Relationship between Cloud Radiative Forcing and Sea Surface Temperatures over the Entire Tropical Oceans

Satellite measurements from January 1985 to December 1989 show that warmer tropical oceans as a whole are associated with less longwave greenhouse effect of clouds and less cloud reflection of solar radiation to the space. The regression slopes of longwave and shortwave cloud radiative forcings against sea surface temperatures averaged from 30°N to 30°S are about −3 and 2 W m−2 K−1, respectively. Relationships of cloud forcings and sea surface temperatures are analyzed for regions with different sizes. As has been reported in previous studies, the magnitude of area-averaged cloud radiative forcing for both longwave and shortwave radiations increases with sea surface temperatures in the equatorial eastern Pacific and is insensitive to sea surface temperatures over the tropical Pacific basin. Yet, when the region extends beyond the tropical Pacific, the magnitude decreases with sea surface temperatures. This phenomenon is shown to relate to changes in clouds over the tropical Indian Ocean and Atlantic, where sea surface temperatures increased but clouds decreased during the 1987 El Niño event. Relevance of the results to other climate changes is discussed.