Extratropical influence on the tropics

It has not been recognized until early 2000's 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 by adding Arctic sea ice can shift the inter-tropical convergence zone (ITCZ) southward, away from the cooled northern hemisphere. I have extensively worked on the mechanism by which how the extratropical thermal forcing can affect the tropical precipitation. In this line, with my collaborators, D. Frierson and Y.-T. Hwang, we have attributed the 20th century tropical rainfall change, especially 1980s African drought, to Northern Hemisphere air pollution.

My studies have progressively evolved from discussing the zonally averaged picture to explaining the regional patterns. More recently, I demonstrated how extratropical radiative forcing can alter the equatorial Pacific SST pattern and modulate the Walker circulation. We may expect the accompanying shifts in deep convection sites over the equatorial Pacific to trigger trains of Rossby waves propagating in both hemispheres. Thus, extratropical forcing in one hemisphere will exert global impacts well into the opposite hemisphere. My goal is to reveal the global teleconnection pattern driven from the extratropics.

Energetics framework illustrating how extratropical energy perturbations can impact the tropical circulation and precipitation distribution. Adopted from Kang et al. (2008)

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.

Mechanism by which Antarctic ozone hole affects the subtropical precipitation. Adopted from Kang et al. (2011)