Surface Buoyancy Forcing and the Mixed Layer of the Western Pacific Warm Pool: Observations and 1D Model Results

Abstract

The broad, shallow body of warm (>29°C) water found in the western tropical Pacific Ocean plays an important role in the coupled ocean-atmosphere dynamics and thermodynamics associated with the El Niño-Southern Oscillation phenomenon. Thus, it is important to understand the processes that maintain and perturb that warm pool. Measurements from a buoy moored in the center of the warm pool during the TOGA Coupled Ocean-Atmosphere Response Experiment show that the exchange of mass between the ocean and atmosphere is as important as the exchange of heat. Rain forms a shallow, buoyant layer that does not mix with the water below except during infrequent strong wind events. Using a one-dimensional mixed layer model, it is demonstrated that the rate of local precipitation governs the mixed layer depth and can thus alter the rates of change in sea surface temperature during both warming and cooling periods. The observed mixed layer depth in the warm pool is at a depth that allows for maximum warming by capturing a significant portion of the shortwave radiation, but is shallow enough to avoid entrainment cooling. Either increased rain rates or reduced wind forcing yields a shallow mixed layer, which may be associated with cooling of the sea surface temperature in spite of a net positive heat flux at the sea surface because penetrating shortwave radiation is lost across the base of the shallow, fresh mixed layer. Decreased rain rates or increased wind forcing can turn on entrainment cooling of the surface mixed layer.