Abstract
Abstract. We present and analyze high-resolution observational data of thermohaline structure and currents acquired in the Gulf of Finland (Baltic Sea), using an autonomous buoy profiler and bottom-mounted acoustic Doppler current profiler during July–August 2009. Vertical profiles of temperature and salinity were measured in the upper 50-m layer with a 3 h time resolution, and vertical profiles of current velocity and direction were recorded with a 10 min time resolution. Although large temporal variations of 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 persisted for 4–15 days and were dominated by certain physical processes: upwelling, relaxation of upwelling, estuarine circulation and its wind-induced reversal, and downwelling. Vertical profiles of current velocities supported the concept of synoptic-scale, quasi-stationary periods of hydrophysical fields, characterized by distinct layered flow structures and current oscillations. To estimate the contribution of different processes to the changes in stratification, a simple conceptual model was developed. The model accounts for heat flux through the sea surface, wind mixing, wind-induced transport (parallel to the horizontal salinity gradient) in the upper layer, and estuarine circulation. It reproduced observed changes in vertical stratification reasonably well. The largest discrepancies between observations and model results were found when water motions across the Gulf and associated vertical displacements of isopycnals (upwelling or downwelling) were dominant processes.
Highlights
Vertical stratification of the water column in oceans and seas is a key factor shaping the distribution and transport of substances
The water column in deeper areas of the Gulf consists of three layers: upper mixed layer (UML), cold intermediate layer (CIL), and the near-bottom layer, which is saltier and slightly warmer than the CIL
Vertical thermohaline structure measured by the buoy profiler in the upper 50-m layer was quantitatively described by estimating UML depth, base of the thermocline (BT), thickness of the thermocline, and depth of the strongest density gradient
Summary
Vertical stratification of the water column in oceans and seas is a key factor shaping the distribution and transport of substances. The Gulf of Finland, a 400-km-long and 48–135km-wide sub-basin of the Baltic Sea, is dominated by fresh water discharge at its eastern end and free water exchange with the Baltic Proper through its western border (Alenius et al, 1998) This creates both horizontal and vertical gradients of salinity. The water column in deeper areas of the Gulf consists of three layers: upper mixed layer (UML), cold intermediate layer (CIL), and the near-bottom layer, which is saltier and slightly warmer than the CIL These water layers are separated by pycnoclines: the thermocline at 10–20-m depths and the permanent halocline at 60–70-m depths. Based on analysis of vertical profiles of temperature and salinity collected in the Gulf from 1987–2008, long-term average parameters of vertical thermohaline structure in summer (June–August) have been estimated (Liblik and Lips, 2011). The main aim is to define distinct stratification patterns in the water layer from the sea surface to a depth of 40–50 m, including the seasonal thermocline, and to explain under what conditions these patterns occur
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