OBSERVATION OF EQUATORIALLY TRAPPED WAVES IN THE PACIFIC USING GEOSATALTIMETER DATA

A spatial integration filtering method is applied to process 4.5 years of Geosat altimeter sea level anomalies (SLA) recorded from 1 April 1 985 to 17 September 1989. The integral limits are chosen as +/- 1.5 de grees latitude centered at the equator and +/- 4 degrees longitude cen tered 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 pe riodical signals with different time scales. Processing the SLA time s eries with a Short Time Fourier Transform (STFT) yields the lime-frequ ency spectra of the SLA which show a frequency spectral splitting-comb ining phenomenon in the periods during and after the 1986-1987 El Nino -Southern Oscillation (ENSO) event. Analysis of non-linear wave-wave i nteraction indicates that this phenomenon results from amplitude modul ation of the short period oscillations in the 60-100 day band by a lon g period oscillation with a period of about 500 days. The wave compone nts of the short period oscillations are further analysed using the eq uatorial wave modes to fit the meridional distribution of the SLA. The results indicate that the downwelling Kelvin wave mode before and dur ing the peak phase of 1986-1987 ENSO and the upwelling Kelvin wave mod e during the 1988 La Nina are the dominant components. The first and s econd Rossby wave modes play important roles for entire time series an d 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 2 5% higher than those in non-ENSO periods.