Time-Dependent Internal Energy Budgets of the Tropical Warm Water Pools
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The exchange of internal energy between the warm water pools of the tropical oceans and the overlying atmosphere is thought to play a central role in the evolving climate system of the earth. Spatial displacements of the warm water pools are observed on annual and interannual time scales, the latter most notably in the Pacific in association with ENSO. Whether such variations are also associated with net changes in pool energy content is investigated. Extending the work of Niiler and Stevenson and Walin who considered the time mean energy budgets for volumes bounded by an isotherm, the time-dependent version of their equation is analyzed in which the main terms involve the time variations of pool volume and average temperature, net energy exchange between the pool and overlying atmosphere, and the turbulent ocean fluxes across the pool boundaries. The dominant signal in the mean seasonal energy budgets of the warm pools is an approximate balance between the annual variation of air pool heat exchange and the time-varying energy storage; the inferred turbulent ocean heat flux per unit area across the bounding surface of the warm pools is relatively steady through the year. Interannual variations of the warm pools are characterized by changes in pool volumes and temperature on ENSO and longer time scales with indications of an out-of-phase relationship between pool pseudo-energy content and the Southern Oscillation index. The ability to diagnose the varying turbulent ocean fluxes exiting the warm water pools on these time scales was impeded by incompatibilities between ocean temperature data and several air-sea flux climatologies. For the unscaled Coupled Ocean-Atmosphere Data Set (COADS) flux product that yields sensibly downgradient ocean heat flux estimates, strong positive correlation between air pool heat flux and inferred turbulent ocean flux at the pool base on an interannual time scale is found. But, given the uncertainties in the air-sea fluxes, it is difficult to firmly attribute these bottom flux changes to variations in ocean mixing processes. Though disappointing in the short term, it is suggested that time-dependent warm pool energy budget analyses constitute powerful diagnostics for validating future air-sea flux climatologies.