Observational Evidence for Vertical Diffusion Driven by Internal Waves of Tidal Origin in the Oslofjord

Abstract

In an earlier paper (Stigebrandt, 1976), a model was proposed for internal wave-induced vertical mixing in a sill fjord. The observational evidence for the theory at that time was rather sparse, but the void is materially filled by observations presented in this note. Current measurements show, in accordance with the theory, that progressive internal waves radiate out from the Dröbak sill in the Oslofjord. An experiment with tracer dye below the sill depth in the same fjord showed that the coefficient of vertical diffusion of the tracer was an order of magnitude less than the overall coefficient for vertical diffusion of density during the same period. This finding, indicating large horizontal inhomogeneities in the mixing field, gives strong support to the mixing model proposed in the earlier paper referred to above.