The influence of diabatic modifications of density at the sea surface on the steady state circulation in ocean gyres is explored. A potential vorticity equation for gyre-scale, forced circulations in the presence of a density source is derived. A scaling of the problem implies that the diabatic changes in density produce a forced circulation of the same order of magnitude as those induced by the wind. This conclusion depends to some extent upon the relative depth of penetration for the two forcing terms. Since the model itself cannot predict these penetration depths, the depth of the observed thermoclines in the oceans is chosen for both. The importance of this assumption is discussed. Although the equations for the continuously stratified case are derived the actual problem solved is a layered equivalent to the more general case. Three patterns of forcing are considered, based on an investigation of the observed distribution of density flux as a function of latitude in the ocean. The typical distribution of the density flux divergence opposes the Sverdrup response to the wind in the southern portions of subtropical gyres. In the regions in which the density flux divergence is positive the forced response is anticyclonic. Therefore in the subtropical gyres the northern portions have stronger circulation in the situation in which there is both wind and density flux. A pair of two gyre models suggest that the diabatic forcing will cause an expansion of the subtropical gyre across the wind stress maximum. The northern gyre is strengthened (weakened) in the case where the surface fluid is lightened (made more dense). This would suggest that there may be a difference between the subpolar gyres in the Atlantic and Pacific caused by the difference E-P and heat budgets over the two basins. The models suggest that subtropical fronts may arise in part because of the change of sign in the density forcing at mid-latitudes. Finally, the gyres computed from the model are shown to be unstable to a long-term thermohaline mode whose structure may in part be determined by the potential vorticity field predicted in the present calculations.