Equatorial wave dynamics are essential in most oceanic models for reproducing aspects of the El Ni¿o--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 Ni¿o and the ensuing 1988--1989 La Ni¿a. This was made possible after extensive qualtity control and specific processing of the recently improved Geosat geophysical data records pertaining to the 17-day Exact Repeat Mission. In particular, the processing was made so that the Geosat-derived ZCA best fit near-surface zonal currents from three equatorial moorings at 165¿E, 140¿W, and 110¿W. The Geosat-derived 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 and where Kelvin and first symmetric Rossby waves explain most of the variance in ZCA, in similar proportion. The 1986--1987 El Ni¿o 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 consistent with wind forcing, and all give rise to notable eastward ZCA for almost an entire year. During the El Ni¿o--La Ni¿a transition period there is the occurrence of two downwelling Rossby waves originating from the eastern Pacific at times of favorable wind forcing. These Rossby waves switch the basin-wide ZCA from eastward to westward, terminating the warm event. Then, the 1988--1989 La Ni¿a 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 apparently related to the enhancement of the trade wind in the western half of the basin. These 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 Kelvin wave forcing and sign of Kelvin wave contribution versus Rossby wave contribution to ZCA in the western Pacific are discussed, and we conclude that neither pleads in favor of the delayed action oscillator mechanism. ¿American Geophysical Union 1994