Stratification and wind mixing in the Southern Kattegat

Abstract

Abstract Changes in salt stratification were investigated by using temperature and fluorescence measurements as tracers. By doing this, information on the short term history of the water column was revealed. Further the dominant processes in mixing and stratification of the upper metres of the water column in the southern Kattegat were identified. Due to the salt content, density increased with depth over most of the water column during the period of measurement., A 50 km intrusion of warm, phytoplankton-rich water from the Great Belt outlet was identified. The density increase was maintained due to the buoyancy input given by the combined effect of a horizontal density gradient and vertical shear. Hence the vertical temperature inhomogeneities lasted for 3–7 days and were not mixed even under an event of strong wing mixing. In this period the nutrient and salt transport are uncoupled, since most of the mixing energy is used to redistribute salt within the nutrient-poor surface layer. A reduction of the horizontal density gradient reduced the buoyancy input and allowed for the homogenization of the upper 15 metres within a few hours. One sharp interface was created. After the formation of the sharp interface salt and nutrient transport can be calculated by using the same entrainment function for both salt and nutrient transport. Only during approximately 5 % of the time there was a direct entrainment of bottom water and only 30% of the available mixing energy could be used to give this entrainment.