Interannual variability of the equatorial Pacific in the 1960's

A linear numerical model forced by winds estimated from ships for each month from January 1961, to December 1970, is used to study the interannual variability of the equatorial Pacific. Model pycnocline variability at the Galapagos Islands is very similar to the observed variability of sea level. The maximum significant cross correlation of the two records is near zero lag. The 1963, 1965, and 1969 El Niño events are characterized by a persistently deep pycnocline. The model pycnocline variability at Talara, Peru, leads the observed SST variability by 2 months. The lag structure of pycnocline variability cross correlations indicates that the variability at the equator is related to the excitation of internal Kelvin and Rossby waves. The onset of the 1965 and 1969 El Niño events was triggered by a large amplitude downwelling Kelvin wave excited by relaxation of the easterlies west of the dateline. None of the El Niño events of the 1960's were related to anomalous relaxations of the wind field over the central Pacific. In addition, the seasonal intensification of the southeast trades over the central Pacific was not as strong as during non‐El Niño years. The subsequent cessation of the remotely forced seasonal upwelling caused the pycnoline to be depressed throughout the El Niño year. During the southern summer, reestablishment of the semiannual variability of the southeast trades over the central equatorial Pacific excited a seasonal downwelling Kelvin wave. This second major downwelling impulse resulted in the double peak downwelling signature observed in sea level records. The minor El Niño of 1963 was soley due to the cessation of the semi‐annual wind stress variability east of 180°. The absence of remotely forced upwelling Kelvin waves kept the pycnocline deeper than normal following the seasonal downwelling at the outset of the year. There was not a relaxation of the wind field west of the dateline prior to the 1963 El Niño.