Hadley Cell Expansion
As global temperatures rise, the temperature difference between the poles and the equator is likely to decrease, expanding the cell of air circulation adjacent to the equator known as the Hadley Cell. One effect this has is that mid-latitude regions like the Mediterranean and the Southwestern US are likely to see an increase in sea level pressure—which corresponds to drier weather.
The Hadley Cell is the most prominent tropical circulation feature. It extends through the entire depth of the troposphere from the equator to the subtropics (at about the 30° latitude line) over both hemispheres. The cell develops in response to intense solar heating near the equator.
Trends and projections
Model projections indicate that the Hadley Circulation will shift its downward branch poleward in both the Northern and Southern Hemispheres, causing drying in the subtropics as a result.
Scientists have already observed a poleward shift of the southern Hadley Cell.
“[D]rying in the Mediterranean, southwestern USA and southern African regions are consistent with projected changes in the Hadley Circulation.”