Understanding Hadley Cell Expansion vs. Contraction: Insights from Simplified Models

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. Using an idealized general circulation model, we show 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 sensitive primarily to the thermal forcing's meridional structure and are less sensitive to its vertical 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 recently observed trends of rapid tropical widening.

The shift of the HC edge is explained using a very simple model in which the Transformed Eulerian Mean (TEM) circulation acts to diffuse heat meridionally. In this context, the HC edge is defined as the downward maximum of residual vertical velocity in the upper troposphere, max(w*); this corresponds well with the conventional Eulerian definition of the HC edge. In response to a positive thermal forcing, there is anomalous diabatic cooling, and hence anomalous TEM descent, on the poleward flank of the thermal forcing. This causes the HC edge, i.e. max(w*), to shift towards the descending anomaly, so that a narrow forcing causes HC contraction and a wide forcing causes HC expansion.

Note: This material has been published in J. Climate, 2013, and was presented at the 2012 AGU Fall Meeting. http://journals.ametsoc.org/doi/abs/10.1175/JCLI-D-12-00598.1