Influence of Equatorial Dynamics on the Pacific North Equatorial Countercurrent

Abstract

The Pacific North Equatorial Countercurrent (NECC) is generally not well simulated in numerical models. In this study, the causes of this problem are investigated by comparing model solutions to observed NECC estimates. The ocean model is a general circulation model of intermediate complexity. Solutions are forced by climatological and interannual wind stresses, τ = (τx,?τy), from Florida State University and the European Centre for Medium-Range Weather Forecasts. Estimates of the observed NECC structure and transport are prepared from expendable bathythermograph data and from the ocean analysis product of NOAA/National Centers for Environmental Prediction. In solutions forced by climatological winds, the NECC develops a discontinuity in the central Pacific that is not present in the observations. The character of the error suggests that it arises from the near-equatorial (5°S?5°N) zonal wind stress, τx, being relatively too strong compared to the y derivative of the wind stress curl term, (curlτ)y, associated with the intertropical convergence zone. This is confirmed in solutions forced by interannual winds, which exhibit a wide range of responses from being very similar to the observed NECC to being extremely poor, the latter occurring when near-equatorial τx is relatively too strong. Results show further that the model NECC transport is determined mainly by the strength of (curlτ)y, but that its structure depends on near-equatorial τx; thus, NECC physics involves equatorial as well as Sverdrup dynamics. Only when the two forcing features are properly prescribed do solutions develop a NECC with both realistic spatial structure and transport.