Evidence for slow mixing across the pycnocline from an open-ocean tracer-release experiment

THE distributions of heat, salt and trace substances in the ocean thermocline depend on mixing along and across surfaces of equal density (isopycnal and diapycnal mixing, respectively). Measurements of the invasion of anthropogenic tracers, such as bomb tritium and 3He (see, for example, refs 1 and 2), have indicated that isopycnal processes dominate diapycnal mixing, and turbulence measurements have suggested that diapycnal mixing is small3,4, but it has not been possible to measure accurately the diapycnal diffusivity. Here we report such a measurement, obtained from the vertical dispersal of a patch of the inert compound SF6 released in the open ocean. The diapycnal diffusivity, averaged over hundreds of kilometres and five months, was 0.11 ± 0.02 cm2 s−1, confirming previous estimates1–4. Such a low diffusivity can support only a rather small diapycnal flux of nitrate into the euphotic zone; it justifies the neglect of diapycnal mixing in dynamic models of the thermocline25–27, and implies that heat, salt and tracers must penetrate the thermocline mostly by transport along, rather than across, density surfaces.