Variability and change of the sea surface temperature threshold for convection

Many studies have discussed the existence of a tropical sea surface temperature (SST) threshold in the vicinity of 26.5ºC, above which atmospheric deep convection is commonly observed. Both global climate models and high-resolution tropical cyclone models suggest that this SST threshold rises under global warming. Some observations for the past few decades, however, show that tropical tropospheric warming has been nearly uniform vertically, suggesting that the troposphere may have become more unstable and casting doubts on the possibility that the SST threshold increases substantially with global warming. Based on an analysis of satellite observations of rainfall for the past 30 years, we determine that the SST threshold has undergone an upward trend of approximately 0.1°C per decade over the last 30 years that has paralleled the increase in tropical mean SST. This increase in the SST threshold likely owes to moist adiabatic lapse rate adjustment of the tropical troposphere; thus our results provide support for the growing body of evidence that, in contrast with some observational indications, the tropical troposphere has warmed in a way that is consistent with theoretical expectations and with climate model simulations. These global climate models suggest that as the tropical oceans warm, the SST threshold for convection will continue to rise in tandem with the tropical mean SST, thereby resulting in little change in the fraction of the tropical oceans that is convectively active. Here we explore possible coupling mechanisms between atmospheric convection and tropical SSTs that may contribute to this close coupling between convective threshold and tropical mean SST and that may help to explain this near invariance of convectively active area.