On the possibility of spontaneous patch formation in the Gulf of Finland

We address a novel option to identify areas of spontaneous patch formation using the concept of finite-time compressibility (FTC) of surface velocity fields. This concept makes it possible to systematically account for the correlations of convergence of surface velocity fields and the underlying Lagrangian transport in the surface layer. Areas with high levels of FTC naturally arise if a transient localized convergence area (e.g. downwelling) moves synchronously with the surface current. We evaluate the FTC levels in the Gulf of Finland by means of tracking changes to the geometry of a large set of triplets of passively advected water parcels by surface currents extracted from 3D simulations for the period 1987-1991 using the OAAS model with a spatial resolution of 1 nautical mile. The focus is on seasonal variations of areas in which the FTC regularly exceeds the threshold for clusterization of surface floats in ideal Kraichnan flows. Six such areas are located along the coast and roughly coincide with frequent downwelling areas whereas three are located in the central region of the gulf. The areas near the southern coast of the gulf and at the entrance to the Neva Bight are present all year round whereas other areas only emerge during certain seasons.