson, Thomas R., Thomas L Delworth, Keith W Dixon, and Ronald J Stouffer, 1999: Model assessment of regional surface temperature trends (1949-1997). Journal of Geophysical Research, 104(D24), 30,981-30,996.
Analyses are conducted to assess whether simulated trends in SST and land surface air temperature from two versions of a coupled ocean-atmosphere model are consistent with the geographical distribution of observed trends over the period 1949-1997. The simulated trends are derived from model experiments with both constant and time-varying radiative forcing. The models analyed are low-resolution (R15, ~4º) and medium-resolution (R30, ~2º) versions of the Geophysical Fluid Dynamics Laboratory (GFDL) coupled climate model. Internal climate variability is estimated from long control integrations of the models with no change of external forcing. The radiatively forced trends are based on ensembles of integrations using estimated past concentrations of greenhouse gases and direct effects of anthropogenic sulfate aerosols (G+S). For the regional assessment, the observed trends at each grid point with adequate temporal coverage during 1949-1997 are first compared with the R15 and R30 model unforced internal variability. Nearly 50% of the analyzed areas have observed warming trends exceeding the 95th percentile of trends from the control simulations. These results suggest that regional warming trends over much of the globe during 1949-1997 are very unlikely to have occurred due to internal climate variability alone and suggest a role for a sustained positive thermal forcing such as increasing greenhouse gases. The observed trends are then compared with the trend distributions obtained by combining the ensemble mean G+S forced trends with the internal variability "trend" distributions from the control runs. Better agreement is found between the ensemble mean G+S trends and the observed trends than between the model internal variability alone and the observed trends. However, the G+S trends are still significantly different from the observed trends over about 30% of the areas analyzed. Reasons for these regional inconsistencies between the simulated and the observed trends include possible deficiencies in (1) specified radiative forcings, (2) simulated responses to specified radiative forcings, (3) simulation of internal climate variability, or (4) observed temperature records.
Knutson, Thomas R., and Robert E Tuleya, 1999: Increased hurricane intensities with CO2 -induced global warming as simulated using the GFDL hurricane prediction system. Climate Dynamics, 15, 503-519.
The impact of CO2 -induced global warming on the intensities of strong hurricanes is investigated using the GFDL regional high-resolution hurricane prediction system. The large-scale initial conditions and boundary conditions for the regional model experiments, including SSTs, are derived from control and transient CO2 increase experiments with the GFDL R30-resolution global coupled climate model. In a case study approach, 51 northwest Pacific storm cases derived from the global model under present-day climate conditions are simulated with the regional model, along with 51 storm cases for high CO2 conditions. For each case, the regional model is integrated forward for five days without ocean coupling. The high CO2 storms, with SSTs warmer by about 2.2° C on average and higher environmental convective available potential energy (CAPE), are more intense than the control storms by about 3-7 m/s (5%-11%) for surface wind speed and 7 to 24 hPa for central surface pressure. The simulated intensity increases are statistically significant according to most of the statistical tests conducted and are robust to changes in storm initialization methods. Near-storm precipitation is 28% greater in the high CO2 sample. In terms of storm tracks, the high CO2 sample is quite similar to the control. The mean radius of hurricane force winds is 2 to 3% greater for the composite high CO2 storm than for the control,and the high CO2 storms penetrate slightly higher into the upper troposphere. More idealized experiments were also performed in which an initial storm disturbance was embedded in highly simplified flow fields using time mean temperature and moisture conditions from the global climate model. These idealized experiments support the case study results and suggest that, in terms of thermodynamic influences, the results for the NW Pacific basin are qualitatively applicable to other tropical storm basins.
Knutson, Thomas R., and Syukuro Manabe, 1998: Model assessment of decadal variability and trends in the Tropical Pacific Ocean. Journal of Climate, 11(9), 2273-2296.
In this report, global coupled ocean-atmosphere models are used to explore possible mechanisms for observed decadal variability and trends in Pacific Ocean SSTs over the past century. The leading mode of internally generated decadal (>7 yr) variability in the model resembles the observed decadal variability in terms of pattern and amplitude. In the model, the pattern and time evolution of tropical winds and oceanic heat content are similar for the decadal and ENSO timescales, suggesting that the decadal variability has a similar "delayed oscillator" mechanism to that on the ENSO timescale. The westward phase propagation of the heat content anomalies, however, is slower and centered slightly farther from the equator (~12° vs 9° N) for the decadal variability. Cool SST anomalies in the midlatitude North Pacific during the warm tropical phase of the decadal variability are induced in the model largely by oceanic advection anomalies. An index of observed SST over a broad triangular region of the tropical and subtropical Pacific indicates a warming rate of +0.41°C (100 yr)-1 since 1900, +1.2°C (100 yr)-1 since 1949, and +2.9°C (100 yr)-1 since 1971. All three warming trends are highly unusual in terms of their duration, with occurrence rates of less than 0.5% in a 2000-yr simulation of internal climate variability using a low-resolution model. The most unusual is the trend since 1900 (96-yr duration); the longest simulated duration of a trend of this magnitude is 85 yr. This suggests that the observed trends are not entirely attributable to natural (internal) variability alone, although natural variability could potentially account for much of the observed trends. To quantitatively explore the possible role of greenhouse gases and aerosols in the observed warming trends, two simulations (using different initial conditions) of twentieth-century climate change due to these two radiative forcings were analyzed. These simulate an accelerated warming trend [~2°C (100 yr)-1] in the triangular Pacific region beginning around the 1960s and suggest that nearly all of the recent warming in the region could be attributable to such a thermal forcing. In summary, the authors' model results indicate that the observed warming trend in the eastern tropical Pacific is not likely to be solely attributable to internal (natural) climate variability. Instead, it is likely that a sustained thermal forcing, such as the increase of greenhouse gases in the atmosphere, has been at least partly responsible for the observed warming.
Knutson, Thomas R., and Syukuro Manabe, 1998: Model assessment of decadal variability and trends in the tropical Pacific Ocean In The Ninth Symposium on Global Change Studies and Namias Symposium on the Status and Prospects for Climate Prediction, Boston, MA, American Meteorological Society, 216-219.
Knutson, Thomas R., Robert E Tuleya, and Yoshio Kurihara, 1998: Simulated increase of hurricane intensities in a CO2-warmed climate. Science, 279(5353), 1018-1020.
Hurricanes can inflict catastrophic property damage and loss of human life. Thus, it is important to determine how the character of these powerful storms could change in response to greenhouse gas-induced global warming. The impact of climate warming on hurricane intensities was investigated with a regional, high-resolution, hurricane prediction model. In a case study, 51 western Pacific storm cases under present-day climate conditions were compared with 51 storm cases under high-CO2 conditions. More idealized experiments were also performed. The large-scale initial conditions were derived from a global climate model. For a sea surface temperature warming of about 2.2°C, the simulations yielded hurricanes that were more intense by 3 to 7 meters per second (5 to 12 percent) for wind speed and 7 to 20 millibars for central surface pressure.
Knutson, Thomas R., Syukuro Manabe, and D Gu, 1997: Simulated ENSO in a global coupled ocean-atmosphere model: Multidecadal amplitude modulation and CO2 sensitivity. Journal of Climate, 10(1), 138-161.
An analysis is presented of simulated ENSO phenomena occurring in three 1000-yr. experiments with a low-resolution (R15) global coupled ocean-atmosphere GCM. Although the model ENSO is much weaker than the observed one, the model ENSO's life cycle is qualitatively similar to the "delayed oscillator" ENSO life cycle simulated using much higher resolution ocean models. Thus, the R15 coupled model appears to capture the essential physical mechanism of ENSO despite its coarse ocean model resolution. Several observational studies have shown that the amplitude of ENSO has varied substantially between different mutidecadal periods during the past century. A wavelet analysis of a multicentury record of eastern tropical Pacific SST inferred from Delta 18O measurements suggests that a similar multidecadal amplitude modulation of ENSO has occurred for at least the past three centuries. A similar multidecadal amplitude modulation occurs for the model ENSO (2-7-yr band), which suggests that much of the past amplitude modulation of the observed ENSO could be attributable to internal variability of the coupled ocean-atmosphere system. In two 1000-yr CO2 sensitivity experiments, the amplitude of the model ENSO decreases slightly relative to the control run in response to either a doubling or quadrupling of CO2. This decreased variability is due in part to CO2-induced changes in the model's time-mean basic state, including a reduced time-mean zonal SST gradient. In contrast to the weaker overall amplitude, the multidecadal amplitude modulations become more pronounced with increased CO2. The frequency of ENSO in the model does not appear to be strongly influenced by increased CO2. Since the multidecadal fluctuations in the model ENSO's amplitude are comparable in magnitude to the reduction in variability due to a quadrupling of CO2, the results suggest that the impact of increased CO2 on ENSO is unlikely to be clearly distinguishable from the climate system "noise" in the near future - unless ENSO is substantially more sensitive to increased CO2 than indicated in the present study.
Knutson, Thomas R., Robert E Tuleya, and Yoshio Kurihara, 1997: Exploring the sensitivity of hurricane intensity to CO2-induced global warming using the GFDL Hurricane Prediction System In 22nd Conference on Hurricanes and Tropical Meteorology, Boston, MA, American Meteorological Society, 587-588.
Knutson, Thomas R., and Syukuro Manabe, 1995: Time-mean response over the tropical Pacific to increased CO2 in a coupled ocean-atmosphere model. Journal of Climate, 8(9), 2181-2199.
The time-mean response over the tropical Pacific region to a quadrupling of CO2 is investigated using a global coupled ocean-atmosphere general circulation model. Tropical Pacific sea surface temperatures (SSTs) rise by about 4 degrees - 5 degrees C. The zonal SST gradient along the equator decreases by about 20%, although it takes about one century (with CO2 increasing at 1% per year compounded) for this change to become clearly evident in the model. Over the central equatorial Pacific, the decreased SST gradient is accompanied by similar decreases in the easterly wind stress and westward ocean surface currents and by a local maximum in precipitation increase. Over the entire rising branch region of the Walker circulation, precipitation is enhanced by 15%, but the time-mean upward motion decreases slightly in intensity. The failure of the zonal overturning atmospheric circulation to intensify with a quadrupling of CO2 is surprising in light of the increased time-mean condensation heating over the "warm pool" region. Three aspects of the model response are important for interpreting this result. 1) The time-mean radiative cooling of the upper troposphere is enhanced, due to both the pronounced upper-tropospheric warming and to the large fractional increase of upper-tropospheric water vapor. 2) The dynamical cooling term, - omega delta theta/ delta p, is enhanced due to increased time-mean static stability ( - delta theta/delta p). This is an effect of moist convection, which keeps the lapse rate close to the moist adiabatic rate, thereby making - delta theta/ delta p larger in a warmer climate. The enhanced radiative cooling and increased static stability allow for the enhanced time-mean heating by moist convection and condensation to be balanced without stronger time-mean upward motions. 3) The weaker surface zonal winds and wind stress in the equatorial Pacific are consistent with the reduced zonal SST gradient. The SST gradient is damped by the west-east differential in evaporative surface cooling (with greater evaporative cooling in the west than in the east). This evaporative damping increases with increasing temperature, owing to the temperature dependence of saturation mixing ratios, which leads to a reduction in the SST gradient in the warmer climate.
Knutson, Thomas R., and Syukuro Manabe, 1994: Impact of increased CO2 on simulated ENSO-like phenomena. Geophysical Research Letters, 21(21), 2295-2298.
The impact of a CO2-induced global warming on ENSO-like fluctuations in a global coupled ocean-atmosphere GCM is analyzed using two multi-century experiments. In the 4xCO2 experiment, CO2 increases by a factor of four in the first 140 years and then remains constant at 4xCO2 for another 360 years; in the control experiment, CO2 remains constant at 1xCO2 for 1000 years. The standard deviation of tropical Pacific SST fluctuations (7°N-7°S, 173°E-120°W; 2 to 15 year timescales) is 24% lower in the 4xCO2 experiment than in the control experiment; for the model's Southern Oscillation Index, a 19% decrease occurs, whereas for central tropical Pacific rainfall, a 3% increase occurs. An important feature of the control simulation is the internally generated modulation of variability on a multi-century timescale, which is comparable in magnitude to the changes occurring with 4xCO2. We conclude that despite an order 5 K warming of the tropical Pacific, and order 50% increase in time-mean atmospheric water vapor under 4xCO2 conditions, ENSO-like SST fluctuations in the coupled model do not intensify, but rather decrease slightly in amplitude.
Knutson, Thomas R., and Syukuro Manabe, 1994: Impact of increasing CO2 on the Walker circulation and ENSO-like phenomena in a coupled ocean-atmosphere model In The Sixth Conference on Climate Variations, Boston, MA, American Meteorological Society, 80-81.
Knutson, Thomas R., and K M Weickmann, 1987: 30-60 day atmospheric oscillations: Composite life cycles of convection and circulation anomalies. Monthly Weather Review, 115(7), 1407-1436.
Life cycles of the 30-60-day atmospheric oscillation were examined by compositing 30-60-day filtered NMC global wind analyses (250 mb and 850 mb) and outgoing long-wave radiation (OLR) for the years 1979-1984. Separate composite life cycles were constructed for the May-Oct. and Nov.-April seasons by using empirical orthogonal function analysis of the large-scale divergent wind field (250-mb velocity potential) to define the oscillation's phase. Monte Carlo simulations were used to assess the statistical significance of the composite OLR and vector wind fields. Large-scale (wavenumber one) tropical divergent wind features propagate eastward around the globe throughout the seasonal cycle. The spatial relationships between these propagating circulation features and OLR are shown by using sequences of composite maps. Good agreement exists between areas of upper air divergence and areas of convection inferred from the OLR satellite data. Convection anomalies are smaller over tropical Africa and South America than over the Indian and western Pacific oceans. Anomalies of OLR are nearly negligible over cooler tropical sea surfaces. Fluctuations in summer monsoon region convection are influenced by the global-scale eastward-moving wave. The oscillation's vertical structure varies with latitude. In the Tropics, upper level and lower level tropospheric wind anomalies are about 180° out of phase. Poleward of about 20°, there is no pronounced phase shift between levels. In tropical and subtropical latitudes, analysis of the nondivergent circulation composites at 250 mb reveals cyclones to the east of the convection and anticyclones alongside or west of the convection. While convection anomalies are most pronounced in the summer hemisphere Tropics, the tropical and subtropical features are most prominent in the winter hemisphere. There is some evidence of symmetry of cyclonic and anticyclonic circulations about the equator. A subset of the composite extratropical vector wind fields were statistically significant (95% level) at 850 and 250 mb in the winter hemisphere (25-85° latitude), based upon a Monte Carlo simulation. During the Nov.-April season, the East Asian jet is retracted toward Asia when positive 30-60-day convection anomalies are occurring over the equatorial Indian Ocean. The eastward shift of convection into the western and central Pacific is accompanied by a series of circulation features over northern Asia and an eastward extension of the East Asian jet. During the May-Oct. season, the shift of large-scale tropical convection anomalies from the Indian Ocean and Indian monsoon regions to the tropical western Pacific is followed (10-15 days later) by the occurrence of strengthened westerlies over southern Australia. In contrast, the extratropical "response" in the summer hemisphere for both the May-Oct. and Nov.-April seasons was not statistically significant.
Knutson, Thomas R., K M Weickmann, and J E Kutzbach, 1986: Global-scale intraseasonal oscillations of outgoing longwave radiation and 250 mb zonal wind during Northern Hemisphere summer. Monthly Weather Review, 114(3), 605-623.
Intraseasonal fluctuations of satellite-based observations of outgoing longwave radiation (OLR) and NMC analyses of 250 mb zonal wind (U250) are described based on global data from nine Northern Hemisphere summers (May- October). Cross-spectral analysis of the 28-72 day spectral band is used to establish statistically significant relationships for the entire data period. Hovmoller diagrams are used to examine individual events and to estimate the oscillation's time scale and propagation characteristics. Intraseasonal OLR fluctuations have their greatest amplitude in the Indian monsoon region and north of the equator in the western tropical Pacific. These two regions have out-of-phase fluctuations and appear to be linked by OLR anomalies propagating eastward (at 3-6 m s -1) along the equator between 50 degrees and 160 degrees E. The OLR oscillation has a preferred time scale of 30-60 days during May-October, based on a sample of more than 30 events. The initiation near the equator of northward-propagating (1-2 m s- 1) OLR anomalies in the Indian monsoon region is also associated with the eastward-propagating equatorial OLR anomalies. The U250 intraseasonal fluctuations have a prominent zonal wavenumber-one structure throughout the tropics with the exception of the Northern Hemisphere tropics over the Atlantic, Africa, and the Indian monsoon region. The U250 anomalies propagate eastward along 0 degrees - 10 degrees S at about 6 m s- 1 from 40 degrees to 160 degrees E and at about 15 m s- 1 from 160 degrees E to 0 degrees W. These longitudinal changes in the oscillation's ground speed may be due in part to longitudinal changes in the zonal wind basic state. The 28-72 day U250 anomalies along 30 degrees S (50 degrees S) are out of phase (in phase) with the tropical U250 anomalies over most of the Pacific and Indian Ocean sectors. The phase relationships between tropical OLR and U250 anomalies seem dynamically consistent, generally showing 250 mb u-component divergence flanking regions of convection. Although the eastward propagation of OLR anomalies along 5 degrees N-5 degrees S is not continuous around the globe, areas of significant coherence scattered throughout the tropics exhibit a zonal wavenumber-one phase structure. In these remote regions, OLR anomalies may be dynamically linked by an eastward-propagating tropical circulation feature.
Miyakoda, Kikuro, Joseph J Sirutis, and Thomas R Knutson, 1986: Experimental 30-day forecasting at GFDL In Workshop on Predictability in the Medium and Extended Range, ECMWF, 17-50.
This is the report of the 30-day forecast experiment conducted at GFDL. The first part is a summary of 8 January case studies, using a finite difference GCM without the anomalous boundary forcings of sea surface temperature (SST). The experiment reveals that the forecast skill of 10-day mean variables is marginal at the end of a month, but that the removal of systematic bias (climate drift) from the original forecasts raises the skill scores appreciably, producing useful one-month prognoses. However, the climate drift is alarmingly large; for example, the forecast error for the 500 mb geopotential height due to the drift is 64% of the total root mean square error. The second part of the paper discusses the forecasts incorporating the observed SST instead of the climatological SST. A series of forecasts was carried out for the most dramatic El Niño event of January 1983. In this study, forecasts were improved for the tropics by using the observed SST, whereas the impact for the extratropics was not beneficial. Four possible causes for the adverse effect of tropical SST were examined, i.e., the cumulus parameterization, the accuracy of SST, the initialization, and the tropical land surface condition. Preliminary investigations suggest that the forecast tropical divergence fields are quite different from those observed, in particular with respect to the components of large scale divergence associated with the 40-50 day oscillation. It is likely that the current initialization of the GFDL forecast system is deficient in treating this distinct tropical oscillation.