Ocean–Atmosphere Interaction in the Making of the Walker Circulation and Equatorial Cold Tongue

Abstract

Abstract The climate over the equatorial Pacific displays a pronounced asymmetry in the zonal direction that is characterized by the Walker circulation in the atmosphere and the cold tongue in the ocean. An intermediate coupled ocean–atmosphere model is used to investigate the driving force and the ocean–atmosphere interaction mechanism for the generation of the zonal asymmetry. In the far eastern Pacific, the upwelling at the equator is weak because zonal winds are blocked by the Andes. The off-equatorial upwelling induced by southerly cross- equatorial winds is thus crucial for cooling the eastern Pacific. A realistic cold tongue appears in the coupled model only when this southerly wind forcing is included. The southerly winds cause the sea surface temperature to fall in the east, enhancing the zonal heat contrast and hence intensifying easterly winds across the basin. These anomalous easterlies induce more equatorial upwelling and raise the thermocline in the east, amplifying the initial cooling by th...