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


Susan L. Howard1, Laurence Padman1, and Jason Hyatt2


1 Earth & Space Research, 1910 Fairview Ave. East, Suite 102, Seattle, WA 98102-3620

2 Dept. of Physical Oceanography, Clark 332, MS #21, Woods Hole Oceanographic Institution, Woods Hole, MA 02543



The Southern Ocean Global Ecosystem Dynamics (SO GLOBEC) 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.  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 mixing processes that subsequently transport heat, salt and nutrients from the UCDW up to the surface mixed layer. The study makes use of CTD and vessel-mounted acoustic Doppler current profiler data collected during 5 research cruises between March and September 2001.  The dominant cause of upward turbulent fluxes appears to be shear-driven mixing associated with near-inertial baroclinic waves excited by wind stress variations.  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.  Preliminary modeling studies suggest that baroclinicity of diurnal tides might contribute to vertical mixing near the shelf break, but further studies are needed to confirm this.  A previous suggestion that double-diffusive convection provides significant heat fluxes in this region is not supported by our analyses of the bulk properties of the pycnocline: the area-averaged double-diffusive heat flux is estimated at ~0.2-0.4 W m-2 over the central and inner continental shelf, and ~0.4-0.9 W m-2 over the continental slope.  



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