Modes of Stratification in a Semi-enclosed Bay at the North Sea–Baltic Sea Transition

Conductivity, temperature and depth (CTD) measurements were recorded weekly at a fixed position in the shallow water of Århus Bay over a period of 1 year. Variations in stratification and salinity are described in this study. Outflow of low salinity water from the Baltic Sea, and an estuarine return flow of high salinity bottom waters originating in the North Sea were observed. Strong stratification occurs during spring, summer and late autumn, whereas stratification is weak or absent during winter. A stratification model in terms of the potential energy of the water column (φ) has been applied including wind, solar insolation, estuarine circulation and current terms. Observations of stratification are very well described by this model, although considerable differences were found to occur during a period of very strong stratification. The interface is placed at a water depth of about 8 m during spring and early summer, and at a depth of about 12 m during autumn. A comparison of frequently used stratification parameters shows a highly positive correlation between φand the density difference between surface and bottom water. A positive correlation betweenφand Brunt–Väisälä frequency was also found in periods of stable oceanographic and meteorological conditions, whereas no correlation was found in periods of unstable conditions