Ocean Wave Dynamics and the Time Scale of ENSO

A reexamination of the coupled delayed-action oscillator model of Suarez and Schopf for the El Nino/Southern Oscillation (ENSO) phenomenon is made, by deriving it using a parameterized atmosphere and explicit linear ocean wave dynamics. The derivation attempts to clarify the role of boundary reflections, damping, and scale sensitivity in determining the characteristic timescale of the model. Making the assumption that SST anomalies are related to thermocline perturbations in the central to eastern part of the basin, and that wind anomalies are related to SST anomalies, ocean wave dynamics are invoked to solve for the relationship between wind anomalies and the relevant thermocline displacement. A perturbation to SST causes wind anomalies which drive Kelvin waves eastward, thereby increasing the SST perturbation. The wind perturbations also generate Rossby waves in the ocean, which propagate westward, eventually reflecting from the western boundary as Kelvin waves. The thermocline displacements of these waves have the opposite sense to those of the directly driven Kelvin waves and form the basis for the delayed, negative feedback in the delayed-action oscillator. By solving the wave dynamics explicitly, we are able to conclude that: 1) The delayed action oscilator—in its simplest form with no eastern boundary reflection and a single Rossby wave reflected from the western boundary—forms the basic oscillator mechanism. 2) Very little of the Rossby wave energy propagating to the western boundary needs to be reflected into the Kelvin wave in order for the system to oscillate (as little as 20% in some cases). 3) The zonal extent of the wind field response to SST anomalies has almost no influence on the solutions. 4) Broadening of the meridional shape of the winds, which inparts more of the Rossby wave energy to higher meridional modes has the net effect of lengthening the delay without a large impact on the amplitude of the returning signal. 5) The role of the eastern boundary is relatively unimportant.