Thermodynamic coupling and predictability of tropical sea surface temperature
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2004 by the American Geophysical Union. Air-sea coupling involves the exchange of both momentum and heat between the atmosphere and the ocean. Dynamic coupling, which relates to the momentum exchange, is believed to play the dominant role in the tropics, especially in phenomena such as the El Nino-Southern Oscillation in the tropical Pacific. However, thermodynamic heat exchange between the atmosphere and the ocean can also play a significant role in air-sea coupling. This is especially true in the tropical Atlantic, where dynamic coupling may be of secondary importance. In this study, the role of this thermodynamic air-sea coupling is studied using an atmospheric general circulation model coupled to a slab ocean model. Two thermodynamic feedback mechanisms are considered: the reduced thermal damping mechanism and the wind-evaporation-SST (WES) feedback. It is shown that thermodynamic coupling leads to amplification and increased persistence of surface wind variability in the deep tropical Atlantic region. This effect is anisotropic, being stronger in the meridional component than in the zonal component of the surface wind. These features cannot be explained by the isotropic reduced thermal damping mechanism, and indicates a possible role for the WES feedback. Predictability experiments using observed December sea surface temperature (SST) initial conditions were also carried out. These show that thermodynamic coupling can lead to forecasts of north tropical Atlantic SST that are significantly better than persistence forecasts during the boreal spring. These results mean that thermodynamic coupling certainly leads to a richer, more complex set of interactions than a local, Hasselmann-type of red-noise model would imply.
