The isopycnal region is treated in a quasi-isopycnal fashion, in which layers do not vanish at outcrops, but retain a thin minimum thickness at all grid points. This is similar to the "massless" layer treatment of Bleck and Boudra (J. Phys. Ocean., 1981) , in contrast to the disappearing layer treatment of Oberhuber (J. Phys. Ocean., 1993). The isopycnal layering provides for far better control of diapycnal mixing and obviates the need for expensive tensorial treatment of diffusive processes in the ocean. The treatment of horizontal mixing within the model is implemented with high order Shapiro filtering which is reduced to a flux form. The mixed layer is treated with a bulk turbulence scheme derived from Kraus-Turner (Tellus, 1967) and Niiler-Kraus (The Sea, 1977) and as implemented in time-integral fashion in Schopf and Cane (J. Phys. Ocean., 1983).

An orthogonal curvilinear coordinate is used to provide global coverage without singularities at the North Pole, and to provide coordinate stretching near the equator. As in the vertical, the equations are derived and implemented in a completely general form. Regular and stretched latitude-longitude grid, and a global grid with the North Atlantic and Arctic treated with a rotated latitude-longitude grid have been implemented.

The Poseidon model employs the "natural" surface boundary conditions (Huang, J. Phys. Ocean., 1993) in which fluxes of heat, salt and mass are specified, as opposed to fluxes of heat and salinity only. In general, the flux of salt is zero, and salinity is changed through the influence of freshwater mass fluxes associated with evaporation and precipitation.

The current version of the OGCM uses a reduced-gravity treatment for the pressure field. For tropical basin simulations, reduced gravity models have been shown to do a highly effective job in simulating both the mean and variability of the upper ocean state (e.g., Schopf and Cane, J. Phys.Ocean., 1983; Gent and Cane, J. Comput. Phys., 1989; Zebiak and Cane, Mon. Wea. Rev., 1987; Murtugudde et al., Mon. Wea. Rev., 1995).

The OGCM was originally designed for global, long-term integrations in a coupled ocean-atmosphere GCM. The initial coupled results were conducted with a 7-layer reduced-gravity version in which a T-S relationship was assumed, so density was computed as a function of temperature only. The model has since undergone tuning and performance analysis for detailed simulation of the tropical pacific. Salinity (and its effect on the equation of state) has been added, and examination of the model sensitivity to vertical mixing has been explored in a 14-layer version of the model (Yu et al., J. Phys. Ocean., 1997; Yu and Schopf, J. Phys. Ocean., 1997). These changes have a large impact on the simulation of the equatorial undercurrent and the surface heat balance. A further set of experiments is underway in which the model is driven with the full FSU wind products over the period from 1980-1995. The resultant simulation of the equatorial undercurrent on the equator at 140°W is shown, along with the long term mean from the TOGA current meter mooring at that site.