Abstract:
This paper begins to explore a previously neglected mechanism for
abyssal ocean mixing using bottom boundary layer dynamics. Abyssal
mixing and the associated upward buoyancy fluxes are necessary to
balance the sinking of dense waters at high latitudes and to close the
global overturning circulation. Previous studies have concentrated on
the hypothesis that the primary mechanism for this mixing is breaking
internal waves generated by tidal flows over rough topography.
However, intriguing observations, particularly from the Brazil Basin
Tracer Release Experiment, suggest that mixing in the flank canyons
of the Mid-Atlantic Ridge generated when strong mean flows interact
with the many sills and constrictions within the canyons may represent
a dynamically important amount of abyssal mixing. The energy
pathways and mechanisms of this mixing are much less clear than in
the case of breaking internal waves. This study attempts to clarify
this by suggesting an analogy with an idealized diffusive boundary
layer over a sloping bottom. This boundary layer is characterized by
up-slope flows powered by the buoyancy flux in the fluid far from
the boundary. Here we explore the energy budget of the boundary
layer, and find that the diffusive boundary layer provides flows that
are generally consistent with those observed in submarine canyons. In
addition, we derive the vertical velocity in the far-field fluid, analogous
to an Ekman pumping velocity, that these boundary layers can induce
when the bottom slope is not constant. Finally, we present both theoretical
and numerical models of exchange flows between the bottom
boundary and the far-field flow when the bottom slope is not constant.
These exchange flows provide a mechanism by which boundary-driven
mixing can affect the overall stratification and buoyancy fluxes of the
basin interior.
