  We explore the possibility to identify areas of intense patch formation from
floating items due to systematic convergence of surface velocity fields by
means of a visual comparison of Lagrangian Coherent Structures (LCS) and
estimates of areas prone to patch formation using the concept of Finite-Time
Compressibility (FTC, a generalisation of the notion of time series of
divergence). The LCSs are evaluated using the Finite Time Lyapunov Exponent
(FTLE) method. The test area is the Gulf of Finland (GoF) in the Baltic Sea. A
basin-wide spatial average of backward FTLE is calculated for the GoF for the
first time. This measure of the mixing strength displays a clear seasonal
pattern. The evaluated backward FTLE features are linked with potential patch
formation regions with high FTC levels. It is shown that areas hosting frequent
upwelling or downwelling have consistently stronger than average mixing
intensity. The combination of both methods, FTC and LCS, has the potential of
being a powerful tool to identify the formation of patches of pollution at the
sea surface.
