Short-term variations of thermohaline structure in the Gulf of Finland

We present and analyze high-frequency observational data of thermohaline structure and currents acquired in the Gulf of Finland (Baltic Sea) using an autonomous buoy profiler and a bottom mounted acoustic Doppler current profiler in July–August 2009. Vertical profiles of temperature and salinity were measured in the upper 50-m layer with a time resolution of 3 h and vertical profiles of current velocity and direction were recorded with a time resolution of 10 min. Although high temporal variations of the vertical temperature and salinity distributions were revealed, it was possible to define several periods with quasi-stationary vertical thermohaline structure. These quasi-stationary stratification patterns lasted from 4 to 15 days and were dominated by certain hydrophysical processes – upwelling, relaxation of the upwelling, wind induced reversal of the estuarine circulation, estuarine circulation, and downwelling. Vertical profiles of current velocities supported the concept of synoptic-scale quasi-stationary periods of hydrophysical fields. The periods with distinct layered flow structures and current oscillations with the prevailing period of 26 h were revealed. A simple model, where the heat flux through the sea surface, wind mixing, wind induced transport (parallel to the horizontal salinity gradient) in the upper layer and estuarine circulation were taken into account, simulated the observed changes in the vertical stratification reasonably well. The largest discrepancies between the observations and model results were found when water movement across the Gulf and associated vertical displacement of isopycnals (upwelling or downwelling) were dominant processes.