3. The Main Hydrographic Characteristics of the Baltic Sea

Strong inputs of river water, the topographic succession of sills and basins, and the narrow entrance to the North Sea are the main features that determine the hydrography of the Baltic Sea. An estuarine circulation is established which transports low-salinity water towards the North Sea. This surface current consists of river water that is modified by mixing with the high-salinity deeper water layers originating from the North Sea. Consequently, a permanent halocline is formed in the Baltic Proper at a depth of ~60 m. In addition to preventing full mixing of the water column, the presence of the halocline widely inhibits the transport of oxygen into deeper water layers. A thermal stratification of the water above the halocline develops during spring and summer such that a warm surface layer forms at depths above 20–30 m. In autumn and winter, the cooling of the surface water and the increasing winds cause erosion of the thermocline such that full mixing of the water column down to the permanent halocline is finally restored. The below-halocline water of the deep basins in the central Baltic Sea may be subjected to stagnation for many years. Water renewal by lateral inputs of high-salinity, oxygenated water occurs irregularly, at intervals of up to 10 years, and only under specific meteorological conditions.