Clouds and cloud radiative forcing over tropical Indian ocean and their relationship with sea surface temperature

Abstract

Earth radiation budget experiment (ERBE) radiative fluxes and International Satellite Cloud Climatology Project (ISCCP) C-2 cloud parameters for the four representative months of January, April, July and October and for the period 1985–1988 are used to study the cloud–radiation interaction over the tropical Indian Ocean (20°S–20°N, 50°E–100°E), and its relationship with sea surface temperature (SST). Some important results are compared with those over tropical west Pacific Ocean (20°S–20°N, 130°E–180°E). Over Indian Ocean, both the shortwave cloud radiative forcing (SWCRF) and longwave cloud radiative forcing (LWCRF) are found to be correlated most with high cloud amounts (HCA) among the various cloud types. HCA and cloud radiative forcings in turn show a positive relationship with SST above ~26.4°C. During July and October, after reaching a maximum value at ~29°C, the relationship is found to be negative. In the rising portion of the HCA–SST relationship, at certain SST threshold value (27.4°C for July) the probability of occurrence of high convective clouds suddenly rises above 50%. During January and July above this SST threshold value the SWCRF is found to be significantly larger than LWCRF, thus causing large negative net cloud radiative forcing. This is found to be associated with the sudden rise in the spatial extent (cloud amount) and the optical depth of the high clouds above the SST threshold values.