Study of formation process of cold intermediate layer based on reanalysis of Black Sea hydrophysical fields for 1971–1993

A reanalysis of hydrophysical fields for 1971–1993 is used to study the formation mechanisms of the cold intermediate layer (CIL): the advective mechanism (associated with the advection of cold waters formed in the northwestern shelf (NWS)) and the convective mechanism (caused by wintertime convection inside cyclonic gyres in the central part of the sea). We consider the periods of alternating atmospheric conditions: the mild winter of 1980–1981, normal winter of 1987–1988, and cold winter of 1992–1993. Interannual features of replenishment and renewal of “old” CIL waters caused by these mechanisms are identified. In particular, cooled shelf waters sink along the continental slope and merge with “old” CIL waters during the mild winter of 1980–1981 more than 1 month later than during the cold winter 1992–1993 and more than 3 weeks later than during the normal winter of 1987–1988. The Sevastopol anticyclonic gyre and the northwest branch of the Black Sea Rim Current markedly influence the transformation of entrained cold NWS waters transported to the southwest and the central part of the water area. The local formation process of cold intermediate waters is found to be caused by the wintertime penetrating convection over domelike isosurfaces of temperature and salinity arising due to rising constant halocline (pycnocline) at the centers of cyclonic gyres because of the intensification of the wintertime circulation. Anomalously cold surface water, characterized by increased density, gradually sinks. An analysis of TS indices indicates that the transformed cold water spreads out over isopycnic surfaces with time, being entrained in cyclonic circulation and spreading throughout the sea, thus renewing “old” CIL waters.