A 3-dimensional planetary geostrophic (PG) ocean general circulation model in spherical coordinates is formulated to examine the thermocline structure and thermohaline circulation of an idealized ocean basin. The model equations consist of full prognostic temperature and salinity equations and diagnostic momentum equations. A simple linear friction is used to close the barotropic circulation at the western boundary. An extensive sensitivity study is conducted with different model parameters and processes. The results are also compared with those obtained using the Bryan-Cox primitive equation model. For the steady state case, the PG model can reproduce the primitive equation model results, and displays a similar sensitivity for a variety of model parameters, but with much lower computational cost. With higher vertical diffusivity and lower horizontal resolution than primitive equation models, the PG model simulates comparable currents and thermocline depth. This difference is attributed to the large horizontal eddy viscosity used in primitive equation models, but absent in the PG model formulation. The model also illustrates the crucial role of convective overturning in providing a source of cold, dense water at depth. Implications of these results to 2-dimensional zonally averaged models are discussed. In particular, we show that parameterizations used in zonally averaged models to relate the east-west pressure difference to the north-south pressure gradient are not valid when convective overturning is turned off. Finally, the model can be used to efficiently investigate time-dependent transient problems of interest in climate studies, such as the effect of seasonally varying surface forcings, without the need to use asynchronous time-stepping techniques.