More Evidence for Boundary Mixing in the Deep Ocean

In “Some evidence for boundary mixing in the deep ocean” Armi (1978) showed that mixing in the deep ocean can be characterized as two complimentary processes: vertical mixing within ~50m thick layers at boundaries and topographic features and lateral advection and eventual smearing of these mixed layers along isopycnal surfaces. Note that these two processes are four-dimensional: three dimensions in space plus one dimension in time, not one-dimensional or two-dimensional or steady. Here, we have analyzed many recent profiles from the CCHDO (CLIVAR and Carbon Hydrographic Data Office) data set, along with global bathymetry and topography at 15 arc seconds (SRTM15+), that convincingly provides “more evidence” for this concept. We find that, density steps bounding mixed layers show up in rough topography areas, like sea mount provinces, while, density profiles over relatively smooth topography, abyssal planes, do not exhibit these layered features. This layering-topography relationship indicates that topography and flow associated either with steady currents, mesoscale eddies or tides is the dominant physical mechanism driving mixing in the deep ocean which is often disguised or parameterized as a vertical eddy diffusivity.