This paper uses the results from two multiseason numerical model simulations of Lake Michigan hydrodynamics to examine the relative effects of wind stress curl, topography, and stratification on large-scale circulation. The multiseason simulations provide a period long enough to encompass the full range of atmospheric and thermal conditions that can occur in the lake. The purpose of this paper is to diagnose the relative importance of various mechanisms responsible for the large-scale circulation patterns by analyzing the vorticity balance in the lake on a monthly timescale. Five different model scenarios are used to isolate the predominant mechanisms: (1) baroclinic lake, spatially variable wind stress; (2) barotropic lake, spatially variable wind stress; (3) baroclinic lake, spatially uniform wind stress; (4) barotropic lake, spatially uniform wind stress; and (5) barotropic lake, linearized equations, spatially uniform wind stress. By comparing the results of these five model scenarios it is shown that the cyclonic wind stress curl in the winter and the effect of baroclinicity in the summer are primarily responsible for the predominantly cyclonic flow in the lake. Topographic effects are also important but are not as significant as wind stress curl and baroclinic effects. Nonlinear effects are much smaller.