Observations of diapycnal upwelling within a sloping submarine canyon.

Small-scale turbulent mixing drives the upwelling of deep water masses in the abyssal ocean as part of the global overturning circulation1. However, the processes leading to mixing and the pathways through which this upwelling occurs remain insufficiently understood. Recent observational and theoretical work2-5 has suggested that deep-water upwelling may occur along the ocean's sloping seafloor; however, evidence has, so far, been indirect. Here we show vigorous near-bottom upwelling across isopycnals at a rate of the order of 100 metres per day, coupled with adiabatic exchange of near-boundary and interior fluid. These observations were made using a dye released close to the seafloor within a sloping submarine canyon, and they provide direct evidence of strong, bottom-focused diapycnal upwelling in the deep ocean. This supports previous suggestions that mixing at topographic features, such as canyons, leads to globally significant upwelling3,6-8. The upwelling rates observed were approximately 10,000 times higher than the global average value required for approximately 30 × 106 m3 s-1 of net upwelling globally9.

Rapid cross-density ocean mixing at mid-depths in the Drake Passage measured by tracer release.

Diapycnal mixing (across density surfaces) is an important process in the global ocean overturning circulation. Mixing in the interior of most of the ocean, however, is thought to have a magnitude just one-tenth of that required to close the global circulation by the downward mixing of less dense waters. Some of this deficit is made up by intense near-bottom mixing occurring in restricted 'hot-spots' associated with rough ocean-floor topography, but it is not clear whether the waters at mid-depth, 1,000 to 3,000 metres, are returned to the surface by cross-density mixing or by along-density flows. Here we show that diapycnal mixing of mid-depth (∼1,500 metres) waters undergoes a sustained 20-fold increase as the Antarctic Circumpolar Current flows through the Drake Passage, between the southern tip of South America and Antarctica. Our results are based on an open-ocean tracer release of trifluoromethyl sulphur pentafluoride. We ascribe the increased mixing to turbulence generated by the deep-reaching Antarctic Circumpolar Current as it flows over rough bottom topography in the Drake Passage. Scaled to the entire circumpolar current, the mixing we observe is compatible with there being a southern component to the global overturning in which about 20 sverdrups (1 Sv = 10(6) m(3) s(-1)) upwell in the Southern Ocean, with cross-density mixing contributing a significant fraction (20 to 30 per cent) of this total, and the remainder upwelling along constant-density surfaces. The great majority of the diapycnal flux is the result of interaction with restricted regions of rough ocean-floor topography.

Long-lived vortices as a mode of deep ventilation in the Greenland Sea.

The Greenland Sea is one of a few sites in the world ocean where convection to great depths occurs-a process that forms some of the densest waters in the ocean. But the role of deep convective eddies, which result from surface cooling and mixing across density surfaces followed by geostrophic adjustment, has not been fully taken into account in the description of the initiation and growth of convection. Here we present tracer, float and hydrographic observations of long-lived ( approximately 1 year) and compact ( approximately 5 km core diameter) vortices that reach down to depths of 2 km. The eddies form in winter, near the rim of the Greenland Sea central gyre, and rotate clockwise with periods of a few days. The cores of the observed eddies are constituted from a mixture of modified Atlantic water that is warm and salty with polar water that is cold and fresh. We infer that these submesoscale coherent eddies contribute substantially to the input of Atlantic and polar waters to depths greater than 500 m in the central Greenland Sea.