Direct observations of a medium-intensity inflow into the Baltic Sea

In January/February 2004 a ship cruise into the Arkona Sea (Western Baltic) happened to start just prior to a medium-size inflow event and to last until a few days after the inflow. The bottom layer of saline inflow water in the southern Sound has a current speed of up to 1 m/s. The current bifurcates at Kriegers Shoal, where only the smaller branch follows the contour lines of the bathymetry at the western rim of the basin, while the main flow proceeds on the steepest path north of Kriegers Shoal. The pycnocline tilt due to geostrophy matches with current observations in both branches. Old bottom water in the basin is lifted up by the denser inflow and pushed forward to Bornholm Gate. Inflow from the Darss Sill at first consists of low-salinity Baltic surface water and a small amount of higher salinity water at the bottom. After a few days the water over the sill is mixed and moves east, establishing a strong subsurface current at the southern rim of the Arkona Basin. Neither the inflow through the Sound nor the water masses from the Darss Sill reached the Bornholm Gate in the east of the Arkona Sea before the end of the survey. The saline outflow observed there consists of bottom water that had been present in the Arkona Sea prior to the new inflow. Overflow water spreading on the sea bottom is modified by entrainment from the low salinity water body above. Best guesses of the entrainment rate, somewhat tentative due to salinity variations at the entrance, are in line with formulae from the literature.