The response of a coupled model consisting of a statistical atmosphere coupled to a simple tropical Pacific Ocean model is used to understand oceanic wave dynamics associated with the El Niño and the Southern Oscillation (ENSO) cycle. The statistical atmosphere is simple enough to allow for the structure and position of the wind stress anomalies to be externally specified. In a control simulation, where the structure of the wind stress anomaly is determined from observations, the model produces a regular five year oscillation. This simulation is consistent with the so-called delayed oscillator theory in that sub-surface wave dynamics determines the slow time scale of the oscillation and surface layer processes are found to be relatively unimportant. Kelvin and Rossby wave propagation along the equator with periods considerably shorter than the simulated oscillation period is detected. How these relatively fast waves are related to the simulated five year oscillation is discussed.
In order to understand the mechanism responsible for the five year period in the control simulation, two sets of sensitivity experiments were conducted. The first set of experiments focused on how the meridonal structure of the wind stress anomaly influences the ENSO period. Relatively broad (narrow) meridional structures lead to relatively long (short) periods. While the gravest Rossby wave appears to be important in these simulations, it is found that the maximum variability in the thermocline is associated with off equatorial Rossby waves. The second set of sensitivity experiments were designed to examine how these off equatorial Rossby waves influence the ENSO cycle. Without the effects of the off equatorial Rossby waves at the western boundary, the model produces a two year oscillation regardless of the meridonial structure of the wind stress anomaly. The mechanism by which these off equatorial Rossby waves influence the ENSO period is described. Based on these experiments, it is shown that the reflection of the gravest Rossby wave off the western boundary is required to produce oscillatory behavior, but the period of the oscillation is determined by the off equatorial Rossby waves and the latitude at which they are forced.
last update: 6 June 1996
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