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

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 the southerlies. A simple analog model is presented to illustrate this coupled ocean?atmosphere feedback originally proposed by Bjerknes. From an oceanographic point of view, the equatorial cold tongue is caused by easterly winds. In the coupled model, much of these easterlies arises as part of the Bjerknes feedback and can be attributed to the southerly wind forcing in the east. Were the earth climate symmetric about the equator, cross-equatorial wind would vanish, and the cold tongue would be much weaker and have a very different zonal structure than is observed today.