Mixing processes in dense overflows with emphasis on the Faroe Bank Channel overflow

Abstract

The amount and distribution of mixing and entrainment that the overflows across the Greenland-Scotland Ridge encounter influence the ventilation of the deep North Atlantic. Constituting about 30% of the total overflow (about 6 Sv) across the Greenland-Scotland Ridge, the continuous, swift overflow through the deepest passage from the Nordic Seas to the North Atlantic Ocean, the Faroe Bank Channel, is a major overflow in the region.

The mixing processes of the Faroe Bank Channel overflow are explored by combining results from observations, including the first direct turbulence measurements, numerical simulations of the overflow, and an idealized process study. The observations show an overflow characterized by strong lateral variability in entrainment and mixing, a transverse circulation actively diluting the bottom layer, and a pronounced vertical structure composed of an about 100m thick stratified interface and a comparably thick well-mixed bottom layer. The turbulent overflow is associated with intense mixing and enhanced turbulent dissipation rate near the bottom and at the plumeambient interface, but with a quiescent core.

Results from numerical simulations of the overflow with second order turbulence closures are compared to the observations. Turbulent dissipation rate and eddy diffusivity profiles inferred from the observations are used in refining the parameters of the turbulence closure. In the bottom-most 50-60 m, where the Richardson number is small and the production of turbulent kinetic energy is well-resolved, the model reproduces the observed vertical structure of enhanced dissipation rate and eddy diffusivity exceptionally well. In the interfacial layer and above the plume-ambient interface, however, the model does not resolve the mixing. A further investigation of the observations, addressing the role of the transverse circulation and internal waves in mixing in the stratified interface, shows that the transverse circulation effectively contributes to mixing of the overflow plume. Dissipation rates are more than doubled in the interfacial layer due to the transverse flow. In the ambient above the overflow plume, internal wave breaking is the dominant mechanism for dissipation of turbulent energy. In the interfacial  layer the main mechanism of mixing is the shear-instability and entrainment associated with the swift gravity current, enhanced by the secondary circulation. However, the internal wave continuum is energetic in the interfacial layer and may contribute to mixing.

To investigate the influence of unresolved small scale topography on the flow of a stratified fluid, a 2-m resolution, non-hydrostatic, three-dimensional numerical model is used. The drag and associated mixing on the stratified flow over real, 1-m resolution, complex topography (interpolated to model resolution) are studied. The results show that a significant drag can be exerted on the flow of a stratified layer overlaying a well-mixed layer (resembling the bottom and interfacial layer of the Faroe Bank Channel overflow) over rough topography. A parameterization of the internal wave drag is developed and implemented, and provides satisfactory results in terms of the domain integrated turbulent kinetic energy levels.