Role of sea surface temperature and wind convergence in regulating convection over the tropical Indian Ocean

Abstract

The notions that (a) the frequency of occurrence of clouds is little affected by the sea surface temperature (SST) over the warm tropical oceans when it is maintained above ∼27.5°C and (b) over these regions the cloudiness is mainly governed by the large‐scale atmospheric dynamics, are well‐founded. This study investigates the association among SST, atmospheric wind divergence, vertical wind and cloudiness (convection) over the Arabian Sea, the Bay of Bengal and the tropical Indian Ocean during different seasons. Analysis carried out on data from all the above regions during 1996–2008 shows that the cloudiness and cloud top altitude increase with SST for SST > 26.5°C and become saturated for SST in the range of 28.5–29.75°C. Further increases in SST reduce cloudiness. Similar variations are obtained by analyzing the data during the Asian summer monsoon season (June–September) alone. In contrast, the cloudiness and cloud top altitude increase with SST even for SST > 29.5°C during winter (December–February). The base value of SST for a significant enhancement in cloudiness during winter is found to be ∼1°C larger than that during the summer monsoon season. Seasonal variations in the SST‐dependence of cloudiness over warm oceanic regions are primarily due to changes in the large‐scale atmospheric circulation which plays a pivotal role in regulating cloudiness, especially through surface‐level moisture convergence, mid‐tropospheric vertical winds, upper tropospheric divergence and orographic effects in the upwind direction. Regions of the warmest SST, maximum surface wind convergence and the largest cloudiness are generally not collocated.