Dynamics of the Abyssal Ocean Circulation Driven by Bottom-Intensified Mixing on Slopes

The large-scale circulation of the abyssal ocean is primarily driven by diapycnal mixing. Traditionally, this mixing has been thought to invariably cause water to rise, and the dynamical response to such an upwelling is easily understood with linear vorticity dynamics. Observations suggest, however, that mixing is most vigorous near the ocean bottom, caused by breaking internal waves that are generated by flows over jagged topography. Such bottom-intensified mixing instead drives downwelling in much of the water influenced by the mixing, and it confines upwelling to thin boundary layers. Bottom-intensified mixing is thus expected to induce a very different dynamical response than previously considered. We here explore these dynamics using a planetary-geostrophic model, which despite dynamical and geometric simplifications captures the gross structure of the observed stratification. The circulation that arises in response to bottom-intensified mixing has a large degree of compensation between up- and downwelling on slopes. Net upwelling only occurs in a thin layer in the deepest water, where boundary layer flows converge, fluid is exchanged with the western boundary, and waters are carried meridionally in western boundary currents. While the zonally-integrated overturning vanishes above, the compensated up- and downwelling on slopes still exchanges fluid with the bottom layer and is integral part of the overturning circulation.