J. Climate, 26, 4304-4321.

# Understanding Hadley cell expansion vs. contraction: insights from simplified models and implications for recent observations

Neil Tandon
Department of Applied Physics and Applied Mathematics, Columbia University, New York, NY.

Edwin P. Gerber
Courant Institute of Mathematical Sciences, New York University, New York, NY.

Adam H. Sobel and Lorenzo M. Polvani
Department of Applied Physics and Applied Mathematics and Department of Earth and Environmental Sciences, Columbia University, New York, NY.

Abstract

This study seeks a deeper understanding of the causes of Hadley Cell (HC) expansion, as projected under global warming, and HC contraction, as observed under El Nino. The authors present a series of experiments in which they apply thermal forcings to an idealized general circulation model. It is shown that a thermal forcing applied to a narrow region around the equator produces "El Nino-like" HC contraction, while a forcing with wider meridional extent produces "global warming-like" HC expansion. These circulation responses are mostly insensitive to the vertical structure of the thermal forcing and are much more sensitive to its meridional structure. If the thermal forcing is confined to the midlatitudes, the amount of HC expansion is more than three times that of a forcing of comparable amplitude that is spread over the tropics. This finding may be relevant to recent trends in tropical widening, which comprehensive models generally underpredict.

The shift of the HC edge can be understood in a very simple way in terms of changes in the transformed Eulerian mean (TEM) circulation. In this context, the HC edge is defined as the maximum in residual vertical velocity in the upper troposphere, $\omega^*_{max}$; this corresponds well with the conventional Eulerian definition of the HC edge. Then, a toy model is constructed in which the TEM circulation simply diffuses heat meridionally. This diffusion produces anomalous diabatic cooling, and hence anomalous TEM descent, on the poleward flank of the thermal forcing. This results in a shift of $\omega^*_{max}$, and thus a shift of the HC edge towards the descending anomaly.