Dynamics of Upwelling and Downwelling in a Channel Basin of the Baltic Sea

We consider upwelling and downwelling dynamics in an idealized ocean model configuration of the Western Gotland Basin in the Baltic Sea, featuring a gently sloping bottom in the west and a steep bathymetry in the east. Typical transient wind conditions and seasonally variable stratification are examined. Upwelling and downwelling jets develop at the coastal boundaries and interact through cross-shore boundary-layer flows. Initial evolution of the coastal jets is consistent with linear theory. The front position and the onset of instability is governed by the wind forcing, with a weak dependence on seasonal stratification. The unstable growth rates and wavelengths over the slope depend on the relative orientation of the slope and isopycnals, consistent with theory. The upwelling jets become baroclinically unstable during the wind-forced phase, whereas instability onset for downwelling on the slope is after 2–3 weeks (during the relaxation phase). The downwelling on the steep side is consistently stable. The regime with unstable upwelling on the slope side with concurrent stable downwelling on the steep side is more frequent (southwesterly winds: 30% occurrence) and leads to strong cross-shore transport. Unstable downwelling on the slope with upwelling on the steep side is a rarer event (northwesterly winds: 10% occurrence) and generates strong vertical mixing on the slope, with implications for oxygen and nutrient fluxes on the inner shelf along the Swedish coast. Baroclinic eddies contribute to elevated vertical mixing in the surface layer.