The energetics of El Nino and La Nina

Data from a realistic model of the ocean, forced with observed atmospheric conditions for the period 1953-92, are analyzed to determine the energetics of interannual variability in the tropical Pacific. The work done by the w inds on the ocean, rather than generating kinetic energy, does work against pressure gradients and generates buoyancy power, which in turn is responsi ble for the rate of change of available potential energy (APE). This means interannual fluctuations in work done by the wind have a phase that leads v ariations in APE. Variations in the sea surface temperature (SST) of the ea stern equatorial Pacific and in APE are highly correlated and in phase so t hat changes in the work done by the wind are precursors of El Nino. The win d does positive work on the ocean during the half cycle that starts with th e peak of El Nino and continues into La Nina; it does negative work during the remaining half cycle. The results corroborate the delayed oscillator mechanism that qualitatively describes the deterministic behavior of ENSO. In that paradigm, a thermocl ine perturbation appearing in the western equatorial Pacific affects the tr ansition from one phase of ENSO to the next when that perturbation arrives in the eastern equatorial Pacific where it influences SST. The analysis of energetics indicates that the transition starts earlier, during La Nina, wh en the perturbation is still in the far western equatorial Pacific. Althoug h the perturbation at that stage affects the thermal structure mainly in th e thermocline, at depth, the associated currents are manifest at the surfac e and immediately affect work done by the wind. For the simulation presente d here, the change in energy resulting from adjustment processes far outwei ghs that due to stochastic processes, such as intraseasonal wind bursts, at least during periods of successive El Nino and La Nina events.