Dense waters sink into the abyss at high latitudes north and south (downward arrows). The bottom waters are lifted up to depths of about 2,000 metres by mixing processes (wiggly arrows), and return to high latitudes at these intermediate depths, eventually rising to the surface via the Southern Ocean (southward and upward pointing arrows), closing the circulation loop; shading indicates the extent of the Southern Ocean. Horizontal arrows at the surface indicate the path of waters back to high latitudes. North of the Southern Ocean, the red line indicates the heights of the tallest topographic features below which mixing is strong. In the Southern Ocean, the red line indicates the topography of the Drake Passage, to illustrate topography at latitudes at which deep water is pulled to the surface by winds (the Roaring Forties). Solid blue regions indicate the deepest points at each latitude, based on a 0.25°-resolution bathymetry data set. Waterhouse et al.¹ confirm that this scenario is consistent with available observations of ocean mixing. (Adapted from ref. 9.)