Turbulence and Mixing in a Rotating Shallow Bottom Gravity Current

The Baltic Sea consists of a system of shallow brackish basins, connected to the salty waters of the North Sea by a system of narrow straights. The strong lateral density gradient across the connecting region intermittently triggers bottom gravity currents that may travel over hundreds of kilometers along the bottom topography towards the deep central basins of the Baltic Sea. This configuration, and the relatively easy accessibility with oceanic instrumentation, makes the Baltic Sea an ideal natural laboratory for the study of rotationally influenced bottom gravity currents. Here, we discuss recent data from an extensive field campaign that took place in spring 2015, when one of the largest inflows of dense bottom waters ever recorded arrived in the central parts of the Baltic Sea. Measurements from this exceptional event include densely-spaced simultaneous turbulence microstructure and velocity (ADCP) transects across the gravity current as well as high-resolution moorings positioned along its pathway. These data reveal a vigorously turbulent dense bottom layer of 10-20 m thickness that travels, strongly affected by rotation, along the lateral slopes of the basin, and merges with its lower flank into a pool of dense bottom water. We use these data to analyze the mixing parameters and dynamics of this flow, compare them to the large-scale oceanic overflows, and briefly discuss the bio-geochemical implications of the oxic waters transported and mixed by the gravity current into the otherwise sulfidic deep layers of the central Baltic Sea.