Investigating processes of stratification and de-stratification in a high resolution model of the German Bight.

Insight into processes contributing to the onset and breakdown of stratificationenhances our understanding of the ecosystem dynamics of shallow seas. In theGerman Bight, a region of freshwater influence (ROFI), stratification is mainlyinfluenced by tidal mixing, fresh water river inflow and wind forcing. The physicalregime of the German Bight in the ROFI region switches between stably stratified tosemi-diurnal stratification. In order to investigate the processes contributing to thisvariability, a three dimensional high resolution numerical model is setup basedon the General Estuarine Transport Model (GETM). The model has a horizontalresolution of 300 m in the inner German Bight and incorporates 30 vertically adaptivelayers. The model results are first compared against observations, showing that the model is ableto reproduce the observed physical regime. The model results are then used to calculate theterms in a dynamic equation for the potential energy anomaly (φ). These termsinclude the horizontal advection, depth-mean straining, non-mean straining, verticaladvection and vertical mixing. The contribution of these processes to stratification isassessed under different conditions in the ROFI region. The results show that inthe semi-diurnal stratification condition, the depth-mean straining and horizontaladvection terms are the most important processes. Both of these processes are in phaseleading to stratified conditions during low water and mixed conditions during highwater. On the other hand, in the stably stratified condition, vertical advection is asimportant as depth-mean straining and horizontal advection. In this condition, thedepth-mean straining and horizontal advection are out of phase and therefore oppose eachother, while the depth-mean straining and vertical advection are in phase. Thisresults in minimum stratification (minimum φ) occurring at maximum ebb andmaximum stratification (maximum φ) occurring at slack after ebb (low water). [ABSTRACT FROM AUTHOR]