Why is There an Evaporation Minimum at the Equator?*

Abstract

At all longitudes oceanic evaporation rates are lower on the equator than at latitudes to the north and south. Over the oceanic cold tongues this is related to the presence of cold water and divergence of heat by the ocean circulation. Herein is investigated why there is also a minimum over the Indo-Pacific warm pool. Model results confirm the recent suggestion of Sobel that deep convective clouds over the warm pool reduce the amount of solar radiation coming into the ocean that the evaporation has to balance. The results also confirm that this is only a partial explanation. Less evaporation over the warm pool than in the trade wind regions is also caused by an interaction between the ocean heat transport and the distribution of surface wind speeds. Low wind speeds over the warm pool reduce the latent heat flux and increase the SST, and stronger wind speeds in the off-equatorial regions of the Tropics increase the latent heat flux and cool the SST. Consequently, the wind speed distribution increases the meridional temperature gradient and increases the poleward ocean heat transport. Low latent heat fluxes over the warm pool can be sustained because the incoming solar radiation is partially offset by ocean heat flux divergence. Large values under the trade winds are sustained by ocean heat flux convergence. Climate models are used to show that, in equilibrium, wind speeds can only influence the latent heat flux distribution through their coupling to the ocean heat transport. In the presence of ocean heat transport, advection of moisture in the atmospheric boundary layer from the subtropics to the equator also increases the evaporation under the trade winds, but this has a much smaller effect than the wind speed or the cloud-radiation interactions.