9/23/2023 0 Comments Deep water 2006Dense deep water is drawn upward along steeply tilted isopycnals (surfaces of constant density), driven by divergence of wind-driven Ekman transport and surface buoyancy forcing, enabling the return of deep water to the surface with minimal diapycnal mixing 6, 7. The vigor of the Southern Ocean return limb derives from the dynamics associated with the existence of an open circumpolar pathway around Antarctica in Drake Passage latitudes 5. Observations suggest that as much as 80% of the World Ocean deep water returns to the surface in the Southern Ocean with the remainder reaching the sea surface through upwelling to the thermocline in low latitudes 2, 4. In contrast, the specific locations where these waters return back to the sea surface to complete the circuit are poorly known. These processes are well documented, with the northern sites well mapped and the southern sites, in coastal polynyas, increasingly so 3. The global overturning circulation moves waters around the world’s oceans, connecting surface and deep waters through two interlinked overturning cells, one with sinking in the far northern North Atlantic and adjacent Nordic Seas and the other with sinking along the Antarctic coastline 1, 2. The timescale for half of the deep water to upwell from 30° S to the mixed layer is ~60–90 years. Deep water reaches the upper ocean predominantly south of the Antarctic Circumpolar Current, with a spatially nonuniform distribution. Upwelling is greatly enhanced at five major topographic features, associated with vigorous mesoscale eddy activity. The analysis reveals that the northern-sourced deep waters enter the Antarctic Circumpolar Current via southward flow along the boundaries of the three ocean basins, before spiraling southeastward and upward through the Antarctic Circumpolar Current. Here we show detailed upwelling pathways in three dimensions, using hydrographic observations and particle tracking in high-resolution models. However, the exact pathways and role of topography in Southern Ocean upwelling remain largely unknown. Upwelling of global deep waters to the sea surface in the Southern Ocean closes the global overturning circulation and is fundamentally important for oceanic uptake of carbon and heat, nutrient resupply for sustaining oceanic biological production, and the melt rate of ice shelves.
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