Mixing Regimes in the Amundsen Basin of the Arctic Ocean
Guthrie, John D.
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We investigate the role of background mixing in the Arctic Ocean and how it has changed with changing forcing and ice cover. We have examined shear measurements made over more than 30 years to determine the variation in internal wave energy and mixing in different regions of the Arctic Ocean. To improve estimates of thermal diffusivity and heat fluxes in the deep basins of the Arctic Ocean from the base of the mixed layer down to the Atlantic Water layer, we have presented results from three different data sets collected in the central Arctic Ocean as well as leveraging earlier microstructure observations. Using expendable current profilers in addition to moorings in the Canada Basin, we use shear variance to estimate diapycnal diffusivity in the central Arctic Ocean. We find little change in time despite drastic changes in sea ice extent during the same time period. However, some spatial variability is present with stronger mixing at depth in the Eurasian Basin and weaker mixing at depth in the Canada Basin. This is attributed to stronger under-ice dissipation in the Canada Basin removing energy from the internal wave field due to the stronger near-surface stratification resulting in that region. Using temperature microstructure profiles, the regimes of turbulent mixing and double diffusion in the Amundsen Basin during spring have been explored, with double diffusion dominating in 2013 and weak turbulence dominating in 2014. We have provided an updated version of a well-known heat flux parameterization for double diffusion that agrees to within 10% of the observed values. We have also shown that a convecting scaling exponent necessary in relating the Nusselt number and the Rayleigh number needs downward revision. In the absence of a thermohaline staircase, we calculate turbulent diffusivity. Heat fluxes for each mixing regime have been compared and contrasted, and we have proposed an upper bound on the level of turbulent diffusivity necessary to prevent thermohaline staircase formation. The decreased turbulence in 2013 is related to large-scale circulation changes and a shift in the location of the Transpolar Drift.
- Oceanography