Expansion of the Hadley cell under global warming
Jian Lu, NCAR
Consistent weakening and poleward expansion of the Hadley circulation are diagnosed in climate change simulations of the IPCC AR4 project. Associated with this widening is a concomitant poleward expansion of the subtropical dry zone. Simple scaling analysis supports the notion that the poleward extent of the Hadley cell is set by the location where the thermally driven jet first becomes baroclinically unstable. The expansion of the Hadley cell is caused by an increase in the subtropical static stability, which pushes poleward the baroclinic instability zone and hence the outer boundary of the Hadley cell.
Contrasting the mechanisms responsible for the expansion of the Hadley cell under global warming versus the expansion driven by the La Nina-like equatorial Pacific cooling unravels distinct sources for the two conditions ---an extratropical source for the former and a tropical source for the latter.
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