THE FREQUENCY AND DURATION OF EPISODES OF COMPLETE VERTICAL MIXING INTHE CLYDE-SEA

A filling box model for the Clyde Sea in which buoyancy input by surfa ce heating and freshwater inflow is opposed by stirring associated wit h surface wind stress, internal tide mixing and convection, has been d riven using observed boundary conditions to simulate stratification in the area, for the period 1985-1990. The results show a well-defined s easonal cycle in stratification and cross-sill exchange. The model ind icates two contrasting regimes: the summer regime during which cross-s ill exchange is low and inflow is not sufficiently dense to sink below sill level, and so deep water conditions are only changed through mix ing; and the winter regime when cross-sill inflow ventilates the whole water column and the deep water is replaced by North Channel water. T he results suggest that at the time of switch-over between the summer and winter regimes, usually in November, the system becomes susceptibl e to an episode of complete vertical mixing as the water column stabil ity is low due to an inversion in the temperature structure. Should th e system fail to mix completely at this time, the high cross-sill infl ow rates result in a reduction in temperature stratification and an in crease in water column stability. Observations of an episode of comple te vertical mixing, made in November 1990, show that in the four-day p eriod when the water column was mixed, deep water temperatures dropped by 1.5 degrees C. The competition between the re-stratifying influenc e of the baroclinic flow and convectively driven mixing at this time i s examined in terms of a simple energetics model. It is demonstrated t hat the persistence of the completely mixed state is largely the resul t of convection caused by intense surface cooling, sustained by the la rge pre-existing heat reservoir in the deep water, which allowed a dai ly mean cooling rate >1 kW m(-2).