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 dye experiment in a continental-slope canyon during the BLT-Recipes program revealed very rapid BBL-focussed upwelling, endorsing this notion. Here, we elucidate the dynamical connection between the mixing and 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 periodically 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 net along-boundary upwelling. Our findings substantiate the view that deep-ocean upwelling can predominantly occur along the ocean's sloping boundaries.