Sensitivity of mixed layer heat budgets to wind forcing: A case study for the equatorial Pacific cold tongue
Résumé
Ocean General Circulation Models (OGCMs) are very useful tools to analyze and quantify heat budget terms governing the sea surface temperature variability. However, model results are strongly dependant on the momentum forcing used. In this study, five simulations of a climate-type OGCM, forced by different wind stress products from atmospheric reanalyses, satellites and in situ data are performed. They are analyzed in the eastern tropical Pacific from 1993 to 2000, in the Pacific cold tongue, a key region where all terms of the heat budget are important. The differences in thermodynamical and dynamical oceanic parameters relevant for mixed layer budgets are first documented in the five simulations, and validations against observations are presented at annual and intraseasonal scales. It is then shown that the different components of the oceanic heat budget are quantitatively greatly affected by the choice of the wind forcing, and the amplitude of the terms explaining sea surface temperature variations can double from one simulation to the other. In contrast, the relative contributions of the terms, i.e. mean advection, atmospheric forcing, mixing at the base of the mixed layer and contributions of eddies are the same in all simulations forced by different wind stress products. This is true for the mean 1993-2000 period, at seasonal timescales and during interannual events such as the abrupt cooling observed in the cold tongue in May 1998. This paper thus suggests that model users can be confident in the physical processes governing sea surface temperature variability whatever the wind stress used to force their model is, but they must be very cautious when estimating quantitatively heat budgets terms with one simulation only.