Equatorial Upwelling in the Central Pacific Estimated from Moored Velocity Profilers

Abstract

Horizontal divergence and vertical velocity (w) are estimated at 0°, 140°W using an array of five subsurface moored acoustic Doppler current profilers deployed from May 1990 to June 1991 during the Tropical Instability Wave Experiment. The record-length mean flow is divergent within the near-surface region and convergent within the thermocline, with maximum convergence located at the high speed core of the Equatorial Undercurrent (EUC). This pattern of divergence results in upwelling at and above the EUC core (with maximum value of 2.3 × 10−5 m s−1 located at 60-m depth) and downwelling below the core. The relative slopes in the zonal plane between the mean velocity vectors and the isotherms suggest a net diffusive heat flux. Assuming that this occurs vertically, an entrainment velocity parameterization provides an estimate of the “diapycnal vertical velocity” profile that reverses sign at the EUC core depth. Several kinematical and dynamical consistency checks are developed on both the time-dependent and the mean motions to supplement a discussion of errors for the mean w profile. The time-dependent fluctuations in w may be an order of magnitude larger than the mean values, and on synoptic timescales w may be directed either up or down over the entire upper 250-m region sampled.