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

► We develop a modified measure of finite-time flow compressibility of sea surface reflecting clustering of tracers. ► We track offshore areas which are likely to form clusters of floats. ► Finite-time flow compressibility maps for the Gulf of Finland are calculated for selected seasons. ► High finite-time compressibility identified near the River Neva mouth ► Extensive seasonal variability in finite-time compressibility in the Gulf of Finland