On the finite-time compressibility of the surface currents in the Gulf of Finland, the Baltic Sea

Abstract

We study the effects of flow compressibility of a two-dimensional (2D) velocity field on the transport and mixing of substances floating on the sea surface overlying 3D circulation. The test area is the Gulf of Finland, the Baltic Sea, where large variations in flow compressibility are likely. The key development is the introduction of a modified measure of finite-time compressibility that accounts for time correlations of realistic flows and is directly related to the ability of clustering of passive tracers in some regions of the sea surface. This measure is evaluated based on 3D velocity fields calculated using the Rossby Centre Ocean Model (Swedish Meteorological and Hydrological Institute) for 1991 and the TRACMASS code for tracking Lagrangian trajectories. The maps of finite-time compressibility are calculated using a variable integration time of 12–72h and compared with a similar map of average surface divergence. The level of finite-time compressibility reaches the threshold of likely formation of patches usually in coastal regions but also in certain elongated offshore areas. The spatial distributions of this measure reveal extensive seasonal-scale variations, with the most persistent areas of high finite-time compressibility in the windy season (October–December) near the River Neva mouth.