A variable sea surface temperature threshold for tropical convection

Abstract

Sea surface temperatures (SSTs) contribute to modulation of deep convection over tropical oceans. Using a threshold value of outgoing long-wave radiation (OLR) as a proxy for deep tropical convection, regional and temporal relationships between SST and deep convection and its variability are examined. Based on this approach, an SST threshold of 27.5°C for deep convection across the global tropics is identified; this is in agreement with the critical SST of around 27°C for the onset of tropical deep convection identified in previous studies. Monthly and regional variations in the critical SST threshold (SSTMON-CRIT) required for the onset of deep tropical convection are also identified. SSTMON-CRIT exhibits interannual and seasonal variations for each of the monsoon regions analysed here. It varies between regions consistent with the mean SST of that region, so is higher in the Pacific than the Atlantic. Further, within a region, the critical SST threshold for convection is cooler in the wet season than the dry. The threshold SST in each region is generally a good proxy for convective activity in the wet season, but this connection breaks down in the dry season. The impact of atmospheric divergence in suppressing convection is much more evident in the dry season. Thus, the applicability of SST as a predictor of convection varies with atmospheric forcing, by region and by season and so we conclude that an absolute SST threshold is not a robust metric for tropical deep convection. Theses results have implications for understanding the distribution of tropical cyclogenesis in varied climate regimes.