Harvard Climate Seminar
The Department of Earth and Planetary Sciences and the Paulson School of Engineering and Applied Sciences presents "Anthropogenic Forcing on Widening of the Hadley Circulation" with Yongyun Hu, Professor, Dept. of Atmospheric and Oceanic Sciences, School of Physics, Peking University, Beijing.
Abstract: Poleward expansion of the Hadley circulation has been an important topic in climate change studies in the past decade, and one of the critically important issues is how it is related to anthropogenic forcings. Using simulations from CMIP5 simulations, we study influences of anthropogenic forcings on the width and strength of the Hadley circulation. It is found that significant poleward expansion of the Hadley circulation can be reproduced in CMIP5 historical all-forcing simulations although the magnitude of trends is much weaker than observations. Simulations with individual forcings demonstrate that increasing greenhouse gases (GHGs) and stratospheric ozone depletion all cause poleward expansion of the Hadley circulation, whereas anthropogenic aerosols do not have significant influences on the Hadley circulation. In CMIP5 projection simulations for the 21st century, ozone recovery competes with increasing GHGs in determining the width of the Hadley circulation, especially in austral summer. In both historical and projection simulations, the strength of the Hadley circulation shows significant weakening in winter in both hemispheres.
Descending branches of the Hadley circulation cause subtropical dry zones in both hemispheres. Widening of the Hadley circulation leads to shifts of the dry zones toward middle latitudes.