FY 1994

Mean zonal momentum balance in the upper and central equatorial Pacific Ocean

Johnson, E.S., and D.S. Luther

J. Geophys. Res., 99(C4), doi: 10.1029/94JC00033, 7689–7705 (1994)

We examine the mean zonal momentum balance in the tropical mid-Pacific using a year of acoustic Doppler current profiler velocities and conductivity-temperature-depth profiler densities from the Hawaii-to-Tahiti Shuttle Experiment. All significant contributions from the mean, annual cycle, and higher-frequency flow fields are determined with the exception of the vertical stresses. We find that even neglecting vertical stresses, the zonal momentum equation is in rough balance at 90–117-m depth at all latitudes from 4°S to 10°N. While the formal error bars are large, this rough balance is reproducible over four to five independent latitudes and so is probably real. The balance at 90-m depth is geostrophic to within 5° of the equator. Closer to the equator, meridional mean convergence and meridional eddy stresses contribute important forces to balance the mean pressure gradient. Nearer the surface, the zonal momentum equation is dominated by eastward pressure gradients near the equator and eastward Coriolis forces from a strong, northward Ekman flow poleward of 2°N. In the vertical integral these forces roughly balance the surface wind stress; thus vertical stresses suffice to close our momentum budget. We conclude that on average vertical stresses arising from the wind forcing do not penetrate deeper than 90 m into the tropical ocean. This contradicts an earlier study of the equatorial zonal momentum budget but is consistent with turbulent dissipation measurements on the equator. Previous findings of stronger, deeper dissipation on the equator are probably due to the stronger, deeper mean shear there rather than to a locally altered stress profile. Vertical turbulent viscosities derived from our observations agree with previous observations on the equator but contradict the conventional, Richardson number parameterization off the equator.