Estimates of vertical mixing rates in the pycnocline

Jason Hyatt, Susan Howard and Laurie Padman

We describe the small-scale mixing processes which transport heat, salt and nutrients from the warm, salty, nutrient rich Upper Circumpolar Deep Water up to the surface mixed layer. The study makes use of CTD and vessel-mounted ADCP collected during NBP01-03 and NBP01-04 survey cruises, along with some data from the 2001 Gould process stations. This data is applied to some published parameterizations in order to make estimates of vertical heat and salt fluxes. The dominant cause of upward turbulent fluxes appears to be shear-driven mixing, possibly associated with near-inertial baroclinic waves. The mean vertical diffusivity associated with this process is estimated at ~1 x 10^‑5 m² s^-1, corresponding to a heat flux into the base of the mixed layer of 1‑2 W m^‑2. A previous suggestion that double-diffusive convection (DDC) provides significant heat fluxes in this region is not supported by our analyses of the bulk properties of the pycnocline: the area-averaged DDC heat flux is estimated at ~0‑0.4 W m^-2 over the central and inner continental shelf, and ~0‑0.9 W m^-2 over the continental slope. Overall, this analysis yields effective vertical diffusivities which are quite low, almost background values, and motivates further investigation of vertical mixing processes beyond the parameterizations.