Significant Contribution of Stratospheric Water Vapor to the Poleward Expansion of the Hadley Circulation in Autumn Under Greenhouse Warming

Abstract

Widening of the Hadley circulation and associated poleward shifts of subtropical dry zones have drawn extensive studies in the past decade. It is found that the poleward expansion of the Hadley circulation has strong seasonality and peaks in autumn in both Hemispheres in response to quadrupling CO2. Here we find that the poleward expansion in autumn is closely related to the increase of stratospheric water vapor (SWV). The SWV increase radiatively cools the stratosphere especially in the polar lower stratosphere, which consequently leads to widening of the Hadley cell in autumn. The SWV effect is affirmed in a set of “SWV-locking” experiments. It is found that the SWV increase leads to a poleward expansion of the Hadley circulation in autumn in both Hemispheres, which contributes about 30% of the total expansion due to quadrupling CO2 in autumn.