Comparison of diapycnal diffusivity measured by tracer and microstructure techniques
Estimates of vertical diffusivity inferred from microstructure measurements in the 2 thermocline of the open ocean (approximately 0.1 cm(2) s(-1)) are typically an order of magnitude smaller than values obtained with basin-wide tracer balances (approximately 1 cm(2) s(-1)). To evaluate this seeming discrepancy between these two methods, a comparison 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 the microstructure-based diapycnal diffusivity was calculated from the dissipation of turbulent kinetic energy measured with a high-resolution temperature profiler. The microstructure measurements revealed that the boundary layer above the sediment is the most turbulent zone in the hypolimnion. Based on two assumptions for the functional form of the buoyancy flux in the bottom boundary layer, horizontally averaged microstructure 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 two techniques yield the same diapycnal diffusivity if the effects of boundary mixing are included in the basin-wide comparison. This conclusion implies that basin-wide diapycnal diffusivity in the ocean thermocline is indeed an order of magnitude larger than mixing in the ocean interior. This is consistent with the results of recent tracer and microstructure measurements conducted in the thermocline of the open ocean.