EVALUATION AND DIAGNOSIS OF SURFACE CURRENTS IN THE NATIONAL CENTERS FOR ENVIRONMENTAL PREDICTIONS OCEAN ANALYSES

Ensemble average currents from the 15 m depth level of the National Ce nters for Environmental Prediction's (NCEP) analyses of the tropical P acific Ocean are evaluated against surface mixed layer current observa tions obtained from an extensive set of satellite-tracked drifting buo ys. These averages display many climatological characteristics of the region but are not intended to serve as a climatology because the data from the analyses are trimmed to match the time-space distribution of the observations. Substantial discrepancies between the analyses and the observations are revealed. First, the near-equatorial meridional c urrents and divergence have approximately twice the magnitude in the a nalyses as in the observations. This discrepancy is largely independen t of whether temperature profile data art assimilated or not and is at tributed to the parameterization of vertical viscosity. Second, the zo nal flow in both the North Equatorial Countercurrent (NECC) and the So uth Equatorial Current (SEC) is much stronger in the analyses than in the observations, especially in the western Pacific. This discrepancy is associated with assimilation of temperature profile data. It arises because salinity is an active variable in the underlying analysis mod el but is not controlled by boundary fluxes or other observations. Und er the uncontrolled influence of advection and strong horizontal diffu sion the salinity distribution becomes nearly homogeneous. Consequentl y, the analyses do not account for observed temperature-salinity corre lations when density is computed following assimilation of temperature profile data. This leads to erroneous pressure gradients that drive e xcessively strong geostrophic currents and force large accelerations n ear the western boundary. Our results indicate that it is important to consider the consequences on the density structure of neglecting sali nity during the assimilation of temperature data. We recommend that su rface salinity observations from drifting buoys and volunteer observin g ships be initiated to improve the ocean analyses.