Comparing Eulerian and Lagrangian eddy census for a tide-less, semi-enclosed basin, the Baltic Sea.

Eddies in the global and coastal ocean play a key role in mixing and transport processes. Here, we present an eddy census for the Baltic Sea covering the years 2008–2010. The eddy tracking was carried out with three different eddy detection methods (an Eulerian streamline based and two Lagrangian coherent structure-based methods). The near surface velocity fields were generated with GETM (General Estuarine Transport Model), an eddy-resolving ocean model (internal Rossby radius ≈ 5 km, grid resolution ≈ 0.6 km). The general shape of the statistics for the three detection algorithms agree well for lifetime and propagation distance. The most striking differences between the methods were visible for the eddy size, with much smaller eddy-radii detected by the Lagrangian methods. Although we only find minor variations in the monthly numbers of tracked eddies in total, we found large variations in the eddy core vorticity, with higher rotation and smaller radii during winter. This was caused by the changing stratification over the course of the year. Additionally, we found spatial changes in the ratio of cyclonic to anticyclonic rotating eddies and seasonal differences in the spatial distribution of eddies. [ABSTRACT FROM AUTHOR]