Dynamics of vortical structures in the Baltic Sea

A fully calibrated and validated 3D hydrodynamic model is applied to the Baltic Sea to investigate the vortical structures in the Baltic Sea in the period 2000-2009. The novelty of the study is the detection and dynamics of large scale mesoscale vortices both in horizontal and vertical planes by using the instantaneous velocity vectors. This study provides some new insights into the type and dynamics of vortical structures in the Baltic Sea. In the horizontal plane, vortices with typical length and velocity scale of 5-12 km and 5-10 cm/s are present. They are of open type three-dimensional structures having two important features. They extend to 90% of the flow depth and have a clear periodicity of one to seven days. The vortices rotate both in clockwise and anti-clockwise directions. The baroclinic Rossby deformation radii are in the same range. The maximum numbers of mesoscale vortices are within the 15-m thick surface layer that diminishes towards the bottom layer. In the vertical planes, there is a multi-layered structure with the layer numbers varying from 2 to 4. Each layer is characterized by a set of distinct vortices. Taylor-Gortler type vortices appear within the surface layers. The middle flow layers are featured by distorted large-scale vortices. Boundary trapped vortices and large local circulation patterns feature the deeper bottom flow layers. There are two underlying mechanisms of vortex generation: one is shear layer instability of the Kelvin Helmholtz type and the other Taylor-Gortler. A conceptual general vortex model is proposed that applies to all basins of the Baltic Sea in vertical planes. Three types of structures are possible in the mixed layer, pycnocline, and the deep layer. In these are Taylor-Gortler vortices, mesoscale coherent structures, and trapped boundary vortices, respectively.