US National Oceanic and Atmospheric Administration
Decadal Climate Variability and Change in the Mediterranean Region
Climate Program Office
NOAA's Office of Oceanic and Atmospheric Research, Silver Spring, Maryland
The Mediterranean region is among the “Hot Spots” projected to experience major climatic changes in the twenty-first century as a result of the global increase in greenhouse gas (GHG) concentrations. However the way in which these changes may initially become manifest in the Mediterranean will also depend on internal decadal variability and its impacts on climate in this region. Here, we present an analysis of the main decadal climate variations that have influenced past climatic conditions in the Mediterranean/South Europe region since the midnineteenth century. Decadal variability is discussed in the context of forced climatic changes from increased GHG.
Results point to significant connections between Mediterranean climate and decadal and multi-decadal variability in the Atlantic. Namely, a significant influence of the North Atlantic Oscillation on Mediterranean precipitation and a relationship between regional temperatures and the Atlantic Multi-decadal Oscillation which may imply a certain degree of decadal regional predictability. CMIP3 projections indicate that in the longer term “forced” regional climatic changes from GHG increases would bring significantly drier conditions over land and major changes in Mediterranean Sea water cycle.
1. Observed twentieth century changes
An analysis of observed twentieth century long-term changes in the water cycle of Mediterranean land areas is presented by Mariotti et al. (2008) (see Figure 1). A weak albeit significant long-term negative precipitation trend is found over Mediterranean land areas. The Palmer Drought Severity Index (PDSI), which reflects the combined effects of precipitation and surface temperature changes, shows a progressive and substantial drying of Mediterranean land surface since 1900 consistent with a decrease in precipitation and an increase in surface temperature. The inter-decadal PDSI fluctuations are similar to those of precipitation, with wetter 1960s compared to the drier 1940s.
Consistently with PDSI behaviour, a number of Mediterranean rivers for which long-time series are available also show long-term decreases in discharge during the twentieth century. Such river discharge decreases are likely in part due to intensified water use.
2. Role of the North Atlantic Oscillation
The linkages between decadal variability of seasonal precipitation anomalies (DVSP) over the Mediterranean and the NAO are explored in Figure 2 (see Mariotti and Dell’Aquila, 2011). In DJF, Mediterranean-mean DVSP anomalies evoke a SLP correlation pattern clearly reminiscent of the NAO, a well known major influence on Mediterranean precipitation during winter (i.e. a positive NAO tends to correspond to drier conditions in the Mediterranean and vice-versa). This connection at decadal time-scales is confirmed by the spatial correlation of the NAO index with DJF precipitation in the Mediterranean domain. Based on this map, the NAO explains over 25% - 30% of decadal DJF precipitation variance in a region spanning parts of Spain, Marocco, Southern France, Italy and the Balkans. The NAO has been shown to affect Mediterranean precipitation by modulating SLP directly over the Mediterranean and by reorganizing large-scale moisture fluxes into the region [Mariotti and Arkin, 2007]. Drier (wetter) decades in the Mediterranean largely correspond to higher (lower) than usual regional SLP and to a positive (negative) NAO phase. The correlation of decadal variability of JJA precipitation anomalies in the Mediterranean with the Summer North Atlantic Oscillation (SNAO) of [Folland et al., 2009] is also shown in Fig.2. The SNAO has significant positive correlation with JJA precipitation in Italy and parts of the Balkans, explaining up to 25% - 30% of decadal precipitation variability in these regions.
3. Linkages with the Atlantic Multi-decadal oscillation
An analysis of the linkages between decadal surface air temperature (Ta) anomalies in the Mediterranean and in surrounding regions is shown in Figure 3. In JJA Mediterranean-mean Ta presents a coherent pattern of positive correlation with surface air temperature in the North Atlantic, with highest values in the eastern North Atlantic off the North African and European coasts. The correlation of Ta over the North Atlantic/European sector with the Atlantic Multidecadal Oscillation index shows that in JJA the AMO/Ta pattern over the Atlantic extends to parts of western Europe and the Mediterranean. This is in stark contrast with what is found in DJF, when the AMO correlation pattern is confined to the North Atlantic with no significant correlation with Ta over Europe, nor the Mediterranean (compare Fig. 3 C-D). The analysis of Mediterranean-mean decadal variability of Ta anomalies also shows contrasting characteristics in JJA and DJF: multi-decadal AMO-like Ta variations in JJA (Mediterranean-mean Ta/AMO index correlation is 0.77 over the period 1850-2009); decadal variation of the anomalies, with no-AMO correlation in DJF (compare Fig. 3 E-F). Interestingly, Mediterranean SST significantly correlate with AMO variability throughout the year.
4. “Forced” Mediterranean water cycle changes
Mariotti et al., 2008 study Mediterranean water cycle changes associated with modifications in radiative forcing based on an ensemble of multi-model coupled simulations from the WCRP Coupled Model Intercomparison Project Phase 3 (hereafter CMIP3). Twentieth century simulations, using observed radiative forcings (both natural and anthropogenic), give a progressive “forced” decrease in rainfall in the Mediterranean region during this century, somewhat higher than that derived from observational data (see section 1). CMIP3 simulations also show a tendency for Mediterranean Sea evaporation and E-P water budget to increase toward the end of the twentieth century in response to the radiative forcings (See Fig. 4).
Based on an ensemble of CMIP3 twenty-first century SRES-A1B emission scenario simulations, Mariotti et al. (2008) show that the above mentioned simulated twentieth century precipitation decrease would be followed by a rapid drying from 2020 onwards. Evapotranspiration would also decrease because of the drier land surface, but, as increased surface temperature favours higher evaporation, the rate would be half that of precipitation. While the “forced” drying found by Mariotti et al. (2008) over land is large, projected changes for the Mediterranean Sea are even more dramatic. Unlike the surrounding land region where evaporation decreases, the projected precipitation reduction over the sea is accompanied by a roughly equal increase in evaporation due to increased sea surface temperature (SST) (ultimately due to more energy input from greenhouse warming). The projected increase in the loss of freshwater (E-P) at the sea surface towards the end of the twenty-first century is large, roughly equal to what is typically received in total by the Mediterranean Sea on an annual basis as discharge from neighboring land and as inflow from the Black Sea.
Although there is a high degree of inter-model consistency among the CMIP3 models regarding Mediterranean climate projections, uncertainties need to be carefully evaluated. These include general uncertainties associated with CMIP3 projections due to model errors (e.g. limited model resolution or the parameterisation of physical processes), the effects of “internal” variability (from purely atmospheric modes or modes of coupled interactions within the ocean-atmosphere-land system) and uncertainties in future emission paths and natural radiative forcings (e.g., future volcanic eruptions). The uncertainty in emission paths is likely to play a larger role later in the twenty-first century when differences among scenarios become larger. In addition to the above mentioned general uncertainties, there are also region-specific uncertainties, including potential errors and limitations in the CMIP3 models’ representation of regional climate variability, Mediterranean Sea circulation and regional climate feedbacks.
The role of “internal” variability on future water cycle variations can be best understood by looking at past regional water cycle variability and comparing this with the results from the CMIP3 twentieth century simulations. Observational analyses of twentieth century water cycle variability in this region show long-term trends including a tendency for increased surface aridity and increased Mediterranean Sea evaporation. These changes are not inconsistent with the “forced” changes depicted by the CMIP3 simulations as a result of green-house gas concentrations increases during this century. However, observational precipitation and evaporation records also include decadal variations that are larger than the above-mentioned simulated long-term trends (e.g. for precipitation, observed decadal anomalies are about two orders of magnitude larger than the simulated trend). As much as both observational errors and model errors can contribute to this discrepancy, this also highlights the important role of “internal” variability in determining observed decadal anomalies. Hence, at least in the short term (roughly 10-30 years out), regional decadal anomalies and any potential for decadal predictability is likely to be critically dependent on the regional impacts of decadal modes of variability “internal” to the climate system.
Mariotti, A., and A. Dell’Aquila, 2011: Decadal climate variability in the Mediterranean region: roles of large-scale forcings and regional processes. Climate Dyn., published on-line.
Anav, A. and A. Mariotti, 2011: Projected changes in 21st century Euro-Mediterranean dominant vegetation types. Climate Res., in press.
Mariotti, A., 2010: Recent changes in Mediterranean water cycle: A pathway toward long-term regional hydroclimatic change? J. Climate, 23, 1513-1525.
Sanchez-Gomez, E., S. Somot, and A. Mariotti, 2009: Future changes in the Mediterranean water budget projected by an ensemble of regional climate models. Geophys. Res. Lett., 36, L21401.
Mariotti, A., N. Zeng, J. Yoon, V. Artale, A. Navarra, P. Alpert, and L. Z X Li, 2008: Mediterranean water cycle changes: transition to drier 21st century conditions in observations and CMIP3 simulations. Environ. Res. Lett., 3, 044001 (8pp) doi:10.1088/1748-9326/3/4/044001.
Mariotti, A., and P. Arkin, 2007: The North Atlantic Oscillation and oceanic precipitation variability. Climate Dyn., 28(1), 35-51. DOI: 10.1007/s00382-006-0170-4.
Contact Annarita Mariotti