Drivers of the recent tropical expansion in the Southern Hemisphere: Changing SSTs or ozone depletion?
Tuesday, July 28, 2015: 4:10 PM
Darryn W Waugh, Johns Hopkins University, Baltimore, MD, United States, Chaim Garfinkel, Hebrew University of Jerusalem, Jerusalem, Israel and Lorenzo M Polvani, Columbia University, New York, NY, United States
Abstract:
Observational evidence indicates that the southern edge of the Hadley Cell has shifted southward during austral summer in recent decades. However, there is no consensus on the cause of this shift, with several studies reaching opposite conclusions as to the relative role of changes in sea surface temperatures (SSTs) and stratospheric ozone depletion in causing this shift. Here we quantitatively compare the results of published studies that have used single-forcing model integrations to isolate the role of different factors on the HC expansion, and analyze a new suite of integrations with the Goddard Earth Observing System Chemistry-Climate Model (GEOSCCM). We show that the weight of the evidence clearly points to stratospheric ozone depletion as the dominant driver of the tropical summertime expansion over the period in which ozone hole was formed (1979 to late 1990s), although SST trends have contributed to trends since then. Studies that have claimed SSTs as the major driver of tropical expansion since 1979 have used prescribed ozone fields that significantly underrepresent the observed Antarctic ozone depletion.
