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  • Brown, J.N. and A.V. Fedorov, How much energy is transferred from the winds to the thermocline on ENSO timescales?. J. Climate. Submitted.

The dynamics of El Niño-Southern Oscillation (ENSO) are studied in terms of the balance between energy input from the winds (via wind power) and changes in the storage of available potential energy (APE) in the tropical ocean. Presently, there is broad disagreement in the way models simulate ENSO events and hence a need for simple, physically based metrics to allow for inter-model comparisons. This energy description is a basin-wide, integral, quantitative approach ideal for inter-model comparison that assesses model behavior in the subsurface ocean. Here it is applied to a range of ocean models and data assimilations. The onset of an El Niño is characterized by a decrease in wind power which leads to a decrease in APE, and hence a flatter thermocline. In contrast, La Niña events are preceded by an increase in wind power that leads to an increase in the available potential energy and a steeper thermocline. The wind power alters the APE via buoyancy power, associated with vertical mass fluxes that modify the slope of the isopycnals. Only a fraction of wind power is converted to buoyancy power. The efficiency of this conversion, γ, is estimated in this study at 50-60%. Once the energy is delivered to the thermocline it is subject to small, but important, diffusive dissipation. We estimate that this dissipation sets the e-folding damping rate for the APE, α, of the order of 1 year-1. We propose to use the efficiency γ and the damping rate α as two energy-based metrics for evaluating dissipative properties of the ocean component of general circulation models, providing a simple method for understanding subsurface ENSO dynamics and a diagnostic tool for explaining some key differences between the models.

Last Updated: 2008-12-16

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