GEOSAT-DERIVED SEA-LEVEL AND SURFACE CURRENT ANOMALIES IN THE EQUATORIAL PACIFIC DURING THE 1986-1989 EL-NINO AND LA-NINA

Equatorial wave dynamics are essential in most oceanic models for repr oducing aspects of the El Nino-Southern Oscillation (ENSO) phenomenon. In this paper, observational evidence of first baroclinic equatorial Kelvin and first symmetric meridional Rossby waves is found in Geosat- derived sea level anomalies (SLA) and surface zonal current anomalies (ZCA) in the equatorial Pacific ocean during the 1986-1987 El Nino and the ensuing 1988-1989 La Nina. This was made possible after extensive quality control and specific processing of the recently improved Geos at geophysical data records pertaining to the 17-day Exact Repeat Miss ion. In particular, the processing was made so that the Geosat-derived ZCA best fit near-surface zonal currents from three equatorial moorin gs at 165 degrees E, 140 degrees W, and 110 degrees W. The Geosat-deri ved SLA and ZCA are decomposed into first baroclinic equatorial Kelvin and gravest Rossby modes. The emphasis is then put on the chronology of the ZCA at the equator where the currents are the most energetic an d where Kelvin and first symmetric Rossby waves explain most of the va riance in ZCA, in similar proportion. The 1986-1987 El Nino is mostly characterized by a strong downwelling Kelvin wave in December 1986, a series of downwelling Kelvin waves in March-October 1987, and a strong upwelling Rossby wave in March-September 1987. These waves are consis tent with wind forcing, and ah give rise to notable eastward ZCA for a lmost an entire year. During the El Nino-La Nina transition period the re is the occurrence of two downwelling Rossby waves originating from the eastern Pacific at times of favorable wind forcing. These Rossby w aves switch the basin-wide ZCA from eastward to westward, terminating the warm event. Then, the 1988-1989 La Nina shows the generation of a series of upwelling Kelvin waves which are clearly identified in March /April and September/October 1988. These upwelling Kelvin waves are ap parently related to the enhancement of the trade wind in the western h alf of the basin. These waves maintain the basin-wide westward ZCA for more than a year, although the ZCA is slightly reduced by a front of upwelling Rossby waves in phase with the normal seasonal cycle. The Ke lvin wave forcing and sign of Kelvin wave contribution versus Rossby w ave contribution to ZCA in the western Pacific are discussed, and we c onclude that neither pleads in favor of the delayed action oscillator mechanism.