Mixing estimates from hydrographic measurements in the Deep Western Boundary Current of the North Atlantic

Diapycnal diffusivity and energy dissipation at the western boundary of the North Atlantic have been inferred from more than 100 profiles of combined LADCP and CTD measurements collected during different cruises from 2000 to 2005 at four different latitudes in the North Atlantic. Both diffusivity and dissipation show strong local variability in on/offshore direction. The mean diffusivity on all four sites was elevated, with the maximum (Kρ=(3.8±0.7)×10-4m2s-1) found at 60∘N and decreasing downstream to (1.1±0.3)×10-4m2s-1 at 16∘N. In contrast to the diffusivity, the energy dissipation rate was lowest in the northern latitudes (ɛ=(2.1±0.7)mWm-2 at 75∘N) and increased to (15.3±11.0)mWm-2 at 16∘N. For maximum velocities less than 26cms-1, the dissipation is independent of the velocity and of the order of 3mWm-2. For higher speeds, dissipation increases exponentially. This feature, together with the energy flux direction and the lack of tidal signals indicate that interaction between mean flow and bottom topography is the dominant process for the elevated mixing.