Annular modes and Hadley cell expansion under global warming

Projections of 21st century climate from the latest state-of-the-art climate models consistently call for a poleward expansion of the tropical Hadley cell (HC) and subtropical dry zones (SDZ) in response to increasing levels of atmospheric greenhouse gases. We find that approximately half of the model-simulated HC and SDZ expansion during the next hundred years can be explained by positive trends in the Northern Hemisphere and Southern Hemisphere annular modes (NAM and SAM), implying a close connection between changes in the tropical and extratropical atmospheric circulation. The link between NAM and SAM variability and the SDZ expansion suggests that future changes in the hydrologic cycle are likely to be strongly influenced by atmospheric dynamics.