Diagnostic Model and Analysis of the Surface Currents in the Tropical Pacific Ocean

Abstract

A diagnostic model of the tropical circulation over the 0-30-m layer is derived by using quasi-linear and steady physics. The horizontal velocity is directly estimated from sea surface height (TOPEX/Poseidon), surface vector wind (SSM/I) and sea surface temperature (AVHRR in situ measurements). The absolute velocity is completed using the mean dynamic height inferred from the World Ocean Atlas (WOA). The central issue investigated in this study is the more accurate estimate of equatorial surface currents relative to prior satellitederived method. The model formulation combines geostrophic, Ekman, and Stommel shear dynamics, and a complementary term from surface buoyancy gradient. The field is compared with velocity observations from 15-m-depth buoy drifter and equatorial Tropical Ocean-Atmosphere (TAO) current meters. Correlations with TAO data on the equator are much higher in the eastern Pacific cold tongue than before. The mean field in the cold tongue is also much more accurate, now showing the equatorial minimum that splits the South Equatorial Current into northern and southern branches. The mean current strength is somewhat less than in drifter composites because the mean dynamic topography from WOA remains too smooth. However, the seasonal cycle and interannual variations are robust, especially anomalies on the order of 1 m s 1 during the 1997-98 ENSO. This direct method using satellite measurements provides surface current analyses for numerous research and operational applications.