A new model for ocean circulation, based on Large Scale Semigeostrophic Equations, is derived. The fundamental simplifying assumption is that the temperature of the fluid is independent of the depth. This allows the five primitive equations to be reduced to two nonlinear evolution equations for the temperature and the vertically averaged stream function. While this may be unphysical, the analysis of the model is a first step towards understanding the dynamics of simple coupled atmosphere-ocean models in which a thermally active upper layer is a minimum requirement. The specific case of the evolution of a Gaussian-strip temperature anomaly is considered. A WKBJ analysis gives the asymptotic structure of the solution at high wave number and reveals two generic features of the model: the importance of thermal diffusion and the emergence of small scales. The analytic results are compared with numerical linear and nonlinear solutions.
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12th Conference on Atmospheric and Oceanic Fluid Dynamics