Turbulent oxygen transport across the halocline of the central Baltic Sea: Identification of key physical processes

Anoxia in the central Baltic Sea is caused by the disproportion of the oxygen demand below the Baltic Sea halocline and the capability to transport sufficient amounts of oxygen from the well oxygenated upper water column through the halocline into the deeper Baltic Sea. Despite the fact of anoxia below the halocline, there is a growing evidence for a considerable oxygen transport through the halocline by turbulent mixing at the basin boundaries, i.e. the location where the halocline gets in the vicinity of the seafloor. We used velocity data from moorings and ship based velocity shear microstructure measurements using a MSS profiler. The data was acquired during three different cruises/seasons in the Eastern Gotland Basin to identify key processes responsible for oxygen mixing events across the strong halocline. The MSS was equipped with a fast oxygen sensor allowing to quantify the vertical oxygen flux. We focused on specific events with inertial waves, mean currents, and topographic waves as major dominating processes. During these events properties such as vertical shear, stratification and oxygen fluxes were analysed. With this information we were able to estimate the potential of the processes for the diapycnal oxygen transport. We found that inertial waves do not contribute much to the overall oxygen flux across the halocline, whereas topographic waves increase the oxygen flux considerably. Also the mean current lead to significant oxygen fluxes under certain shear and stratification conditions, suggesting that further attention should be turned to those.