Equatorial undercurrent: Measurements and theories

Abstract

An eastward jet in the equatorial thermocline and, below it, a weaker westward current extending to a depth of about 1200 meters have been observed at practically all longitudes. In the western Pacific the flow in the deep (150 meter) mixed layer above the thermocline is westward at and near the surface and eastward at greater depths when the winds are westward. These shallow currents appear to reverse direction when the monsoons do. At longitudes where a deep mixed layer is absent, the Atlantic and eastern part of the Pacific, the undercurrent in the thermocline is symmetric about the equator, has its downstream flow in geostrophic balance, and is marked by deep penetration of warm water of high oxygen concentration when the winds are light. When the southeast trades gain in strength, the core of the undercurrent moves upwind, its zonal flow becomes ageostrophic, the westward surface flow becomes stronger, the subsurface eastward flow becomes weaker, the ridging of isotherms at the equator becomes more pronounced, and the troughing becomes less pronounced. The evidence in favor of these variations occurring systematically is very tenuous. For a constant‐density model to be relevant to the motion observed in the mixed layer of the western Pacific, it must be nonlinear and the vertical diffusion of momentum must be important at all depths. The results of such models are in reasonable agreement with observations if the winds are westward. To explain the eastward flow in the thermocline below the mixed layer, a stratified model is necessary. According to one such model, density gradients, in the absence of local winds, give rise to an eastward surface current in the equatorial thermocline and below it to a westward current. The meridional circulation is marked by equatorial downwelling. The modification of these ‘thermally’ driven currents by local winds is consistent with the earlier description of the flow at longitudes where a mixed surface layer is absent.