Ocean Boundary Mixing during Ekman Layer Arrest

Abstract

As a water parcel comes into contact with an ocean boundary, energy is dissipated within the boundary layer with some fraction directed into vertical mixing. In a stratified flow this increases the potential energy associated with the density field. It has previously been assumed that mixing efficiencies (ratio of potential energy increase to total energy dissipation) are likely to be small (<10%), which would also imply that boundary mixing is a minor contributor to total ocean mixing. While this is probably true for mixing driven purely by shear driven instability of the boundary layer, the analysis presented here suggests that flows with downslope Ekman transport can generate much larger mixing efficiencies. This is supported by laboratory experiments, which yield mixing efficiencies of 30% (independent of flow parameters), compared to around 3% for flows in the same system with upslope Ekman transport. Simple basinwide estimates suggest that this mechanism can potentially account for a significant fraction of the vertical mixing in the ocean.