The dynamics of upwelling in the Baltic Sea: the interplay of topography and stratification

In the Baltic Sea, wind-driven coastal upwelling and downwelling events are important for the redistribution of nutrients and other tracers vertically as well as laterally. These events develop under favorable wind conditions and are associated with longshore baroclinic coastal jets that evolve on time scales of a few days. An upwelling (or downwelling) event can be decomposed into an active and passive phase. During the active phase, the coastal jet is governed by the interplay between the wind, bottom bathymetry, and stratification. In the active phase, meandering of the jet has been commonly observed in the Baltic Sea, which has been proposed to arise from variations in the vorticity of the coastal jet due to along-shore variations in the bathymetry. As the longshore component of the wind weakens, the coastal jet becomes unstable and starts to decay enacting formations of eddies in the mesoscale (passive phase). The motivation of the present contribution is to analyze the role of baroclinic instability in the passive phase and to examine its interplay with the cross-shore slope, excluding alongshore variations by the bottom bathymetry. We use the MIT General Ocean Circulation Model (MITgcm) to study the formation and decay of coastal jets under upwelling and downwelling favorable conditions in summer. We consider an idealized flat-bottom case as well as more realistic bathymetric transects of the western Gotland Basin. The simulations are analyzed with respect to the energy conversion rates between the mean jet and eddies, the position and structure of the wind-driven coastal jet, and the characteristic scales of the variability. The results highlight the role of both, baroclinic and barotropic instabilities, for the development of the coastal jet and associated amplitudes of up- and down-welling