Dynamics of the Freshwater-influenced Surface Layers in the Skagerrak

The Skagerrak receives large amounts of freshwater, both in the form of rather low-saline waters from the Baltic Sea and the southern North Sea and in pure form from local rivers. These waters, mixed with the underlying Atlantic Water, participate in a variable, mainly wind-driven, cyclonic surface circulation in the Skagerrak. This paper presents a thorough analysis of hydrographic data to provide insight into the distribution and circulation of freshwater in the Skagerrak. From measured salinity and temperature profiles, we compute vertically integrated variables such as the freshwater content, the potential energy and the steric sea level. Furthermore, the density variance is calculated from the density fields. The calculations show that freshwater is accumulated mainly along the Swedish and Norwegian coasts. The computed topography of the steric sea level agrees well with published results for the coast as determined by geodetic methods. This shows that the steric effect gives the dominating contribution to the topography of the mean sea level in the Skagerrak. The computed distribution of freshwater along the coasts is thus largely verified by the geodetically determined mean sea level. We find strong indications of an extensive recirculation of freshwater within the Skagerrak, due to advection/dispersion from the storage along the Norwegian coast. The horizontal distribution of density variance in the upper 10 m mirrors the spreading of low-saline water from the Kattegat. Deeper down there are maxima in density variance along the coasts, reflecting occasional downwelling and strengthening of the baroclinic coastal currents due to time-varying winds. The mean potential energy along the coasts increases monotonously in the cyclonic direction, from the inflow region in the southwest to the outflow region off the western part of the Norwegian Skagerrak coast. The corresponding flow increase is mainly due to incorporation of Atlantic Water into the surface layer. Using a rough energy budget for the Skagerrak, we find that the potential energy of the coastal current system cannot be maintained by wind-driven diapycnal mixing. We suggest that isopycnal downwelling along the coasts provides the necessary energy. The considerable annual variation found in potential energy in the coastal currents covaries with the variation of the wind stress amplitude.