A filling box model for the Clyde Sea in which buoyancy input by surface heating and freshwater inflow is opposed by stirring associated with surface wind stress, internal tide mixing and convection, has been driven using observed boundary conditions to simulate stratification in the area, for the period 1985-1990. The results show a well-defined seasonal cycle in stratification and cross-sill exchange. The model indicates two contrasting regimes: the summer regime during which cross-sill exchange is low and inflow is not sufficiently dense to sink below sill level, and so deep water conditions are only changed through mixing; and the winter regime when cross-sill inflow ventilates the whole water column and the deep water is replaced by North Channel water. The results suggest that at the time of switch-over between the summer and winter regimes, usually in November, the system becomes susceptible to an episode of complete vertical mixing as the water column stability is low due to an inversion in the temperature structure. Should the system fail to mix completely at this time, the high cross-sill inflow rates result in a reduction in temperature stratification and an increase in water column stability. Observations of an episode of complete vertical mixing, made in November 1990, show that in the four-day period when the water column was mixed, deep water temperatures dropped by 1.5 degrees C. The competition between the re-stratifying influence of the baroclinic flow and convectively driven mixing at this time is examined in terms of a simple energetics model. It is demonstrated that the persistence of the completely mixed state is largely the result of convection caused by intense surface cooling, sustained by the large pre-existing heat reservoir in the deep water, which allowed a daily mean cooling rate >1 kW m(-2).