Salinity

The salinity of sea water, given on the practical, dimensionless scale [PSU], is calculated using the 3D CEMBS ecosystem model of the Baltic Sea. The SO SatBałtyk service gives the salinity distributions at the surface and at different depths in the form of maps with a horizontal resolution of 1 km.

The methodology of determining salinity

Salinity is calculated using the 3D CEMBS ecohydrodynamic prognostic model. The boundary conditions at the sea surface are the atmospheric data from the UM prognostic model (ICM University of Warsaw). The model has an open boundary with the North Sea so that changes resulting from inflows of water from that sea can be mapped more accurately. The 3D CEMBS model is equipped with a river inflow module, which supplies information on the amount of fresh water entering the Baltic from 72 rivers as well as the quantities of nutrients it carries. KPP parameterization is applied in order to map vertical mixing as accurately as possible. The model also has a module for assimilating satellite data from the MODIS sensor on the Aqua (EOS PM) satellite. The model results have a horizontal resolution of ca 2 km. In the vertical the model is divided into 21 layers. Each of the first four layers is 5 m thick. The thickness of the other 16 layers increases with depth. The results supplied to the SO SatBałtyk system are interpolated on a grid of 1 km resolution. More information can be found in the article: Activation of the operational ecohydrodynamic model (3D CEMBS) – the hydrodynamic part (Dzierzbicka-Głowacka et al. 2013).

Validation (assessment of accuracy)

The accuracy of the model results has been estimated by comparing them with in situ measurements from buoys and during cruises of R/V Oceania (see the adjoining Figure). The statistical error of salinity in the surface layer of the sea calculated using the 3D CEMBS model is ca 0.26 PSU, whereas the systematic error varies from -0.32 to 0.33 PSU, depending on the measurement station. When full-depth profiles are compared with measurements made during cruises of R/V Oceania (see the adjoining Table), these errors are 0.95 PSU and 0.99 PSU respectively. For particular cruises, the correlation coefficient varies from 0.86 to 0.96. More information can be gleaned from the article mentioned in the previous paragraph.

Interesting phenomena relating to salinity

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.