The Clyde Sea: a Model of the Seasonal Cycle of Stratification and Mixing

Abstract

Mooring observations in the fjordic system of the Clyde Sea show complete vertical mixing in the deep waters during late November 1990 as a result of cooling and wind stirring. ADCP observations before and during mixing indicate that the breakdown of stratification drastically modified the circulation and largely removed the vertical shear associated with the density driven flow. There are no previous documented cases of complete mixing though the available records are limited. A compilation of the historical data indicates that the Clyde system is usually stratified by a combination of thermal and freshwater buoyancy inputs with most pronounced stratification in the summer months when Δσ l may exceed 1·5 kg m -3. Winter stratification is generally weaker with temperature inversions occurring during the cooling phase. The processes controlling stratification are represented in a filling box model of the Clyde system in which the exchange flow with the North Channel is related to the surface density difference across the sill. The stratifying effect of buoyancy inputs as heat and freshwater is opposed by mechanical stirring due to (i) wind-stress and (ii) the barotropic tide. Additional stirring contributions from (iii) convective effects associated with deep-water replacement and (iv) the internal wave mechanism proposed by Stigebrandt (1976) may also contribute. Model runs using only mechanisms (i) and (ii) did not exhibit the rise in bottom temperature observed during the summer season and, as a consequence, over-estimated density stratification. Additional inputs from (iii) and, in particular (iv), using best estimates of the efficiency factors, produced a more realistic balance and gave a reasonable first order account of the seasonal cycle of density stratification. It also provides a fair simulation of the seasonal evolution of temperature and salinity including the temperature inversions observed in the November and December. The results point to a fine balance between the annual mean inputs of buoyancy and stirring in the Clyde system thus allowing the possibility of annual winter mixing.