Effects of Sea Surface Temperature on Tropical Shallow Circulation Strength

Shallow circulations in the tropics are related to the sea surface temperature distribution, longwave cooling above the boundary layer, and shallow convection in both observations and models. To understand the influences sea surface temperature has on the shallow circulation strength, fixed sea surface temperature distributions are applied in experiments using an idealized general circulation model with uniform insolation and no rotation. The experiments are designed to distinguish the effects of mean sea surface temperature from those of sea surface temperature gradients, creating combinations using global mean SSTs from 299 to 305 K and SST ranges from 2.5 to 15 K. In these simulations, a large‐scale, deep circulation develops connecting the coldest to hottest SSTs. Experiments with sufficiently large sea surface temperature mean or range also exhibit a distinct shallow cell above the boundary layer. This shallow cell is associated with a deep moist layer sloping upwards from the coldest to hottest sea surface temperatures. We find that warming the mean sea surface temperature increases the strength and depth of these shallow circulations primarily by increasing shallow convection over intermediate sea surface temperatures, while increasing the sea surface temperature gradient enhances shallow circulation strength primarily by enhancing longwave cooling above the boundary layer.