Boundary Layer Dynamics and Abyssal Mixing in Mid-Ocean Ridge Canyons

Physical oceanographers have known for several decades the total amount of abyssal mixing and upwelling required to balance deep-water formation, but we are still working to understand the mechanisms and locations---how and where it happens. From observational studies, we know that areas of rough topography are important and the hundreds of Grand-Canyon sized canyons that line mid-ocean ridges have particularly energetic mixing.

Diffusive boundary layers and their interaction with canyon-like topography provide a new mechanism for both increasing the total amount of mixing locally and for using that local mixing to affect the stratification far from the bottom. These boundary layers share many important properties with observed flows in abyssal canyons, like increased kinetic energy near topographic sills and strong currents running from the abyssal plains up the slopes of the mid-ocean ridges toward their crests.

These processes, though previously neglected, may be a source of a dynamically important amount of abyssal upwelling, profoundly affecting predictions of the basin-scale circulation. And because these boundary layers can force exchange of large volumes of fluid between the relatively unstratified boundary layer and the far-field fluid, they may alter the stratification of the basin as well. This type of mechanism cannot be captured by the kind of mixing parameterizations used in current global climate models, based on a bottom roughness. Therefore, there is much work still to do to better understand how these boundary layers behave in more realistic contexts and how we might incorporate that understanding into climate models.