Hemispherically asymmetric Hadley cell response to CO removal.

A poleward shift of the Hadley cell (HC) edge in a warming climate, which contributes to the expansion of drought-prone subtropical regions, has been widely documented. The question addressed here is whether this shift is reversible with CO removal. By conducting large-ensemble experiments where CO concentrations are systematically increased and then decreased to the present-day level, we show that the poleward-shifted HC edge in a warming climate does not return to its present-day state when CO concentrations are reduced. While the Southern Hemisphere HC edge remains poleward of its present-day state, the Northern Hemisphere HC edge ends up farther equatorward of its present-day state. Such hemispherically asymmetric HC edge changes are closely associated with the changes in vertical wind shear in the subtropical atmosphere, which result from the long adjustment time of the ocean response to CO removal. Our findings suggest that CO removal may not guarantee the recovery of the subtropical dryness associated with the HC changes.