On the Role of Equatorial Ocean Modes in the ENSO Cycle

Abstract

In order to test certain aspects of the ENSO mechanisms proposed by Suarez and Schopf and by Battisti and Hirst, we force a shallow water ocean with 28 years of observed (FSU) winds and decompose both the atmospheric forcing and ocean response into equatorial modes. The proposed mechanism was verified in the following sense. For each warm phase of the cycle, a downwelling Kelvin mode directly forced by the weakening of the trade winds in the central Pacific was found. The same wind anomaly forces an upwelling gravest Rossby mode which propagates freely westward and reflects as a freely propagating Kelvin mode at the western boundary. This Kelvin mode returns to the scene of the original warming and acts as a retarded forcing (of opposite sign) eventually switching the phase of the oscillation from warm to cold. The cold phase then proceeds, by the same mechanism but with modes of opposite signs, to eventually switch to the warm phase. The contribution of the higher Rossby modes to this process is estimated. It is found that almost all the Kelvin response in the western Pacific results from the reflection of the gravest symmetric equatorial Rossby mode so that the ENSO cycle is defined by the interaction of only two ocean modes, the Kelvin and lowest Rossby mode. These modes are sometimes forced directly by the wind as part of an intrinsic coupled atmosphere-ocean mode and sometimes propagate freely where the wind forcing is negligible. The distinction between these two manifestations of the ocean modes is of the greatest importance and is stressed throughout the paper. The case of the warm event of 1976 is an interesting example of the failure of the switching mechanism, and is discussed in the conclusion.