Simulated Relationships between Sea Surface Temperatures and Tropical Convection in Climate Models and Their Implications for Tropical Cyclone Activity

Abstract

he impact of enhanced atmospheric CO2 concentrations on tropical convection and sea surface temperatures (SSTs) over the global tropics is assessed using five fully coupled atmospheric?oceanic general circulation models (AOGCMs). Relationships between SST and either outgoing longwave radiation or convective precipitation rates are evaluated for three climate states: present day, a doubled-CO2 scenario, and a quadrupled-CO2 scenario. All AOGCMs capture a relationship between present-day outgoing longwave radiation (OLR) and SST and between convective precipitation rate (PRC) and SST: deep tropical convection (DTC)?signified by rapidly decreasing OLR and rapidly increasing PRC rates?occurs above an SST threshold of around 25°C. Consistent across all AOGCMs, as concentrations increase to 2 ? CO2 and 4 ? CO2, the threshold SSTs for DTC to occur shift to 25.5°?28°C and 26.5°?30°C, respectively. Annual PRC rates in the 20°N?20°S region increase for two AOGCMs [Meteorological Research Institute Coupled General Circulation Model, version 2.3.2 (MRI CGCM2.3.2) and ECHAM5/Max Planck Institute Ocean Model (MPI-OM)] with increasing CO2, but PRC in the other three AOGCMs [Geophysical Fluid Dynamics Laboratory Climate Model versions 2.0 and 2.1 (GFDL CM2.0 and CM2.1) and National Center for Atmospheric Research (NCAR) Parallel Climate Model (PCM)] exhibits almost no change. Within this tropical zone, increased CO2 concentrations yield up to a 6.1% increase in the number of locations with monthly averaged PRC exceeding two established DTC thresholds (12 and 14?mm?day?1). These results indicate that, although the SST threshold for DTC is projected to shift with increasing atmospheric CO2 concentrations, there will not be an expansion of regions experiencing DTC. One implication of these findings is that there will be little change in regions experiencing tropical cyclogenesis in future climate states.