An Estimate of Equatorial Upwelling in the Pacific

Abstract

Upwelling in the equatorial Pacific Ocean manifests itself by a tongue of cool water stretching from the Galapagos Islands to the date line. To estimate the rate of upwelling, the mass, heat and salt budgets of the tongue are investigated. The Ekman divergence is determined from wind stress as 84 × 106 m3 s−1. It is compensated by geostrophic convergence of the same magnitude as determined from the zonal pressure gradient. Since the vertical distribution of the two meridional flows is different, a strong vertical circulation results, which leads to upwelling at a rate of ∼50 × 106 m3 s−1. A consideration of the heat budget leads to the conclusion that horizontal advection in the South Equatorial Current does contribute to the cool tongue, but that the contribution of upwelling is much larger. The heat budget also indicates that upwelling comes from depths above the core of the undercurrent and that the source water has temperatures only ∼3°C less than the water flowing out laterally. The seasonal variation of all properties associated with the cool tongue is strong and produces a cross-equatorial flow of water from the summer to the winter hemisphere of ∼20 × 106 m3 s−16.