Séminaire

Low-frequency sea surface temperature (SST) variations in the North Atlantic have substantial impact on regional and global climate. Identification of the mechanisms driving these variations is therefore critical for improving decadal  predictions, but also for assessing future long-term projections of ocean circulation feedbacks. These mechanisms however are still poorly understood; a key challenging issue is the extent to which observed historical SST changes were modulated by internal variability or by external forcing. The role of anthropogenic aerosol forcing is particularly controversial, and often overlooked in this is the potential role of the ocean in generating not only internal variability, but also feedbacks to atmospheric forcing. In this talk, I will show the impact of historical (1850-1975) anthropogenic sulphate aerosols on key features of the Atlantic climate in the coupled Community Earth System model. Using multi-member ensemble experiments, the impacts of emissions from North America and Europe, the major historical emission regions, are contrasted. The results  show that aerosols from either source cause a long-term cooling of North Atlantic SSTs. The SST changes result from a combination of atmospheric aerosol effects -interactions of the aerosols with solar radiation and with clouds cool the surface- and an aerosol-induced strengthening of the Atlantic Meridional Overturning Circulation (AMOC). The response to North American emissions is larger than that to European emissions, since for North American emissions the prevailing wind pattern results in a larger aerosol spread over the Atlantic, collocated with high climatological cloud cover. Emissions from either source cause a southward shift of the Inter-Tropical Convergence Zone, affecting tropical precipitation globally. Preliminary findings on the suppression of Tropical Hurricane frequency and a north-eastward shift of extra-tropical storms over Europe highlight further the relevance of aerosol-induced changes for societies and ecosystems neighbouring the Atlantic. The analysis provides novel insights into the mechanisms of aerosol impact on the Atlantic, and allows more general conclusions on the role of external forcing on the (multi)decadal components of SST and AMOC variability in a state-of-the-art model.

mkhodri@gmail.com