NP seminar: Mixing in the Amundsen Basin of the Central Arctic Ocean: Quiet and Quieter

Speaker: John D. Guthrie, University of Washington

Using temperature microstructure profiles collected during the 2013 and 2014 field seasons of the North Pole Environmental observatory (NPEO), 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 convective 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. We propose that a thermohaline staircase was able to form at depths above 200 m in 2013 due to exceptionally weak mixing that year. The decreased turbulence in 2013 is related to large-scale circulation changes and a shift in the location of the Transpolar Drift which resulted in increased near-surface stratification.