Spring cold water intrusions as the beginningof the cold intermediate layer formation in the Baltic sea

Abstract

Thermohaline intrusions are a typical feature of the Baltic Sea water body. We report observations of vivid colder/saltier intrusion activity in intermediate layers and upper pycnocline of the Gdansk Bay (the south-eastern part of the Baltic Sea) in early spring (March–April, 2013). Extremely low water temperature (down to 1.4–2 °C) and specific water salinity (SÃ7.5–7.8) of the intrusions point at a possible area of the intrusion waters formation: within the upper mixed layer in the Arkona/Bornholm basins at the end of March. Data on water temperature and salinity available from the International Council for the Exploration of the Sea (ICES) data repository, data of the Arkona Becken automated station, meteorological information, and remote sensing data for March–April 2013 are used to confirm the conclusions. It is confirmed once again that the transfer from the two-layered winter water stratification to the three-layered summer one in the Baltic Sea is a consequence of the sea-scale exchange process. The latter is manifested as simultaneous advection in upper and intermediate layers of waters with lower and higher salinity, correspondingly. The reported extremely cold intrusions indicate the beginning of the formation of the Cold Intermediate Layer (CIL), which thus contains waters of the Arkona/Bornholm basins. The main environmental factors driving the observed vivid intrusions in April 2013 are easterly winds and negative buoyancy fluxes due to seasonal solar heating of waters with the temperature below that of the density maximum. Substantial prolongation of the period of seasonal vertical mixing might be the most important process related to the transition of water temperature over the temperature of the density maximum (Tmd) in the Baltic Sea. With the climate warming, the cooling of surface waters below the Tmd becomes rare, limiting deep ventilation of Baltic waters in spring.