Finite-time compressibility as an agent of frequent spontaneous patch formation in the surface layer: A case study for the Gulf of Finland, the Baltic Sea

•Spontaneous increase in surface concentrations systematically analysed.•Joint impact of convergence and Lagrangian transport quantified.•Nine areas likely hosting intense patch generation identified in the Gulf of Finland.•Areas with likely spontaneous patch formation located in areas of frequent downwelling.

We explore the possibilities for spontaneous formation of surface patches with high concentrations of contaminants through time correlations of the convergence field and the Lagrangian transport. The test area is the Gulf of Finland, the Baltic Sea, where surface velocity fields show extensive convergence. The flow properties are extracted from 3D velocity fields simulated for 1987-1991 using the OAAS model with a resolution of 1 mile. The focus is on the spatial distribution of the areas in which the values of finite-time flow compressibility of surface velocity fields exceed the threshold for clustering of floats. The distribution of such areas is asymmetric, with likely areas of patch formation located predominantly in the southern and eastern regions of the gulf. Out of nine areas of likely patch formation, six are located along the coast in regions of frequent downwelling, while three are identified in the central region of the gulf.