Model Estimates of Interannual Stratification Variability on Southern Georges Bank, 1985-1992 and 1995

J. J. Bisagni

One-dimensional potential energy models have long been shown to describe the location of tidal mixing fronts, separating well-mixed from stratified waters on mid-latitude continental shelf regions, such as Georges Bank, from spring-fall. The same models can also be used to describe adjustments of the fronts and changes in density stratification to time-dependent variability of surface heating, tidal stirring and wind mixing over a broad range of time scales. In this study, input time series of five-day averaged vertical heat flux, tidal stirring and wind mixing, together with a simple one-dimensional potential energy anomaly (PE) model incorporating the effects of a bottom boundary layer, spatially-varying bottom roughness and spatially-varying but constant tidal mixing efficiencies are used to examine interannual variability of monthly-averaged stratification on southern Georges Bank from 1985-1992 and 1995. Comparison of PE model results with PE estimates from shipboard CTD measurements from winter-summer 1995 show good agreement. Model results for 1985-1992 show no interannual variability for water depths of less-than 60 m where the water column remains well-mixed throughout each year, in agreement with earlier work. Interannual stratification variability for water depths of 60-80 m (80-100 m) is greatest during August (September) with reduced variability for other months from April-October (April-November). However, despite mean density stratification being low during the critical spring spawning/hatching/larval period of April-June for cod and haddock on southern Georges Bank, the coefficient of variation of interannual stratification variability is equal to ~1 during May-June (April-May) for water depths of 60-80 m (80-100 m), indicating a high degree of variability relative to the 1985-1992 mean stratification in the region. The effect of this variability on larval cod and haddock from the region needs to be assessed.