Hans Burchard, Leibniz Institute for Baltic Sea Research Warnemünde, Rostock, Germany (hans.burchard@io-warnemuende.de)  |  http://www.io-warnemuende.de/hans-burchard-en.html

ABSTRACT

Understanding the vertical overturning circulation of the Baltic Sea has been focus of Baltic Sea research during the last century. Still, the quantitative understanding of turbulent mixing processes in closing the circulation is incomplete. In recent years, direct observations of turbulent mixing and improved numerical modelling helped to increse the understanding of these processes. In the transition area between the North Sea and the Baltic Sea entrainment processes dominate the vertical mixing in the inflowing saline bottom water. The hot spots of these processes are located at the Darss Sill and the Bornholm Channel in the western Baltic Sea. In the central Baltic Sea the horizontal advection of saline water in deep layers below the permanent halocline dominate the temporal changes and associated transports. This is accompanied by the turbulent vertical transport through the halocline into the surface layers. During stagnation periods, the residence time of the deep water in the Eastern Gotland Basin increases roughly by a factor of five. Vertical mixing through the halocline is drastically reduced when inflows are lacking, the potential processes of diapycnal mixing are discussed to the present knowledge. The turbulent motion resulting from breaking internal waves is capable of turbulent transports through the halocline corresponding to the estimates of the salt transport into the surface mixed layer. A large scale tracer release experiment, which has been carried out in the central Baltic Sea a few years ago, revealed the leading role of boundary mixing for the overall vertical turbulent transports. Near-bottom currents induced by inflow events likely enhance vertical mixing. The upward vertical transport of nutrients such as phosphate directly depends on these mixing processes and has substantial environmental consequences such as massive summer blooms of cyanobacteria.