Connecting Mixing to Upwelling along the Ocean's Sloping Boundary

Deep-ocean upwelling, driven by small-scale turbulence, plays a key role in climate by regulating the ocean's capacity to sequester heat and carbon. Recent theoretical studies have hypothesized that such upwelling may primarily occur within a bottom boundary layer (BBL) along the sloping seafloor. A recent dye experiment in a continental-slope canyon as part of the BLT-Recipes program revealed very rapid BBL-focussed upwelling, endorsing this notion. Here, we elucidate the mixing processes governing the upwelling. We show that along-canyon upwelling stems from episodic turbulent mixing cells up to 250 m high, generated by tides sweeping up- and down-canyon. The tidal currents support a vertical shear that advects dense waters over slower-flowing lighter waters, reducing BBL stratification. This triggers instabilities that mix the dense waters with neighboring lighter waters, resulting in a net along-boundary upwelling flow. Our findings substantiate the view that deep-ocean upwelling can predominantly occur along the ocean's sloping boundaries.