The potential of the systematic use of Lagrangian trajectories to identify normally hidden properties and semi-persistent patterns of the dynamics of surface currents is analysed and tested for the Gulf of Finland and the south-western Baltic Sea. These patterns are highlighted using statistical analysis of a large number of Lagrangian trajectories constructed by using horizontal velocity fields in the uppermost layer generated by three-dimensional circulation models. The evaluation of environmental risks and the identification of patterns of rapid Lagrangian transport rely on a specific discretization of the direct problem of the propagation of passive tracers. This discretization contains several time scales and other parameters, the optimum choice of which is analysed based on examples from the Gulf of Finland. It is shown how the dependence of the overall ratio of net and bulk transport can be used for highlighting the nature of the surface dynamics. Semi-persistent patterns of net transport are identified for the Gulf of Finland using the Rossby Centre Ocean Model (RCO) velocity fields and the offline trajectory model TRACMASS for the time period 1987–1991. Rapid transport pathways are mostly aligned along certain coastal segments but pathways across the gulf are evident in transitional months (March-May and August-October). The results are applicable for estimates of the transport of neutrally buoyant substances in the uppermost layer of the sea.
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