Analysis of the structure of currents in the Gulf of Finland using the Okubo-Weiss parameter

We analyze transport properties of surface water flow in the Gulf of Finland, the Baltic Sea, using spatial distributions of the Okubo-Weiss parameter, which reveals the spatial pattern of strain and relative vorticity at a specific time. The calculations are based on surface velocities calculated using the OAAS model with a spatial resolution of 1 nautical mile for 1987 in the framework of BONUS+ BalticWay cooperation. The currents are, on average, strain-dominated, with typical OW parameter values about 0.1. During short instances the OW parameter can abruptly increase by a factor 10. These events are not correlated with wind speed but occur slightly more frequently during the windy autumn season. Substantial areas of strong strain and relative vorticity regularly occur along the coast (due to topographic effects and coastal current fluctuations) and at two offshore areas; (i) in the eastern part near the mouth of the Neva river and (ii) in the north-western part of the gulf. The patterns of strong strain in the western part of the gulf originate from the northern nearshore but at times span down to the southern coast.