Characteristics and variability of the vertical thermohaline structure in the Gulf of Finland in summer

Vertical profiles of temperature and salinity collected in the summers of 1987-2008 were analyzed in order to describe the mean characteristics and variability of the vertical termohaline structure in the Gulf of Finland.Quantitative estimates of the mean characteristics of the upper mixed layer, seasonal thermocline, cold intermediate layer, halocline and deep layer as well as their along-gulf changes were obtained.Both the long-term (inter-annual) and short term variations in the thermohaline structure were related to the changes in the atmospheric forcing.Two distinct periods with statistically different mean temperature and salinity in the deep layer were detected among the analyzed 22 years.The overall vertical salinity (and density) gradient was much stronger and the halocline was sharper in the recent years than in the years 1987-1990.However, the summer mean vertical salinity and density gradients in the seasonal thermocline did not reveal large inter-annual variations.We suggest that a possible shift towards fresher waters in the Baltic Sea due to the climate change would result in the two-layer structure of water column in the deeper areas of the Gulf of Finland in summer.At the same time, a possible increase of sea surface temperature could lead to a strengthening of the vertical density stratification in the seasonal thermocline.