Seasonal fluctuations in the advance of a tidewater glacier and potential causes: Hubbard Glacier, Alaska, USA

Satellite imagery has been used to acquire seasonal terminus positions of tidewater Hubbard Glacier, Alaska, USA, from 1992 to 2006. During this 15 year time period, the width-averaged advance of the entire terminus has been 620m at a mean rate of 35ma. Seasonal fluctuation of the terminus ranges from 150 to 200m on average and varies spatially. A section of the terminus, near a narrow gap where the glacier has now twice closed off 40 km long Russell Fiord, exhibited little to no mean advance during this time period but displayed seasonal fluctuations of 300–500m. Seasonal variability in surface ice speeds and surface sea-water temperatures was also observed; both are potential forcing mechanisms for terminus fluctuations. Seasonal changes in sea-water temperature of 10–128C, as well as seasonal changes in subglacial freshwater discharge, are inferred to influence calving and submarine melting at the terminus, driving seasonal variations. Displacements of the medial moraine separating Hubbard and Valerie Glaciers at the terminus suggest surge-like pulses of the latter, with a periodicity of several years. The timing of these pulses suggests they may influence the Hubbard terminus near Gilbert Point and have implications for future closures of Russell Fiord.