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Seasonal cascades down the coastal slopes and intra-layer convection are considered as the two additional mechanisms contributing to the Baltic Sea cold intermediate layer (CIL) formation along with conventional seasonal vertical mixing. Field measurements are presented, reporting for the first time the possibility of denser water formation and cascading from the Baltic Sea underwater slopes, which take place under fall and winter cooling conditions and deliver waters into intermediate layer of salinity stratified deep-sea area. The presence in spring within the CIL of water with temperature below that of maximum density (Tmd) and that at the local surface in winter time allows tracing its formation: it is argued that the source of the coldest waters of the Baltic CIL is early spring (March-April) cascading, arising due to heating of water before reaching the Tmd. Fast increase of the open water heat content during further spring heating indicates that horizontal exchange rather than direct solar heating is responsible for that. When the surface is covered with water, heated above the Tmd, the conditions within the CIL become favorable for intralayer convection due to the presence of waters of Tmd in intermediate layer, which can explain its well-known features-the observed increase of its salinity and deepening with time.

Abstract. Seasonal cascades down the coastal slopes and intra-layer convection are considered as the two additional mechanisms contributing to the Baltic Sea cold intermediate layer (CIL) formation along with conventional seasonal vertical mixing. Field measurements are presented, reporting for the first time the possibility of denser water formation and cascading from the Baltic Sea underwater slopes, which take place under fall and winter cooling conditions and deliver waters into intermediate layer of salinity stratified deep-sea area. The presence in spring within the CIL of water with temperature below that of maximum density (Tmd) and that at the local surface in winter time allows tracing its formation: it is argued that the source of the coldest waters of the Baltic CIL is early spring (March-April) cascading, arising due to heating of water before reaching the Tmd. Fast increase of the open water heat content during further spring heating indicates that horizontal exchange rather than direct solar heating is responsible for that. When the surface is covered with water, heated above the Tmd, the conditions within the CIL become favorable for intralayer convection due to the presence of waters of Tmd in intermediate layer, which can explain its well-known features -the observed increase of its salinity and deepening with time.