The TRR 181 seminar is held by Dr. Till Martin Baumann (University of Bergen) on June 16, 11 am.

Quantifying Turbulence from Standard Observations: Revisiting Finescale Parameterizations in the Arctic Ocean

Abstract

In the Arctic Ocean, vertical transport of heat and matter by turbulent mixing is ultimately coupled to the sea-ice cover, with immediate and far-reaching impacts on the climate and ecosystem. Unfortunately, direct observations of mixing are difficult, expensive and sparse.
Finescale parameterization (FS) of turbulent energy dissipation rate (ε) allows for the quantification of turbulence from breaking internal waves using standard measurements, such as profiles of hydrography and velocity. While FS proved to be reliable in mid-latitudes, the Arctic Ocean internal wave field is distinct, rendering the applicability of FS uncertain. To test FS in a wide range of Arctic conditions, we compiled data from 8 cruises, including the extensive MOSAiC and N-ICE campaigns. All profiles of hydrography and velocity used to calculate FS were collocated with in-situ measurements of ε obtained from microstructure profilers. FS was applied between 50 m and 450 m below the surface. Results show a good performance of FS, with cruise-average FS-derived ε being within a factor of 5 to observations. However, estimates from individual profiles show large variability. In our data, FS performance is independent of the shear-strain ratio (Rω) and internal wave field bandwidth (N/f), but there is evidence that substantially non-white shear spectra indicate conditions less suitable for FS. The noise level of the velocity profiles influences FS, with lowered current meters generally outperforming ship-mounted current meters. A widely used formulation of FS using only hydrography and a prescribed Rω=10 produced slightly more variability in the results.