An ocean circulation model, developed for the study of mesoscale to gyre-scale circulation and heat transport, is described and tested. The model employs density as vertical coordinate except in the immediate vicinity of possible coordinate surface intersections with the ocean surface. Test simulations are carried out for a double-gyre ocean basin of the type used by Holland (1978), but with better resolution in the upper 1000 m of the model ocean. The final state of a 5-year coarse-mesh (50 km resolution on a 2400 km × 1200 km domain) spin-up is used as the initial state for mesoscale experimentation on a 25 km mesh. Basin energetics, as well as mean mass transport streamfunction and mean potential vorticity fields are given for three mesoscale experiments, in which the magnitude and formulation of lateral dissipation are varied. Each of these experiments, run for approximately five years, exhibits vigorous mesoscale activity associated with western boundary current separation and the free jet. ... Abstract An ocean circulation model, developed for the study of mesoscale to gyre-scale circulation and heat transport, is described and tested. The model employs density as vertical coordinate except in the immediate vicinity of possible coordinate surface intersections with the ocean surface. Test simulations are carried out for a double-gyre ocean basin of the type used by Holland (1978), but with better resolution in the upper 1000 m of the model ocean. The final state of a 5-year coarse-mesh (50 km resolution on a 2400 km × 1200 km domain) spin-up is used as the initial state for mesoscale experimentation on a 25 km mesh. Basin energetics, as well as mean mass transport streamfunction and mean potential vorticity fields are given for three mesoscale experiments, in which the magnitude and formulation of lateral dissipation are varied. Each of these experiments, run for approximately five years, exhibits vigorous mesoscale activity associated with western boundary current separation and the free jet. ...