Thermohaline stratification and instabilities at tidal mixing fronts: results of an eddy resolving model for the German Bight

A three-dimensional numerical model for the German Bight which calculates dynamics and hydrography prognostically is presented. Processes influencing development and mesoscale variability of thermal stratification are evaluated by means of model calculations. Systematic case studies for different wind conditions have been carried out and part of the mesoscale variability in development of thermohaline stratification could be explained in terms of prevailing wind conditions. Model calculations show strong tidal stirring during southwesterly winds, whereby thermocline development in most of the German Bight is suppressed. For moderate winds from westerly or easterly directions, haline stratification develops through wind-induced differential advection. The water column is gravitationally stabilized and, thus, vertical exchange is reduced. Therefore, in the area of an existing halocline, heat input is limited to the surface layer and thermocline development occurs. Characteristic extensions of thermohaline stratified areas, depending on wind direction, are identified. For westerly winds, thermohaline stratification occurs in the region of the Old Elbe Valley. During easterly wind forcing, parts of the shallower East and North Frisian Shelves are stratified in temperature and salinity, additionally. Underlying processes responsible are discussed with the help of examples of horizontal distributions of production and dissipation terms of eddy kinetic energy. Baroclinic instability was identified as one process responsible for development of thermohaline stratification in the German Bight.