Rain and Cloud Perspectives on the Seasonal Cycle over the Western Equatorial Indian Ocean

Convection over the western equatorial Indian Ocean (WEIO) is strongly linked to precipitation over Africa and Australia but is poorly represented in current climate models, and its observed seasonal cycle is poorly understood. This study investigates the seasonal cycle of convection in the WEIO through rainfall and cloud measurements. Rainfall shows a single annual peak in early austral summer, but cloud proxies identify convective activity maxima in both boreal and austral summer. These diverging measures of convection during boreal summer are indicative of a reduction in the intensity of precipitation associated with a given cloud regime or cloud-top height during this time of year but an increase in the overall occurrence of high-top clouds and convectively active cloud regimes. The change in precipitation intensity associated with regimes is found to explain most of the changes in total precipitation during the period from May to November, whereas changes in the occurrence of convective regimes explains most of the changes throughout the rest of the year. The reduction in precipitation intensities associated with cloud regimes over the WEIO during boreal summer appears to be related to large-scale monsoon circulations, which suppress convection through forcing air descent in the midtroposphere and increase the apparent occurrence of convectively active cloud regimes through the advection of high-level cloud from monsoon-active areas toward the WEIO region.