Tropical climate sensitivities: clouds, water vapor, radiation and large-scale circulation

Abstract

The physical mechanisms that affect the tropical sea surface temperature (SST) are investigated using a two box model and a mesoscale model with fixed uniform and sinusoidal SST gradients. Emphasis is placed on the large-scale circulation containing rising motion in regions with high humidity and high clouds and subsidence in regions with low upper tropospheric humidity, a trade inversion and low clouds. Changes in the area of the convective region are investigated as are effects of the high and low clouds. The results show that water vapor feedback in clear skies depends on the height of the moist boundary layer.Radiative cooling in the cold pool constrains the large-scale circulation strength. The strength of the circulation decreases with increasing convective area, because the increase in dry static stability overwhelms the increase in cooling rate in the two box model and the mesoscale model with SST gradients.For simulations with constant fixed SST and zero mean vertical motion, the sensitivities of the top of the atmosphere radiative quantities to SST are similar to observations comparing El Nino and La Nina SSTs and radiative quantities when averaged over an area including the ascending and descending branches of the tropical large-scale circulation. These simulations also show that the clear-sky temperature and water vapor feedbacks are of about equal magnitude in opposite directions, and the lapse rate feedback is negative with a magnitude of approximately 2 W m-2 K-1.Shortwave cloud forcing is related to the strength of the large-scale circulation in the SST gradient simulations. The longwave radiative cooling at the top of stratus clouds enhances the large-scale circulation. Increasing the mesoscale model SST gradient strengthens the large-scale circulation and decreases the average net absorbed shortwave radiation.