Spatial and Seasonal Variations of Sea Surface Temperature Threshold for Tropical Convection

Abstract Tropical rainfall variations are of direct societal relevance and drive climate variations worldwide via teleconnections. The convective rainfall tends to occur when sea surface temperature (SST) exceeds a threshold, SST thr , usually taken to be constant in time and space. We analyze 40-yr monthly observations and find that SST thr varies by up to 4°C in space and with season. Based on local convective instability, we develop a quantitative theory that largely explains the SST thr variations using the climatological state of the tropical atmosphere. Although it is often assumed that spatial variations of tropical upper-tropospheric temperature are small and can be neglected, it is shown that lower climatological values favor a lower SST thr . Similarly, a small increase in climatological surface relative humidity also leads to a decrease in SST thr , as does a lower climatological air–sea temperature difference. Consequently, efforts to understand and predict natural or forced variations in tropical rainfall must account for, in addition to SST, the temperatures aloft and the near-surface humidity and temperature and requires improved understanding of what controls their distribution in space and time.