A barotropic prognostic numerical model of the circulation in the Gulf of Mexico

Abstract

A prognostic vorticity equation, which includes topographic, inertial, lateral and Ekman-type bottom friction and Coriolis terms, is derived from the primitive equations under barotropic conditions. A numerical analog using a double step in time and space is established. Vorticity is predicted from a stream field and then the new vorticity field is relaxed to obtain the new stream field. Initialization of the problem and application of boundary conditions are discussed. The model is applied to the Gulf of Mexico with wind stress, tides, and surface gravity waves neglected. Values of the maximum time step, the bottom and lateral friction coefficients, and the effect of western intensification is determined in a parametric study. A "standard" set of parameters is chosen and used for a 230-day prediction which goes to a quasi-steady state. Part of this standard run is rerun with uniform depth of 1000 m, and topography is shown to be the primary cause of the form of the circulation. Variation of the western intensification in the input is observed to cause the seasonal break-off of an anticyclonic eddy, and a season run of 93 days with variation of input only is made that produces the variation from the circulation patterns of late summer to early winter. This season run agrees qualitatively with observed seasonal patterns in the Gulf of Mexico. The conclusion is that inflow conditions and topography are the primary circulation control.