A free surface tropical ocean model is presented for explicit prediction of the actual topography of sea surface and other variables, for further intercomparative studies among differently formulated oceanic general circulation models (OGCMs), and data assimilation of the observed sea level. Based on an OGCM without the rigid-lid approximation developed at the Institute of Atmospheric Physics (Beijing, China), the dynamic formulation and numerics of the model have some unique features such as explicit treatment of sea level elevation as a prognostic variable, calculation of the departures of thermodynamic variables in time integration by subtracting the standard stratification, introduction of the &sgr; vertical coordinate, total available energy consideration, and a barotropic-baroclinic mode-splitting algorithm for computational efficiency. The model covers the tropical Pacific Ocean between 30 ¿N and 30 ¿S with horizontal grid spacing of 1¿ in latitude and 2¿ in longitude. There are 14 vertical layers with 8 layers in the top 200 m. Prior to further applications of the model, simulations of the annual mean and seasonal cycle of the tropical Pacific Ocean have been carried out to examine performance of this free surface OGCM forced by climatological atmospheric fluxes. It is shown that with the appropriate treatments of the governing equations and numerical methods, the free surface baroclinic OGCM can be computed at least as efficiently as a rigid-lid model. By eliminating the rigid-lid approximation, keeping explicitly the available surface energy and energy conversion related with divergence of vertically integrated flow, and by subtracting the standard stratification, the model would be more accurate, reasonable, and effective in representing the energy cycle in the real ocean.

The misrepresentation of energy conversion caused by the rigid-lid approximation is shown and estimated on the basis of the simulated data. The simulated results show that the model is able to reproduce not only the observed current system and thermal structure and their variations but also the actual sea level in the tropical Pacific, which shares many observed sea surface features. The preliminary comparison with other existing simulations shows some noticeable differences in the simulated circulation in spite of using the same wind stress, which need to be investigated further. ¿ American Geophysical Union 1992