Clouds, radiation, and the diurnal cycle of sea surface temperature in the tropical western Pacific

Abstract

The relationship among clouds, surface radiation flux, and the sea surface temperature (SST) of the tropical western Pacific Ocean over the diurnal cycle is addressed in the context of the Atmospheric Radiation Measurement (ARM) Program scientific objectives for the tropical western Pacific Ocean. An understanding of the relationship between clouds and SST on a variety of time and space scales is needed to understand fully the cloud-radiation feedback in the tropical oceans and the maintenance of the warm pool. Here the diurnal cycle is emphasized. Data from the TOGA COARE Intensive Observation Period is examined and interpreted using an ocean mixed layer model that includes a parameterization of the "skin" temperature, explicit salinity, a surface heat budget that includes the sensible heat flux associated with rain, and the contribution of rain to the surface momentum flux. Using a mix of modeling and observations, three different case studies are examined in detail: clear and calm, clear and windy, and disturbed. For these typical sets of conditions and processes in the tropical ocean warm pool, the upper-ocean structure is clarified so that the skin sea surface temperature, the bulk surface temperature (at a depth of 1 cm), and the temperature at 0.5 and 5 m below the surface (which is the level that buoys and ships routinely observe "surface" temperature) can be interpreted. Sensitivity studies are conducted with the model to investigate the roles of wind speed, precipitation, ocean turbidity, and ocean initial state in modulating the radiation-induced diurnal cycle in SST. It is found that in high insolation, low wind regimes that the skin temperature may be as much as 30°C warmer than the 0.5-m buoy temperature. Spatial distribution of the diurnal amplitude of the SST are calculated for the global Tropics, and speculations are made regarding the implication of the SST variability to the tropical climate.