Observation of equatorially trapped waves in the Pacific using Geosat altimeter data

A spatial integration filtering method is applied to process 4.5 years of Geosat altimeter sea level anomalies(SLA) recorded from 1 April 1985 to 17 September 1989. The integral limits are chosen as ±1.5° latitude centered at the equator and ±4° longitude centered at the each central longitude. These limits are chosen in order to eliminate the random noise and to enhance the equatorially trapped wave signals. The spatial-integration-filtered SLA time series show periodical signals with different time scales. Processing the SLA time series with a Short Time Fourier Transform (STFT) yields the time-frequency spectra of the SLA which show a frequency spectral splitting-combining phenomenon in the periods during and after the 1986–1987 El Niñio-Southern Oscillation (ENSO) event. Analysis of non-linear wave-wave interaction indicates that this phenomenon results from amplitude modulation of the short period oscillations in the 60–100 day band by a long period oscillation with a period of about 500 days. The wave components of the short period oscillations are further analysed using the equatorial wave modes to fit the meridional distribution of the SLA. The results indicate that the downwelling Kelvin wave mode before and during the peak phase of 1986–1987 ENSO and the upwelling Kelvin wave mode during the 1988 La Niña are the dominant components. The first and second Rossby wave modes play important roles for entire time series and occasionally become the dominant components. The mean phase speed of Kelvin wave mode during the 1986–1987 ENSO is 3.0 m s−1 which is 25% higher than those in non-ENSO periods.