The Significance of Cloud–Radiative Forcing to the General Circulation on Climate Time Scales—A Satellite Interpretation

Abstract

Cloud?radiative forcing calculations based on Nimbus-7 radiation budget and cloudiness measurements reveal that cloud-induced longwave (LW) warming (cloud greenhouse influence) is dominant over the tropics, whereas cloud-induced shortwave (SW) cooling (cloud albedo influence) is dominant over mid- and high latitudes. The average SW cloud cooling taken over the area of the globe from 65°N to 65°S is ?27.8 W m?2. This magnitude slightly overcomes LW cloud warming (?25.7 W m?2), resulting in a small net cooling effect of ?2.1 W m?2 over 93% of the earth. A 6-year zonally averaged mean cloudy- and clear-sky net radiation flux analysis shows that there are three distinct regimes in terms of net cloud warming or cooling, that is, warming in the tropics (between 20°N and 20°S) and in the high latitudes (poleward of 55°) and cooling in the extratropical latitudes between 20° and 55° in both hemispheres. These distributions reinforce the intensities of the Hadley and Ferrel meridional circulation cells. This stems from strong warming due to high-level clouds in the tropics and strong cooling due to mid- and low-level clouds at extratropical latitudes. The magnitude of the contribution by cloud forcing is found to be of the same order as eddy heat and momentum flux forcing to the maintenance of the mean meridional circulation. Surface?atmosphere forcing obtained by differentiating the cloud-induced effects from the measured radiative fluxes indicates that an east?west coupled North Africa?western Pacific energy transport dipole is maintained mainly by low-latitude land?ocean contrasts associated with shortwave radiation but supported by cloud controls on tropical longwave radiation. This implies that interannual variations in the net radiation balance associated with these two regions can give rise to fluctuations of the basic dipole structure and thus fundamental changes in low-latitude climate.