On the Annual Cycle in the Tropical Eastern Central Pacific

Abstract

In the tropical eastern central Pacific Ocean, the annual cycle in sea surface temperature (SST), surface winds and pressure, and clouds are alternatively dominated by an antisymmetric (with respect to the equator) monsoonal mode in February and August and a quasi-symmetric equatorial-coastal mode in May and November, both having a period of one year. The monsoonal mode is forced by the differential insulation between the Northern and Southern Hemispheres. The surface wind variation of the monsoonal mode tends to lead SST variation in late spring/fall. The equatorial-coastal mode originates from atmosphere–ocean interaction. Its development is characterized by contemporaneous intensification and spatial expansion (westward and poleward phase propagation). The interaction between the forced monsoonal mode and the coupled equatorial-coastal mode plays a critical role in the annual cycle. From October to February, the decline of the southern winter regime of the monsoonal mode initiates and sustains the amplification of the equatorial-coastal mode, causing annual weakening of the cold tongue. From April to June, the enhancement of the poleward SST gradient associated with the decay of the equatorial-coastal mode initiates the eastern North Pacific summer monsoon. Atmosphere-ocean interaction is directly responsible for the annual weakening and reestablishment of the cold tongue, whereas the annual cycle in insulation regulates the interaction indirectly through the forced monsoonal mode.