Giudici, A., Soomere, T., 2013. Identification of areas of frequent patch formation from velocity fields.

We explore the ability of the formation of patches of substances floating on the sea surface owing to intrinsic features of ocean circulation associated with persistent flow convergence in areas characterised with strong vertical velocities (e.g. hosting downwelling). Their impact on the field of surface floaters can be quantified using so-called flow compressibility of a two-dimensional velocity field. Large values of this measure for idealized, delta-correlated-in-time flows are directly related to the tendency of floating tracers to gather into patches. We employ a modification of this measure, so called finite-time-compressibility to describe the real marine flows in a more consistent way, accounting for the match of areas with high convergence with the Lagrangian transport of the resulting patches. Analysis of this measure for the Gulf of Finland, the Baltic Sea, using surface velocity fields from the OAAS model with a resolution of 1 nm shows that domains where systematic development of patchiness is very likely occur either along straight sections of the coastlines that usually host downwelling. Surprisingly, high values of finite-time compressibility frequently occur throughout the year in two offshore locations and in the windy season near the centre of the widest part of the gulf.