Development of a coastal upwelling front driven by advection and topographic effects in the North Sea–Baltic Sea transition

Upwelling of cold, saline, and nutrient rich water was observed in late September 1999 along an east–west transect in the SW Kattegat. The Kattegat forms part of the transitional zone between the high saline North Sea and the low saline Baltic Sea. The upwelling occurred after an extended period of northward flow and eastern winds in the Kattegat, that changed into a southward flow as wind ceased. The upwelling was the result of a combination of high current speeds and bottom topography whereby the high-speed inflow water was forced towards the surface at the slope. Nutrient data show that the upwelling brought nutrient rich bottom water to the light exposed surface and a related strong (1943.5 ml^–1) bloom of Pseudo-nitzschia pseudodelicatissima was observed. Surface (1.0 m) chlorophyll-a values increased from 3.7 to 10.0 μg 1^–1 during upwelling. A frontal structure developed during the upwelling with raised fluorescence values at the front related to convergence. The present upwelling front was only indirectly related to the wind conditions and the front propagated off shore as compared to other upwelling fronts.