Interannual Variability of Meridional Heat Transport in a Numerical Model of the Upper Equatorial Pacific ocean

Abstract

The interannual heat budget of the Pacific equatorial upwelling zone is studied using a primitive equation, a reduced gravity model of the upper Pacific equatorial ocean. The model is forced with monthly mean FSU winds from 1971 to 1990. A meridional overturning cell transports heat poleward with poleward flowing warm surface water compensated by colder, deeper equatorward flow. A horizontal cell transports heat equatorward as warmer equatorward flow, which enters the equatorial zone near the western boundary to feed the Equatorial Undercurrent, is compensated by colder poleward flow in the eastern basin. The heat transported by transient eddies is equatorward. Heat transport anomalies from the 20-year mean balance the time rate of change of heat content on interannual and seasonal timescales. Anomalies of surface heat flux do not contribute significantly to the interannual heat budget. Although each simulated ENSO event develops differently, similarities in the interannual heat budget emerge. Heat content increases due to an anomalous net equatorward heat transport during the initial stages when the easterlies are anomalously strong. This increase is associated first with an increase in the equatorward heat transport by the horizontal cell due to the increase of the zonal temperature gradient and then with a reduction of the poleward heat transported by meridional overturning as the easterlies slacken. Following the appearance of the maximum SST anomaly, anomalous net poleward heat transport compensates for the subsequent decrease in heat content. The anomalous poloward heat transport is associated first with a reduction of the equatorward heat transport by the horizontal cell as the Equatorial Undercurrent transport decreases, and later with an increase in the meridional overturning as the easterly wind stress returns to normal and the poleward surface flow is anomalously warm. Northward heat transport occurs during the warm phase when there is an export of heat, southward heat transport occurs during the preconditioning phase when there is an import of heat into the equatorial region. This result indicates that a cross-equatorial exchange of warm water occurs on the interannual timescale as first suggested by the sea level analysis of Wyrtki and Wenzel.