Structure and evolution of the cold intermediate layer in the southeastern part of the Baltic Sea by the field measurement data of 2004–2008

The cold intermediate layer (CIL) was distinguished in the series of vertical CTD profiles obtained in 2004–2008 in the Gdansk basin of the Baltic Sea. The CIL is determined as the layer between the maximum absolute values of the vertical gradient of the water temperature during its decrease in the seasonal thermocline and the growth in the ranges of the permanent halocline. The CIL contains both homogenous and high-gradient sublayers (by various parameters); in particular, a layer of abnormally cold water of the temperature below that of the maximum density might be registered right up to July. Three stages were distinguished in the seasonal cycle of the CIL development: the formation by local mechanisms (March–April), the geo-strophic adjustment within the general thermohaline structure of the sea and slow modification (May–August), and the decay (September–January). The annual maximum thickness of the upper quasi-homogeneous layer, as well as that of the CIL, is by no means owing to the winter vertical mixing but to the conditions during the time of the early spring warming (early March). The complicated structure and character of the temporal variability point to the significant role of advection in the formation and evolution of the Baltic CIL.