A REDUCED-GRAVITY, PRIMITIVE EQUATION, ISOPYCNIC OCEAN GCM - FORMULATION AND SIMULATIONS

A reduced gravity, primitive equation, ocean GCM with an isopycnal ver tical coordinate is developed. A ''buffer'' layer is introduced to all ow the mixed layer to detrain mass at arbitrary densities without the coordinate drift or the heat loss suffered by other isopycnal models. The diapycnal velocity is derived from the thermodynamic equation. Neg ative layers are removed by a heat- and mass-conserving convective adj ustment scheme. The model formulation on a beta plane employs an A gri d and allows irregular coastlines and local grid stretching. Simulatio ns with climatological winds and surface heat fluxes based on observed sea surface temperatures. (SSTs) are presented for the Atlantic, the Pacific, and the Indian Oceans, The surface mixed layer is modeled as a constant depth layer, and salinity effects are neglected in this ver sion. The surface heat flux parameterization used here leads to errors in model SSTs, which are reasonable in the Tropics but are higher in the western boundary current regions. The seasonal dependence of the c urrents compare reasonably well with the available observations and ot her model results, though there are differences in the amplitudes of t he currents, The model thermocline reproduces the observed slopes, tro ughs, and ridges in the Tropics. Neglecting salinity effects and lack of a variable depth mixed layer affect the model simulation of the the rmocline at higher latitudes. The cold tongue in the eastern Pacific i s also affected by the assumption of a constant-depth mixed layer, but the warm pod in the west and the zonal slope of the thermocline corre spond well with the observations. The Gulf Stream and the Kuroshio hav e reasonable current speeds but separate slightly earlier than observe d, in contrast to most models that separate late. Seasonal reversal of the Somali Current in the Indian Ocean and the South Equatorial Curre nt in the Pacific Ocean are reproduced, and the Equatorial Undercurren t is stronger than in most models with comparable grid resolution. Eff orts underway to improve model performance are listed along the way.