Wind-forced reversing jets in the western equatorial Pacific

Upper-ocean zonal currents in the western equatorial Pacific are remarkably variable, changing direction both with time and depth. As a part of the Tr opical Ocean and Global Atmosphere Coupled Ocean-Atmosphere Response Experi ment, an enhanced monitoring array of moorings measured the upper-ocean vel ocity, temperature, salinity, and, surface meteorological conditions in the western equatorial Pacific for two years (March 1992-April 1994). Data fro m this array are used to evaluate the zonal momentum balance. Although nonl inear terms (zonal. meridional, and vertical advection) were at times large , reversing jets were primarily due to an interplay between wind forcing an d compensating pressure gradients. In the weakly stratified surface layer, the flow is to a large extent directly forced by local winds. Eastward acce leration associated with westerly wind bursts and westward accelerations as sociated with easterly trades lead to frequent reversals in the surface-lay er flow. However, pressure gradients set up by the wind bursts partially co mpensate the local wind forcing in the surface layer. Below the surface lay er, these pressure gradients tend to accelerate the upper-thermocline how i n a direction opposing the local winds. Consequently, during westerly wind bursts. a reversing jet structure can develop, with a surface eastward curr ent overlying a westward intermediate layer flow, overlaying the eastward E quatorial Undercurrent.