Abstract

The intensity and climatic impacts of oceanic overturning are critically shaped by deep-ocean turbulent mixing, which transforms cold waters sinking at high latitudes into warmer, shallower waters. The effectiveness of turbulent mixing in driving this transformation is jointly set by two factors: the intensity of turbulence near topographic boundaries, and the rate at which well-mixed boundary waters are exchanged with the stratified ocean interior. Yet while many near-boundary turbulent processes have been documented, little is known about the mechanisms regulating boundary – interior exchange. Here, we use observations of the turbulent properties of a major outflow of Antarctic Bottom Water (AABW) from the Weddell Sea to identify a previously-unrecognised mechanism of deep-ocean mixing, by which near-boundary turbulence and boundary – interior exchange are concurrently intensified. The observations included high-resolution fine- and microstructure measurements across the AABW flow through the Orkney Passage, and were obtained with ship-deployed instrumentation and the autonomous underwater vehicle Autosub Long Range under the auspices of the U.K. DynOPO (Dynamics of the Orkney Passage Outflow) project. The cornerstone of the new mechanism is the generation of submesoscale dynamical instabilities by the flow of deep-ocean waters along a steep topographic boundary. The dynamics and large-scale implications of this mode of mixing will be discussed.