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 ther mocline depend on mixing along and across surfaces of equal density (i sopycnal and diapycnal mixing, respectively). Measurements of the inva sion of anthropogenic tracers, such as bomb tritium and He-3 (see, for example, refs 1 and 2), have indicated that isopycnal processes domin ate 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 measuremen t, obtained from the vertical dispersal of a patch of the inert compou nd 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 suppor t only a rather small diapycnal flux of nitrate into the euphotic zone ; it justifies the neglect of diapycnal mixing in dynamic models of th e thermocline25-27, and implies that heat, salt and tracers must penet rate the thermocline mostly by transport along, rather than across, de nsity surfaces.