The importance of the Drogden Sill in the Sound for major Baltic inflows

Saline water from the North Sea, which is able to enter the deep water of the central parts of the Baltic Sea, must unavoidably pass two shallow sill areas: the Darss Sill in the Belt Sea area and the Drogden Sill in the Sound. Both sills restrict considerably the inflow of highly saline and oxygen-rich water known as major Baltic inflows. Long-term observations of salinity and current made at light vessels Gedser Rev and Drogden in the sill areas are used to evaluate the role of the Drogden Sill in major inflow of events. The different contributions of the two passages are documented on the basis of 90 events that took place between 1897 and 1976. Based on the volume and salt transports across the two sills the intensity of major inflows is re-assessed. The significance of each sill varies considerably from event to event. The volume crossing the Drogden Sill during major events is, on average, one third of that crossing the Darss Sill and usually far more salt enters the Baltic across the latter. However, in some cases, the amounts of salt transported across the sills are equal (e.g. January 1993) or the amount crossing the Drogden Sill is even the larger fraction (e.g. January 1925, January 1908). The re-assessed intensity of the 1993 event showed that the inflow in January 1993 must be characterized as very strong owing to the considerable salt transport across the Drogden Sill.