Interannual Geostrophic Current Anomalies in the Near-Equatorial Western Pacific

Abstract

A network of island tide gauges is used to estimate interannual geostrophic current anomalies (GCAs) in the western Pacific from 1975 to 1997. The focus of this study is the zonal component of the current averaged between 160°E and 180° and 2° to 7° north and south of the equator in the mean flow regions associated with the North Equatorial Countercurrent (NECC) and the South Equatorial Current (SEC), respectively. The tide gauge GCA estimates agree closely with similarly derived currents from TOPEX/Poseidon sea level anomalies. The GCAs in the western Pacific relate to a basin-scale adjustment associated with the El Niño?Southern Oscillation, characterized here using empirical orthogonal functions of tide gauge and supporting sea surface temperature and heat storage data. The dominant EOF mode describes the mature phase of ENSO events and correlates (0.8) with the GCA south of the equator. The second mode describes transitions to and from ENSO events and correlates (0.9) with the GCA north of the equator. The typical scenario then is for the NECC to intensify about 6 months prior to the peak of an El Niño, to remain near mean conditions during the peak stage of El Niño, and to later weaken about 6 months following the peak. In contrast, the SEC generally weakens throughout an El Niño displaying eastward anomalies. This equatorial asymmetry in the GCAs is consistent with a similar asymmetry in the wind field over the western Pacific. The phase differences between the NECC and SEC are less apparent during La Niña events. The GCA results provide further evidence that transitional phases of ENSO are more active north than south of the equator in the warm pool region.