Influence of assimilation of subsurface temperature measurements on simulations of equatorial undercurrent and south equatorial current along the Pacific equator

Equatorial Pacific current and temperature fields were simulated with and w ithout assimilation of subsurface temperature measurements for April 1992-M arch 1995 and compared with moored buoy and research vessel current measure ments. Data assimilation intensified the mean east-west slope of the thermo cline along the equator in the eastern Pacific, shifted eastward the longit ude of the mean Equatorial Undercurrent (EUC) maximum speed 800 km to 125 d egrees W, and produced a 25% stronger mean EUC core speed in the eastern Pa cific. In the eastern Pacific the mean EUC core speed simulated with data a ssimilation was slightly more representative of observations compared to th at computed without data assimilated; in the western Pacific the data assim ilation had no impact on mean EUC simulations. Data assimilation intensified the north-south slope of the thermocline sout h of the equator in the western Pacific to produce a thicker and more inten se westward-flowing South Equatorial Current (SEC) in the western Pacific. In the western Pacific the mean SEC transport per unit width simulated with data assimilation was more representative of observations compared to that computed without data assimilation. However, large differences remained be tween the observed SEC transport per unit width and that simulated with dat a assimilation. In the eastern Pacific, the data assimilation had no impact on mean SEC simulations. The temporal variability of monthly mean EUC core speeds and SEC transports per unit width were increased significantly by data assimilation. It also increased the representativeness of monthly mean SEC transports per unit wi dth to the observations. However, the data representativeness of monthly me an EUC core speeds was decreased. Results could be explained by the couplin g between zonal gradient of temperature and EUC and between meridional grad ient of temperature and SEC. Longitudinal variations along the Pacific equa tor of the impact of data assimilation on the EUC and SEC precludes the cho ice of a single site to evaluate the effectiveness of data assimilation sch emes.