The ocean flows downhill near the seafloor and recirculates upward above.

Abstract

The ocean's circulation redistributes heat, salt, biota, dissolved gases, microplastics, and sediments on Earth. The abyssal ocean, in the lowest 1000 m above the seafloor, moves on average with the deeper seafloor to its left in the Northern Hemisphere and to its right in the Southern Hemisphere. This finding has received little attention and its consequences for the abyssal vertical circulation have remained largely unexplored. Here, we show, using current-meter measurements and numerical simulations, that the interior flow, <math xmlns="http://www.w3.org/1998/Math/MathML"><mi>O</mi></math> (100 m) - <math xmlns="http://www.w3.org/1998/Math/MathML"><mi>O</mi></math> (1000 m) above the seafloor, is deflected within the bottom boundary layer, the lowest <math xmlns="http://www.w3.org/1998/Math/MathML"><mi>O</mi></math> (10 m), into a widespread downhill flow. This flow intensifies with the steepness of the seafloor. We further reveal that typical local changes in seafloor steepness lead to a shallow divergence and a deep convergence of this downhill flow. These are connected by an overlying upward recirculation forming closed overturning cells that extend on average over the lowest 1000 m of the ocean. Our study improves the understanding of the oceanic abyssal circulation and the climate-relevant overturning. Future research should focus on quantifying the transports of heat, particles, and dissolved chemicals associated with these abyssal slope overturning cells.