Interannual variability of the equatorial Pacific—Revisited

Abstract

A linear numerical model forced by monthly shipboard wind estimates used to study the interannual response of the tropical Pacific Ocean for 1961–1970 (Busalacchi and O'Brien, 1981) is extended through December 1978. The additional 8 years include the 1972 and 1976 Los Niños and the aborted event of 1975. Model pycnocline variations at several locations are compared with observed sea‐level fluctuations. El Niño events are depicted as periods when the pycnocline is persistently deep along the eastern boundary. Remotely forced equatorial Kelvin waves are responsible for this response. The character of each simulated El Niño is strongly dependent on the relation between zonal wind stress changes in the western and central equatorial Pacific. There are important differences in the location and timing of the wind changes associated with each El Niño. During the 1970's, the easterlies in the central equatorial Pacific had more of an active role in the evolution of El Niño than in the previous decade. In the western tropical Pacific a rapid shoaling of the pycnocline during each El Niño is caused by westward‐propagating Rossby waves. Interannual pycnocline displacements in the central equatorial Pacific are determined by the superposition of Kelvin waves excited to the west and first horizontal‐mode Rossby waves generated to the east.