Subglacial discharge-driven exchange flow over submarine sills in a glacial fjord: LeConte Bay, Alaska

Alexander Hager1, David Sutherland1, Rebecca H Jackson2, Christian Kienholz3, Jonathan D Nash4, Jason M Amundson3 and Roman J Motyka5, (1)University of Oregon, Department of Earth Sciences, Eugene, United States, (2)Rutgers University, New Brunswick, NJ, United States, (3)University of Alaska Southeast, Juneau, AK, United States, (4)Oregon State University, Corvallis, United States, (5)Univ Alaska-Geophysical Inst, Juneau, AK, United States
Bathymetry and subglacial discharge plumes are dominant controls on glacial fjord circulation, and thus advection of heat to glacier termini. Submarine sills at the mouth regulate fjord-shelf exchange flow, which is enhanced by entrainment of ambient fjord water into subglacial discharge plumes. It should therefore be expected that variations in subglacial discharge on diurnal to seasonal timescales are reciprocated by similar fluctuations in sill-modulated exchange flow, an assertion we explore at LeConte Bay, a shallow-silled glacial fjord in Alaska. We pair a suite of observations taken over two years with numerical model experiments to examine the interplay between temporally varying discharge and sill-modulated fjord renewal and exchange flow. Early results suggest LeConte Bay reaches an overmixing limit by mid-June, at which the depth of the outflowing plume is great enough to impose a hydraulic control at the fjord mouth, limiting exchange flow. Variations in discharge at LeConte Bay may therefore affect exchange flow in early summer, but not in late summer when the fjord is overmixed. If true, the overmixed condition would inhibit discharge-driven fjord renewal, despite substantially higher discharge in late summer, and shield the glacier terminus from a mechanism of heat transport.