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Chunzai Wang
,
Robert H. Weisberg
, and
Huijun Yang

Abstract

The thermodynamical process of latent heat flux is added to an analogical delayed oscillator model of the El Niño–Southern Oscillation (ENSO) that mainly considers equatorial ocean dynamics and produces regular, non–phase-locked oscillations. Latent heat flux affects the model sea surface temperature (SST) variations by a positive feedback between the surface wind speed and SST operating through evaporation, which is called the wind speed–evaporation–SST feedback. The wind speed–evaporation–SST feedback in which the atmosphere interacts thermodynamically with the ocean through surface heat flux differs from the conventional zonal wind stress–SST feedback in which the atmsophere interacts dynamically with the ocean through momentum flux.

The combination of equatorial ocean dynamics and thermodynamics produces relatively more realistic model oscillations. When the annual cycle amplitude of the zonal wind in the wind speed–evaporation–SST feedback is gradually increased, the model solution undergoes a transition from periodic to chaotic and then to periodic oscillations for some ranges of the parameters, whereas for other ranges of the parameters the transition goes from periodic to quasiperiodic and then to periodic oscillations. The route to chaos is the intermittency route. Along with such irregularity, the nonlinear interactions between the annual and interannual cycles operating through the wind speed–evaporation–SST feedback also produce a phase-locking of ENSO to the seasonal cycle. The model ENSO onset and peak occur in the boreal winter and spring, respectively, consistent with the observed phase-locking of ENSO in the far eastern Pacific. It is shown that ENSO decadal or interdecadal variability may result from the nonlinear interactions between the annual and interannual cycles in the Tropics.

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