= Solution
Deep currents and <tides> moving across rough topography, including the Mid-Atlantic Ridge, generate <internal waves>. Their breaking produces <turbulent mixing> across <isopycnals>, called <diapycnal mixing>. This transfers <buoyancy> between deep and overlying waters, transforms dense water into lighter water, and contributes to the upward return needed to close the overturning. The required mechanical <energy> ultimately comes from winds and <tides>; roughness is a mechanism for converting this input into mixing, rather than an independent source of <energy>. Mixing is spatially concentrated, and local vertical motion depends on the divergence of the turbulent <buoyancy flux>, so enhanced mixing does not imply uniform local upwelling everywhere over a ridge.https://www.whoi.edu/cms/files/vmix_38943.pdf[The Impact of Abyssal Mixing Parameterizations in an Ocean General Circulation Model, abstract and introduction: tidal internal-wave generation over rough topography and its role in overturning]{ref}
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