Wind control of circumpolar deep water intrusions to the Western Antarctic Peninsula shelf: weather-band and seasonal variability

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While we have previously addressed the role of the mesoscale in upper circumpolar deep water (UCDW) intrusions to the West Antarctic Peninsula (WAP) continental shelf and its origin in the baroclinic instability of the shelf break current (McKee et al, in prep), in this study we consider the advective component of the intrusions.  The sub-inertial current at a mooring on the WAP shelf, immediately adjacent to the eastern wall of Marguerite Trough canyon, is found to be strongly coherent with the along-slope wind stress across a continuum of time scales (weather-band through monthly).  A modal decomposition of three current meter records at different depths reveals that the response is strongly barotropic.  At weather-band time scales, the wind response may be explained by canyon-upwelling theory, which describes a time-dependent response to the perturbation of an initially-geostrophic shelf-break current.  At seasonal time-scales, the intruding current follows the seasonal cycle of the along-slope wind stress.  As the along-slope wind falls to near-zero in April, there is a minimum in the sub-inertial current (negative velocity) accompanying a cooler T_max and a collapse in pycnocline stratification.  This 'reversed' April circulation is interpreted as a net reduction in the canyon-upwelling process and, therefore, a reduction in the importance of the non-linear momentum advection terms.  This is consistent with the apparent flow relaxation towards linear potential-vorticity-conserving f / H contours near steep bathymetry and towards climatological geostrophic contours where bathymetry is flat.