On the annual cycle of the tropical Pacific atmosphere and ocean

Abstract

A simple climate model is proposed for studying the key parameters of the coupled ocean-atmosphere system, monthly mean surface winds and precipitation. In this model, sea-level pressure is thermodynamically determined from sea surface temperature (SST) through a vertically integrated hydrostatic equation in which the vertical mean lapse rate is a function of SST plus a time-independent correction. The surface wind are then computed from the sea-level pressure gradients through a linear surface momentum balance with the anisotropic and latitude-dependent Rayleigh friction coefficients. The precipitation is calculated from a moisture budget by taking into account the effect of SST on convective instability. This model is capable of simulating a realistic annual cycle of the surface winds, sea-level pressure, and precipitation over the tropical Pacific. The response of the tropical atmosphere to symmetric and antisymmetric SST forcing is examined using the simple model. It is found that the annual variation of the symmetric SST mode results in the westward propagation of the surface zonal wind, sea-level pressure, and precipitation along the equator and the antisymmetric SST variation causes the seasonal oscillation of monsoonal circulations and rainfall. The mechanisms of annual variations of the tropical Pacific sea surface temperature are investigated using a reduced-gravity ocean model. It is found that the westward propagation of the equatorial SST anomaly is due to wind-related dynamic processes. Therefore, the westward propagation of atmospheric and oceanic quantities is a manifestation of coupled air-sea interaction. The initiation of anomalous warming or cooling in the eastern Pacific, however, is primarily due to the northward propagation of the SST anomaly in response to solar radiation forcing. As a result, the annual variation of the Pacific SST can be regarded as an interplay between coupled air-sea interaction and solar radiation forcing. The remote forcing of anomalous winds in the western Pacific also shows contribution on the SST variation in the eastern Pacific. The annual variation of cloudiness has effects on the change of SST in the western Pacific and the northeastern Pacific monsoon region.