THE ZONAL MOMENTUM BALANCE OF THE EQUATORIAL UNDERCURRENT IN THE CENTRAL PACIFIC

Current velocity data from an array of subsurface moorings deployed du ring the Tropical Instability Wave Experiment from May 1990 to June 19 91 are used to diagnose the upper-ocean zonal momentum balance at 0 de grees, 140 degrees W. The flow field and associated zonal momentum Bur divergence are fully three-dimensional over the upper 250 m, consiste nt with the earliest descriptions and theoretical ideas of the Equator ial Undercurrent (EUC). Estimates of the vertical stress divergence sh ow dynamical flow regimes that change between the surface and the base of the EUC, being essentially linear (modified by nonlinearity) near the surface, weakly nonlinear at the EUC core, and fully nonlinear bel ow the core. The vertical stress divergence is much larger over the lo wer portion of the EUC than previously reported, but this is consisten t with the observed downstream deceleration of the EUC and the idea th at vertical mixing is important in maintaining the thermostad. Nonline arity becomes increasingly important with decreasing frequency, but te nds to cancel upon vertical integration.