Variability of the Denmark Strait overflow: A numerical study

Abstract

Mesoscale variability in the Denmark Strait overflow is investigated using a version of the Miami Isopycnic Coordinate Ocean Model that includes a recently developed intermittent and vigorous, turbulent and diffusive diapycnal mixing scheme. Earlier idealized modeling work on the subject is further extended in that a realistic replica of the shelf‐slope topography and irregular coastline geometry of the Denmark Strait area is also employed. Compared with earlier studies, our experiments reveal that (1) the introduction of the new diapycnal scheme and the true shelf‐slope topography do not affect the formation of the intense cyclonic eddies south of the sill and along the continental slope and (2) the new diapycnal scheme has a significant effect on the bottom plume, in that the distribution, volume transport and properties of the bottom plume appear to be more realistic and more in line with what is observed. The first conclusion adds support to the vortex‐stretching mechanism suggested in earlier studies as the cause for the formation of the cyclonic eddies. This mechanism is therefore a robust feature and provides an explanation for the observed variability in the Denmark Strait overflow. The second conclusion underscores the importance of including realistic parameterizations of diapycnal mixing in isopycnal coordinate ocean models. Specifically, the results show that the water that descends along the bottom south of the sill becomes lighter and is more voluminous compared to experiments without the new mixing scheme. Even when including the mixing scheme, the bottom plume separates into distinct boluses of enhanced thickness. Moreover, these boluses are also associated with water of anomalously high density, as well as with the cyclonic eddies in the upper water column.