Convective turbulent mixing drives rapid upwelling along the ocean’s bottom boundary

Abstract Deep-ocean upwelling, driven by small-scale turbulent mixing¹,², plays a central role in Earth’s climate by regulating the ocean’s capacity to sequester heat and carbon for decades to millennia³,⁴. Recent theoretical studies⁵⁻⁸ have hypothesized that this upwelling may primarily occur within a thin bottom boundary layer (BBL) adjacent to the sloping seafloor. However, little is known about the mixing processes that might drive such along-bottom upwelling, and the problem remains the subject of an enduring and vigorous debate⁸⁻¹¹. Here, we elucidate the processes governing BBL-focussed upwelling in a typical continental-slope canyon, in which very rapid upwelling is observed¹². We show that upwelling along the canyon stems from episodic cells of convective turbulent mixing up to 250 m in height, generated by tidal currents sweeping up- and down-canyon. Once per tidal cycle, these currents develop a vertical shear that conveys dense waters over slower-flowing lighter waters below, causing the dense waters to convectively mix, lighten and upwell. We argue that this upwelling mechanism is likely to be of wider representativeness, substantiating the view that deep-ocean upwelling can predominantly occur along the ocean’s sloping boundaries.