Resumen de Frontal variability in the German Bight
While regarded for a long time as a completely mixed shelf sea, the German Bight exhibits stable summer stratification with an associated tidal mixing front as well as fronts induced by river discharge. Repeated CTD measurements and data from a current meter mooring over an entire annual cycle are used to describe the evolution and variability of the tidal mixing front in the southern German Bight. The annual cycle of stratification can be devided into three parts, i.e. stable summer stratification with associated front, winter conditions with no front and a period of temporary stratification events in spring with time scales of some days. The observational results are discussed in the light of a potential energy and a numerical shelf sea model and it is shown that the salinity differences induced by river outflow are essential for the early occurrence of temporary as well as for the stability of the longer lasting summer stratification. The important mechanisms are the effect of differential advection which stabilizes the water column and allows for thermal stratification already with small surface heat inputs, and a rearrangement of the salinity pattern which reinforces thermal stratification in summer. This dependence of stratification and front on the presence of salinity gradients contrasts the known, exclusively heat induced tidal mixing fronts around the British Isles which can be characterized by their thermal structure alone. A conceptual model is presented that includes the mentioned salinity effects, and possible consequences of the observed stratification cycle for phytoplankton development are discussed.
