Earth’s surface shows large zonal asymmetries, i.e., surface properties vary greatly in longitude, and these asymmetries greatly impact the dynamics of the overlying atmosphere. These asymmetries can be broadly characterized by whether they primarily affect the atmosphere through mechanical or thermal interaction: for instance, mountains exert a direct drag on the atmospheric flow and thus represent a mechanical forcing. On the other hand, warm/cool ocean currents act as local heat sources/sinks on the atmosphere. Ocean currents create planetary-scale thermal contrasts at the surface: in the Atlantic, ocean circulation carries heat from the Southern hemisphere high latitudes all the way to the Northern hemisphere high latitudes. The net effect is to cool the Southern Hemisphere and warm the Northern Hemisphere; some of this hemispheric asymmetry is compensated by the atmosphere, which transports heat from the Northern to the Southern Hemisphere. Paleoclimate proxies suggest important variations in the strength of this cross-equatorial ocean circulation, in particular since the Last Glacial Maximum. For instance, there may have been periods during which cross-equatorial circulation nearly shut down, causing the Southern Hemisphere to warm and the Northern Hemisphere to cool. These variations were manifested by a large drop in the sea surface temperature in the North Atlantic, and appear to be in phase with equally drastic changes in the deep tropics. A number of mechanisms have been proposed to explain these observed changes, but it remains unclear how climate change over a relatively small area in the high latitudes could prompt such dramatic changes throughout the deep tropics.
Kang et al. explores the sensitivity of the tropical atmospheric circulation to zonally-confined heat source and sink in the Northern and Southern Hemispheres respectively. This idealized surface heating distribution is meant to crudely represent a cross-hemispheric heat flow,associated with cooling in the Northern and warming in the Southern Hemisphere. Cross-hemispheric heating is prescribed at first in the high latitudes and then in the low-latitudes regions; thus providing a handy way to compare the effects of tropical versus extratropical heat sources on the tropical dynamics. Using this setting, the tropical circulation is found to be sensitive to high-latitudes heat sources; In particular, SSTs are shown to decrease in the tropics of the cooled hemisphere, while the rain band in the deep tropics (i.e., the ITCZ) shifts away from the cooled hemisphere. This climate response in the deep tropics is quite zonally symmetric, which is qualitatively consistent with the widespread changes in the deep tropics seen during several shutdown events of the cross-hemispheric Atlantic circulation since the Last Glacial Maximum. Kang et al. show that this global response of the tropics to a localized high latitude heat source implies that forcing imparted to the tropical atmosphere has been homogenized by the strong westerlies of the extratropics. On the other hand, they also demonstrate that an equivalent heat source in the tropics produces a zonally-asymmetric dynamical response, with temperature and precipitation shifts confined to the vicinity of the heat sources. While a direct control of precipitation by stationary waves characterize the response of the deep tropics to a tropical heat source, transient eddy fluxes associated with synoptic weather systems communicate an extratropical forcing to the tropics. Although Kang et al. shows conclusive evidence for a zonally-symmetric response of the tropics to a zonally-confined forcing in the extratropics, the mechanism by which the forcing is being communicated to the tropics remains largely unexplained. Further work is required to better understand and ultimately quantify this extratropical-tropical interaction.
Kang, S.M., I.M. Held and S.-P. Xie (2013): Contrasting the tropical responses to zonally forcing asymmetric extratropical and tropical thermal forcing. Clim. Dyn., DOI 10.1007/s00382-013-1863-02