On the processes that control seasonal variations of sea surface temperatures in the tropical Pacific Ocean

In the tropical Pacific Ocean, the cold phase of both interannual and seasonal sea surface temperature variations is characterized by cold waters off the coast of South America and a pronounced equatorial tongue of cold surface waters in the eastern Pacific. The warm phase, in both cases, is marked by the weakening or complete absence of these features. Despite these striking similarities, very different physical processes are dominant on seasonal and interannual time scales. Interannually, a horizontal redistribution of warm surface waters, the dynamical response of the ocean to changes in the winds, is of primary importance. What matters most seasonally are two local processes: seasonal upwelling associated with a divergence of surface 0. currents, and the seasonal modulation of mixing processes, by heat fluxes, that control to what extent upwelling induced by the mean winds influences sea surface temperatures. These results shed light on the different requirements that coupled ocean-atmosphere models should meet if they are to reproduce both seasonal and interannual variability. The results also make a case for measurements, along a meridian in the eastern tropical Pacific, that focus on the relations between sea surface temperature changes, heat flux variations and mixing processes.