COMPARISON OF DIAPYCNAL DIFFUSIVITY MEASURED BY TRACER AND MICROSTRUCTURE TECHNIQUES

Estimates of vertical diffusivity inferred from microstructure measure ments in the 2 thermocline of the open ocean (approximately 0.1 cm(2) s(-1)) are typically an order of magnitude smaller than values obtaine d with basin-wide tracer balances (approximately 1 cm(2) s(-1)). To ev aluate this seeming discrepancy between these two methods, a compariso n study was conducted in the hypolimnion of Lake Alpnach (Switzerland) over a period of 1 month. Diapycnal tracer diffusivity was estimated from the vertical spreading of SF6 and from the heat budget, whereas t he microstructure-based diapycnal diffusivity was calculated from the dissipation of turbulent kinetic energy measured with a high-resolutio n temperature profiler. The microstructure measurements revealed that the boundary layer above the sediment is the most turbulent zone in th e hypolimnion. Based on two assumptions for the functional form of the buoyancy flux in the bottom boundary layer, horizontally averaged mic rostructure diffusivities and basin-wide tracer diffusivities agree to within a factor of two. We conclude that (1) the apparent paradox is not due to insufficiencies in the microstructure method and (2) the tw o techniques yield the same diapycnal diffusivity if the effects of bo undary mixing are included in the basin-wide comparison. This conclusi on implies that basin-wide diapycnal diffusivity in the ocean thermocl ine is indeed an order of magnitude larger than mixing in the ocean in terior. This is consistent with the results of recent tracer and micro structure measurements conducted in the thermocline of the open ocean.