The Processes Determining the Annual Cycle of Equatorial Sea Surface Temperature: A Coupled General Circulation Model Perspective

Abstract

The origin of the annual cycle of equatorial sea surface temperature (SST) is diagnosed using a global coupled atmosphere?ocean general circulation model (CGCM) that realistically simulates this annual cycle. The simulated heat flux, wind stress, upper ocean thermal structure, and mixed-layer depth, which are critical to the correct simulation of the near-equatorial SST, are compared with observations for realism. Based on this analysis, it is concluded that the model results should be applicable to the actual coupled system in the Indian and Atlantic Oceans. In the Pacific, errors in the simulated zonal wind stress and heat flux imply that, even though the SST annual cycle is realistic, the processes that govern its evolution may be different than those found in nature. The processes controlling the annual cycle of SST in the CGCM are diagnosed with experiments in which the ocean component model is forced with the CGCM surface fluxes of heat, momentum, and freshwater. In the eastern Pacific, the annual cycle of SST is due in large part to upwelling variations. Near the coast of South America, the upwelling is caused in nearly equal parts by the zonal and meridional wind stress, whereas farther to the west the upwelling induced by the zonal wind stress is dominant. The surface heat flux forces a significant portion of the annual cycle in the eastern Atlantic. However, as was found for the eastern Pacific, upwelling is the dominant process determining the annual cycle of SST. The annual cycle of SST in the Indian Ocean is dominated by the response to the varying heat flux. The annual cycle of surface heat flux in the Atlantic and Indian Oceans is due about equally to the latent and shortwave fluxes. In both the Atlantic and Indian Oceans, variations in the magnitude of the surface heat flux are more important than mixed-layer depth variations in determining the SST response to the flux of heat across the ocean surface.