Extratropical influence on the tropics

It has not been recognized until recently that the extratropics can force the tropics. Many studies such as Zhang and Delworth(2005) and Chiang and Bitz (2005) have demonstrated that extratropical thermal forcing induced either by weakening of thermohaline circulation or imposing Arctic sea ice can shift the inter-tropical convergence zone (ITCZ) southward, away from the cooled northern hemisphere. For my PhD thesis, I have worked on the mechanism by which how the extratropical thermal forcing can affect the tropical precipitation, as summarized in the schematic below. In this line, with my collaborators, D. Frierson and Y.-T. Hwang, we have recently attributed the 20th century tropical rainfall change, especially 1980s African drought, to Northern Hemisphere air pollution. This research has garnered much attention from the news media.

Impact of ozone-hole on subtropical rainfall

The Southern Hemisphere mid- to high-latitude circulation has undergone marked climate change over the past few decades. One of the most pronounced features is a poleward displacement of the extratropical jet, which has been accompanied by a poleward shift of mid- to high-latitude precipitation associated with the extratropical storm track. Many studies have shown that these are caused by the ozone-hole. My research demonstrated that the ozone hole has contributed to the observed change in subtropical precipitation, and exposed a sequence of high-to-low latitude causes and effects linking stratospheric polar ozone depletion all the way to subtropical moistening. This study was published in Science and received much attention from the media.