Atmospheric circulation trends, 1950-2000: Relative roles of sea surface temperature forcing and direct atmospheric radiative forcing

SST forcing and direct atmospheric radiative forcing drive distinctive circulation responses that contribute about equally to the global pattern of circulation trends. These distinctive circulation responses are approximately additive and partially offsetting. Atmospheric radiative changes directly drive the strengthening and poleward shift of the midlatitude westerly winds in the Southern Hemisphere (and to a lesser extent may contribute to those over the Atlantic - Eurasian sector in the Northern Hemisphere), whereas SST trends (specifically those in the tropics) are responsible for the intensification of the Aleutian low and weakening of the tropical Walker circulation.

Discrepancies between the atmospheric circulation trends simulated by CAM3 and Community Climate System Model, version 3 (CCSM3), a coupled model driven by the same atmospheric radiative forcing as CAM3, are traced to differences in their tropical SST trends: in particular, a 60% weaker warming of the tropical Indo-Pacific in the CCSM3 ensemble mean than in nature.