The Baltic Sea is brackish. Its deep basins only get fresh oxygen when heavy, salty water from the North Sea flows in through the Danish straits and sinks to the bottom. Large inflows like that are rare, and in December 2014 one of the biggest ever recorded happened. In early December easterly winds lowered the water level of the Baltic. From mid December a series of westerly storms pushed salty Kattegat water through the Great Belt and the Sound, day after day. The salty water spilled over the Darss Sill and the Drogden Sill and sank into the Arkona Basin, where the salinity at the seabed rose from about 16 to 25 grams of salt per kilogram of water. About 4 billion tonnes of salt came in, the third largest inflow since records began in 1880. In the weeks after, the heavy water crept east along the seabed towards Bornholm and the deeper basins beyond. Colour shows the salinity at the seabed, white streaks the surface currents, both from the Copernicus Baltic Sea reanalysis, day by day from 15 November 2014 to 31 January 2015.
Source: Data: E.U. Copernicus Marine Service, Baltic Sea physics reanalysis (BALTICSEA_MULTIYEAR_PHY_003_011), daily at about 2 km. Inflow size: Mohrholz et al. (2015), Leibniz Institute for Baltic Sea Research Warnemünde. Coastline: GLOBE land mask. Visualisation by The Moving Sea
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