Mixing and convection in the Greenland Sea from a tracer-release experiment

Convective vertical mixing in restricted areas of the subpolar oceans, such as the Greenland Sea, is thought to be the process responsible for forming much of the dense water of the ocean interior(1,2). Deep-water formation v aries substantially on annual and decadal timescales(3-5), and responds to regional climate signals such as the North Atlantic Oscillation(6,7); its v ariations may therefore give early warning of changes in the thermohaline c irculation that may accompany climate changes. Here we report direct measur ements of vertical mixing, by convection and by turbulence, from a sulphur hexafluoride tracer-release experiment in the central Greenland Sea gyre. I n summer, we found rapid turbulent vertical mixing of about 1.1 cm(2) s-(1) . In the following late winter, part of the water column was mixed more vig orously by convection, indicated by the rising and vertical redistribution of the tracer patch in the centre of the gyre. At the same time, mixing out side the gyre centre was only slightly greater than in summer. The results suggest that about 10% of the water in the gyre centre was vertically trans ported in convective plumes, which reached from the surface to, at their de epest, 1,200-1,400 m. Convection was limited to a very restricted area, how ever, and smaller volumes of water were transported to depth than previousl y estimated(9). Our results imply that it may be the rapid year-round turbu lent mixing, rather than convection, that dominates vertical mixing in the region as a whole.