2009 Chapman Lecture

Ice, Winds, and River Runoff: Arctic Shelf Processes

Dr. Tom Weingartner
Institute of Marine Science
University of Alaska

Tuesday, November 4, 2009, 3:00 p.m.
Clark Building, Room 507
Reception to follow

This talk provides an overview of seasonal variations on the inner shelf (inshore of the ~20 m isobath) of the Alaskan Beaufort Sea.  The seasonality is driven by the annual thaw/melt cycle, which modifies the influence of the predominantly along-shelf (and upwelling-favorable) winds and river runoff.  For up to 8 months (October-May) of the year a nearly immobile lid of landfast ice covers the inner shelf thus inhibiting the direct influence of the wind on mixing and circulation.  Hence, the underice circulation is weak and appears to be a response to offshore winds, underice friction, and perhaps, spatial variations in the offshore extent of the landfast ice.  In June, the inner shelf remains ice-covered but receives a large, impulsive influx of river runoff.  The river discharge strongly stratifies the inner shelf and creates underice river plumes whose dynamics are likely influenced by frictional contact with the ice cover and a surface buoyancy flux from the melting ice cover.  By late June the ice retreats from the coast, so that winds directly affect the inner shelf.  However, the wind-driven response is substantially altered by the stratification.  Moreover, the accumulation of freshwater from ice melt and runoff, along with relatively weak winds, likely results in energetic mesoscale motions associated with frontal instabilities.  By mid-September, strong storm winds force vigorous along-shelf flows that can easily replace the volume of the inner shelf prior to the onset of the landfast ice.  Consequently, storm winds are probably instrumental in establishing the water mass properties on the inner shelf at the onset of the landfast ice season.

Although the discussion is based on observations drawn from the Alaskan Beaufort Sea, the seasonally varying processes outlined above are likely common to other Arctic ‘interior’ shelves, where the landfast ice cover can occupy ~20% of the shelf area.  These processes remain poorly understood but they are critical in controlling the cross-shelf dispersal of the terrestrial runoff that profoundly influences the stratification and biogeochemistry of the Arctic Ocean basin.

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