Ocean Dynamics, Thermocline Adjustment, and Regulation of Tropical SST

The role of tropical Pacific ocean dynamics in regulating the ocean response to thermodynamic forcing is investigated using an ocean general circulation model (GCM) coupled to a model of the atmospheric mixed layer. It is found that the basin mean sea surface temperature (SST) change is less in the presence of varying ocean heat transport than would be the case if the forcing was everywhere balanced by an equivalent change in the surface heat flux. This occurs because the thermal forcing in the eastern equatorial Pacific is partially compensated by an increase in heat flux divergence associated with the equatorial upwelling. This constitutes a validation of a previously identified “ocean dynamical thermostat.” A simple two-box model of subtropical–equatorial interaction shows that the SST regulation mechanism crucially depends on spatial variation in the sensitivity of the surface fluxes to SST perturbations. In the GCM, this sensitivity increases with latitude, largely a result of the wind speed dependence of the latent heat flux, so that a uniform forcing can be balanced by a smaller SST change in the subtropics than in equatorial latitudes. The tropical ocean circulation moves heat to where the ocean more readily loses it to the atmosphere. Water that subducts in subtropical latitudes and returns to the equatorial thermocline therefore has a smaller temperature perturbation than the surface equatorial waters. The thermocline temperature adjusts on timescales of decades to the imposed forcing, but the adjustment is insufficient to cancel the thermostat mechanism. The results imply that an increase in the downward heat flux at the ocean surface, as happens with increasing concentrations of greenhouse gases, should be accompanied by a stronger equatorial SST gradient. This contradicts the results of coupled atmosphere–ocean GCMs. Various explanations are offered. None are conclusive, but the possibility that the discrepancy lies in the low resolution of the ocean GCMs typically used in the study of climate change is discussed.