Kelvin and Rossby Wave Contributions to the SST Oscillation of ENSO

Abstract

A simple model of interannual SST oscillation is developed using the two wave components of Kelvin and lowest symmetric Rossby waves. The model dynamics is similar to that of A. C. Hirst except the wind stress is simply parameterized in terms of SST. The model is used to investigate the standing oscillation character of SST variation and the transition mechanism by wave reflection at the western and eastern boundaries. The standing oscillation of SST in the central and eastern Pacific and a weak SST variation in the western Pacific are examined in terms of the SST tendencies by oceanic Kelvin and Rossby waves. The Kelvin and Rossby wave contributions to SST are almost completely canceled by each other in the western Pacific. But in the eastern Pacific, the SST variation is determined mainly by the Kelvin wave, and the Rossby wave contribution plays a minor role. The wave reflections at the eastern and western boundaries play a negative feedback mechanism to an ENSO-like oscillation. The variation of zonal-mean thermocline depth associated with the reflected free waves has a phase difference of about 150° from that of zonal-mean SST. The quadrature phase lead of the zonal-mean depth (ocean memory)—resulting from a large cancellation between the zonal means of forced and reflected free waves—to the zonal-mean SST (forcing) produces the ENSO-like oscillation. The western boundary reflection plays a more important role in the oscillation than the eastern boundary does. The dependency of the oscillation period and instability of the model system on the reflection coefficients is also studied in the present study.