Equatorial Kelvin and Rossby waves evidenced in the Pacific Ocean through Geosat sea level and surface current anomalies

Abstract

Equatorial Kelvin and Rossby waves are comprehensively demonstrated over most of the equatorial Pacific basin, through their signatures in sea level and zonal surface geostrophic current anomalies. This was made possible with altimeter data pertaining to the first year of the Geosat (Geodetic Satellite) 17‐day exact repeat orbit (November 8, 1986, to November 8, 1987). To this end, along‐track corrected Geosat sea level anomalies (SLAs), relative to the time period of interest, were first smoothed using nonlinear and linear filters. The original 17‐day time step was then reduced by combining all ascending and descending tracks within 10° longitudinal bands. Finally, SLAs were gridded onto a regular grid, and low‐pass filters were applied in latitude and time in order to smooth out remaining high‐frequency noise. Anomalies of zonal surface geostrophic current were calculated using the first and second derivatives of the SLA meridional gradient, off and on the equator, respectively. Sea level and surface current anomalies are validated in the western equatorial Pacific with in situ data gathered during seven hydrographic cruises at 165°E, and through expendable bathythermograph and mooring measurements. Following their chronological appearance along the 165°E meridian, the major low‐frequency SLAs and zonal surface current anomalies are described and explained in terms of the equatorial wave theory. An equatorial downwelling Kelvin wave, known to be the main oceanic signal of the 1986‐1987 El Niño, is generated in December 1986, concomitant with a strong westerly wind anomaly occurring west of the dateline. The associated propagating equatorial SLAs correspond to an elevation of 15 cm. Independent estimates of this Kelvin wave phase speed are obtained through time‐lag correlation matrix analysis (2.82 ± 0.96 m s−1) and the least squares fit of the SLA meridional structures to theoretical Kelvin wave shape (2.26 ± 1.02 m s−1). Both estimates indicate that the Kelvin wave has the characteristic of a first baroclinic mode. An equatorial upwelling Kelvin wave is then detectable in June 1987. It is characterized by a 10‐cm sea level drop, propagating only from the western to the central equatorial Pacific. A first meridional mode (m = 1) equatorial upwelling Rossby wave crossing the entire Pacific basin from March 1987 (eastern part) to September 1987 (western part) shows up in SLAs and zonal surface current anomalies. Such a Rossby wave corresponds to propagating sea level drops which are extreme (−12 cm) at about 4°N and 4°S latitudes. The consequences on zonal surface geostrophic current are very important since, in the case of the upwelling, it dramatically decreases the three major surface currents (the North and South Equatorial Countercurrents, and South Equatorial Current) by an amplitude similar to their mean annual velocity values. The least squares fit of the Rossby wave SLA meridional structures to its theoretical m = 1 form cogently suggests the dominance of the first baroclinic mode (c = 2.59 ± 0.65 m s−1). This dominance is corroborated by an estimate of the Rossby wave phase speed (1.02 ± 0.37 m s−1), which roughly corresponds to the theoretical phase speed (c/2m + 1) of the m = 1 equatorial Rossby wave. It is suggested that the equatorial upwelling Rossby wave is mostly due to a reflection of an equatorial upwelling Kelvin wave generated in January 1987 near the dateline. Whether or not the overall propagating features are part of the 1986–1987 El Niño or belong to the “normal” seasonal cycle cannot be decided in the absence of longer altimeter sea level time series.