Salinity is a magnitude relating to the quantities of salts dissolved in sea water. These salts include a range of inorganic chemical compounds made up of elements like sodium, potassium, magnesium, bromine, sulphur and carbon, among many others. Like other magnitudes, salinity can be determined in various ways. In the SatBałtyk System, this parameter is calculated on the basis of the 3D CEMBS Baltic Sea ecosystem model and the PM3D hydrodynamic model. The descriptions of the various products will detail the relevant methods and their accuracy.
Inflows of very highly saline water from the North Sea
Under normal circumstances, Baltic Sea waters are stagnant. Inflows from the North Sea are infrequent: significant ones occur very suddenly and at irregular intervals. The exchange of waters between the Baltic and the North Sea at the scale of days and weeks is mainly barotropic, caused by the difference in sea levels. Westerly winds raise the sea level in the Kattegat and lower it in the western Baltic, causing water to flow into the Baltic. The short-term characteristics of water exchange are very variable as regards both salinity and the amount of water transported. Inflows are dependent on the wind field, the atmospheric pressure over the North Atlantic and the amount of water in the Baltic. Inflowing waters maintain the vertical stratification and the salinity decreasing from the Danish Straits towards the Baltic Proper. Oceanic waters are best detectable in the deep parts of the western and southern Baltic, where even small inflows can be registered. The greater the distance from the Danish Straits, the weaker the effect of inflows; only large inflows reach the central Baltic. Such large inflows usually take place every 4-5 years and spread through the Baltic in the layer beneath the halocline. Medium-sized inflows occur several times more often; they are especially important in autumn and winter as their waters are usually better oxygenated than Baltic water and are dense enough to flow below the halocline into the Baltic Proper, where they counteract anoxia.
The spread of river waters
Plumes of low salinity due to the spread of fresh waters from the mouths of large rivers into much more saline sea waters are clearly visible on salinity maps. The shapes and extents of such freshened water plumes depend on the volume of river water entering the sea and on the speed and direction of surface sea currents, which depend largely on the speed and direction of the wind. The figure exemplifies the spread of fresh waters from the River Vistula in the Gulf of Gdańsk.