Tropical Connections to Climatic Change in the Extratropical Southern Hemisphere: The Role of Atlantic SST Trends

Abstract:

In recent decades, significant changes have been observed to the high-latitude Southern Hemisphere (SH) climate, changes which, amongst other factors, have been increasingly attributed to tropical sea surface temperature (SST) forcing. Using an AGCM, we further investigate this possibility by documenting contemporary spring-time relationships between SST trends and the SH extratropical atmospheric circulation. A suite of idealised simulations are analysed wherein the AGCM is forced by underlying SST conditions in which which trends over 1979-2009 are constrained to individual ocean basins (Pacific, Indian, and Atlantic), allowing the impact of each region to be assessed in isolation. When forced with global SST, the model broadly replicates the spatial pattern of geopotential height trends seen in reanalyses. Forcing the model by each ocean basin separately, however, reveals significant differences in the associated teleconnection patterns, with similar structures to reanalyses simulated only when Atlantic SST trends are included. It is revealed that such Atlantic teleconnections are associated with dynamical changes in the zonal Walker Circulation and a corresponding intensification of the local Hadley Cell, the impact of which results in the development of atmospheric Rossby waves. Thus, increased Rossby waves, forced by positive Atlantic SST trends, may have played a role in driving geopotential height trends in the SH extratropics. Consequently, changes to the regional atmospheric circulation promote warming throughout the Antarctic Peninsula/West Antarctica, with a pattern that closely matches observations. As such, Atlantic SST trends, via a teleconnection to the SH extratropics, may have contributed to springtime climatic change in Antarctica.