MEAN ZONAL MOMENTUM BALANCE IN THE UPPER AND CENTRAL EQUATORIAL PACIFIC-OCEAN

We examine the mean zonal momentum balance in the tropical mid-Pacific using a year of acoustic Doppler current profiler velocities and cond uctivity-temperature-depth profiler densities from the Hawaii-to-Tahit i Shuttle Experiment. All significant contributions from the mean, ann ual cycle, and higher-frequency flow fields are determined with the ex ception of the vertical stresses. We find that even neglecting vertica l stresses, the zonal momentum equation is in rough balance at 90-117- m depth at all latitudes from 4-degrees-S to 10-degrees-N. While the f ormal error bars are large, this rough balance is reproducible over fo ur to five independent latitudes and so is probably real. The balance at 90-m depth is geostrophic to within 5-degrees of the equator. Close r to the equator, meridional mean convergence and meridional eddy stre sses contribute important forces to balance the mean pressure gradient . Nearer the surface, the zonal momentum equation is dominated by east ward pressure gradients near the equator and eastward Coriolis forces from a strong, northward Ekman flow poleward of 2-degrees-N. In the ve rtical integral these forces roughly balance the surface wind stress; thus vertical stresses suffice to close our momentum budget. We conclu de that on average vertical stresses arising from the wind forcing do not penetrate deeper than 90 m into the tropical ocean. This contradic ts an earlier study of the equatorial zonal momentum budget but is con sistent with turbulent dissipation measurements on the equator. Previo us findings of stronger, deeper dissipation on the equator are probabl y due to the stronger, deeper mean shear there rather than to a locall y altered stress profile. Vertical turbulent viscosities derived from our observations agree with previous observations on the equator but c ontradict the conventional, Richardson number parameterization off the equator.