Dynamic and Thermodynamic Regulation of Ocean Warming*

The relative roles of clouds, surface evaporation, and ocean heat transport in limiting maximum sea surface temperatures (SSTs) in the western Pacific warm pool are investigated by means of simple and intermediate coupled ocean–atmosphere models. The authors first take an analytical approach by constructing a conceptual two-box model that contains dynamic coupling among the Walker circulation, SST, and ocean thermocline and thermodynamic coupling, which includes shortwave and longwave cloud forcing and latent and sensible heat fluxes at the ocean surface. In a realistic parameter regime, the three mechanisms mentioned above are all essential in limiting the SSTs within the observed range. The lack of any one mechanism would lead to an equilibrium SST that is too high, although unstable warming due to the super greenhouse effect would not occur. The analysis of the surface heat balance from the simple box model indicates that in the western Pacific warm pool, cloud reflection has a dominant effect, followed by evaporation and ocean dynamics. The simple model results are further evaluated numerically by using an intermediate coupled ocean–atmosphere model. With the forcing of the annual-mean solar radiation, this model is capable of simulating a realistic annual mean climate in the tropical Pacific. The authors then introduce an initial SST perturbation and examine how the perturbation evolves with time in the presence of clouds, surface evaporation, and ocean dynamic processes. Four experiments have been designed. In the first three experiments, each of the three processes is studied separately; in the last experiment, they are combined. The intermediate model results indicate that in the western Pacific warm pool, the largest negative feedback comes from the cloud shortwave radiation forcing, followed by the surface evaporation and ocean heat transport. The sensitivity of the model to various initial SST perturbation patterns is also investigated.