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.

Last Updated: 2008-08-22