The response of an ocean general circulation model to specified wind stress is used to understand the role of ocean wave propagation in the evolution of sea surface temperature (SST) in the equatorial Pacific Ocean. The ocean model reproduces observed equatorial Pacific interannual variability of SST and heat content in response to forcing by the observed wind stress. The ocean model is then forced with the same wind stress, but evolving backwards in time. A comparison of the results from these two experiments shows that the low frequency SST anomalies are almost the same in either case, except reversed in time, suggesting that the SST anomalies are in near equilibrium with the wind stress forcing. On the other hand, the equatorial heat content anomalies display slow eastward propagation and therefore differ strongly for the two experiments. It is shown, however, that the ocean model heat content response satisfies a "fast wave limit"; the departures of the heat content anomalies from the zonal mean are very similar, except reversed in time, for the two experiments, and are therefore in equilibrium with the wind stress forcing. Only the zonal mean heat content anomalies of the two experiments differ significantly. Thus, the memory of the model ocean on El Niņo time scales resides in the zonal mean heat content anomalies.
Analysis of the shallow water equations shows that in this fast wave limit, the time tendency of the zonal mean heat content anomalies at the equator is proportional to the meridional gradient of the zonal mean of the curl of the wind stress forcing. Predictions from this relation are verified against the numerical results.
While the SST anomalies are close to being in equilibrium with the wind stress forcing, there are also significant departures from equilibrium. These departures are shown to be in equilibrium with the zonal mean heat content.
The results suggest a new mechanism for coupled ocean-atmosphere variability and El Niņo in the tropical Pacific.
last update: 21 October 1994
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