Effect of deep convection on the regulation of tropical sea surface temperature

Abstract

THE distribution of tropical sea surface temperatures (SSTs) is negatively skewed: a 'warm pool' with SSTs greater than 300.5 K covers roughly half the tropical oceans (15° N to 15° S), whereas SSTs as low as 293 K are observed in regions of equatorial and coastal upwelling and persistent stratus cloud cover1. SSTs are thus within 2–3 K of the highest values over a large area of the tropical ocean, leading some authors to suggest that some physical process may act to limit SSTs to below about 303 K. Ramanathan and Collins2 have proposed a 'thermostat' mechanism in which ocean warming produces enhanced deep convection, leading to the formation of extensive cirrus cloud canopies which shield the troposphere and ocean surface from incoming solar radiation. Here I suggest that a mechanism of this sort may not be required to explain the SST distribution. I argue that large-scale dynamical processes will act to maintain uniform tropical tropospheric temperatures to within about 2 K, and that, in the absence of horizontal temperature contrasts in the atmosphere, a negatively skewed SST frequency distribution is bound to develop through equilibration between the atmosphere and spatially varying SSTs. In addition, although cirrus clouds reduce the solar insolation at the Earth's surface in regions of deep convection, they would not necessarily prevent SSTs from exceeding 305 K in the face of extensive greenhouse warming.