Intense surface currents in the tropical Pacific during 1996-1998

Tropical Pacific Ocean near-surface currents and their momentum and tempera ture balances are investigated using several data sets, including drifting buoy velocities and TOPEX/Poseidon altimetry. The data sets are combined to produce monthly surface current estimates on a uniform grid for the 6-year period 1993-1998, using multivariate optimal interpolation. The analysis s hows dramatic changes in current from December 1996 through August 1998 in response to the recent El Nino-Southern Oscillation event. Eastward current anomalies of similar to1 m s(-1) were recorded in December 1996 in the wes tern Pacific, generated by sporadic westerly wind bursts. By April 1997 the se equatorial anomalies reached the eastern boundary, and in the summer of 1997 a band of strong eastward flow formed across the basin. This circulati on pattern persisted until the beginning of 1998, when a westward equatoria l jet appeared in January-April. Interestingly, the reversal of flow occurr ed prior to the restoration of the trade winds. The timing of these events as well as the results of previous dynamical studies raise questions about the relative importance of terms in the zonal momentum and temperature bala nces. We address the first of these questions by examining the applicabilit y of a three-term linear zonal momentum balance on the equator. In all case s we focus on anomalies from the time mean. Our results show that local acc eleration is reasonably well balanced by the difference between zonal press ure gradient anomaly and wind-induced momentum flux. This three-term balanc e displays significant basin-wide variations and is consistent with the con clusion that the equatorial Pacific is not in equilibrium with local wind f orcing due to the presence of propagating waves. Examination of the tempera ture balance shows that at the beginning of El Nino the warming in the cent ral Pacific is mainly supported by horizontal temperature transport, while the vertical heat exchange and transport become important in the eastern ha lf of the equatorial Pacific during the transition from El Nino to La Nina.