The dynamics of upwelling in the Baltic Sea studied by a combined SAR/IR satellite data and circulation model analysis

Up- and downwelling are typical phenomena of the Baltic Sea. Because of the complex coastline and many islands, wind from any direction causes up- and downwelling near the coast. The extent of upwelling is scaled by the internal Rossby radius which is about 2-10 km in the Baltic Sea. During summer and autumn when the sea surface is warm, upwelling can be observed as a local temperature drop of several degrees by infrared satellite measurements. Cold water from below the thermocline is lifted upwards and eventually reaches the surface, where it replaces a well-mixed and considerably warmer upper layer. Upwelling is forced by sudden storms or strong wind events form different directions, with typical time scales ranging from a few days up to weeks. Satellite data indicate that the horizontal scales of coastal upwelling in the Baltic Sea are of the order of 100 km alongshore and some 10-20 km in the direction out from the coast. Sometimes upwelled water is spread several tens of kilometers out into the basin, forming filaments of cold water.
Cold upwelled water can impose significant changes in the stability of the marine boundary layer as well as in the surface water density relative to surrounding waters. Lower wind stress caused by increased stability over colder and denser water lead to lower sea roughness creating areas of lower signal values in synthetic aperture radar (SAR) imagery. The appearance of upwelling on SAR and sea surface temperature (SST) images can have varied correlation because of other factors affecting SAR imaging. To further investigate upwelling events and their filaments detected in SAR/IR satellite images, a high resolution coupled sea ice-ocean model of the Baltic Sea has been applied. Over upwelling areas the wind stress is significantly reduced if the mean wind speed is below a certain threshold. The combined analysis is applied to two upwelling situations occurred in July 2006 along the coasts of the Baltic States and in June 2008 along the Polish coast close to Hel peninsula. From the numerical model corresponding surface and barotropic currents associated to upwelling structures can be derived. Upwelling structures and filaments are related to topographic features controlled by the vorticity dynamics of the coastal jet.