Secondary circulation in shallow ocean straits: Observations and numerical modeling of the Danish Straits

We study the secondary circulation in the Danish Straits using the unstructured-grid hydrodynamic model SCHISM covering the North Sea and Baltic Sea. The resolution in the straits is up to ~100 m. Since the large-scale atmospheric variability controls the transport in these straits, we focus on the processes with subtidal time scales. We compare the in- and outflows in the straits to flood and ebb flows in estuaries and analyze similarities and differences. Very prominently, the outflow and inflow phases of the Danish Straits feature substantial differences to the tidal straining in estuaries. With a resolution of ~100 m, new transport and mixing pathways, previously unresolved, appear fundamental to the strait dynamics. The variability of the strait bathymetry leads to a strongly differing appearance of secondary circulation. Helical cells, often with a horizontal extension of ~1 km, develop in the deep parts of the channels. A comparison between the high-resolution simulation and a simulation with a coarser grid of ~500 m in the straits suggests that the coarser resolution overestimates the stratification and misrepresents the transport balance. Axial velocities and transport through the Sound are underestimated by ~12%. These differences are explained by the underdeveloped secondary circulation in the coarse-resolution simulation and the associated changes in mixing along the straits. In conclusion, the use of ultrafine resolution grids is essential to adequately resolve secondary flow patterns and two-layer exchange.