Mixing in the pycnocline over the western Antarctic Peninsula shelf during Southern Ocean GLOBEC

The Southern Ocean Global Ecosystem Dynamics program studied the continental shelf region in the vicinity of Marguerite Bay, on the western side of the Antarctic Peninsula, to determine the factors that contribute to Antarctic krill survival over winter. Subsurface intrusions of upper circumpolar deep water (UCDW) onto the shelf provide much of the nutrient flux into the region. Here we describe the small-scale processes that contribute to upward diapycnal fluxes of heat, salt, and nutrients from the UCDW to the surface-mixed layer. The study makes use of conductivity–temperature–depth and vessel-mounted acoustic Doppler current profiler data collected during three research cruises between April and September 2001. Near-inertial baroclinic waves generated by wind stress provide most of the shear across the sharp pycnocline at the base of the mixed layer. The mean vertical diffusivity associated with shear instability is estimated at ⩽1×10−5 m2 s−1, corresponding to a heat flux into the base of the mixed layer of <2 W m−2. A previous suggestion that double-diffusive convection (DDC) provides significant upward heat fluxes (of order 10 W m−2) in a nearby region is not supported by our analyses of the present data set, which indicates almost no contribution to diapyncal fluxes from DDC.