| Summary: | Jakobshavn Isbrae is a major outlet glacier in West Greenland that lost its protective ice shelf in 2002 and has been speeding up and retreating since. We image its grounding line for the first time with a ground portable radar interferometer deployed in 2016 and detect its migration at tidal frequencies. The southern half of the glacier develops a floating section (3 km x 3 km) that migrates in phase with the tide up to a distance of 1.5 km, which is far more than expected from flotation. We attribute the migration to kilometer-scale seawater intrusions, 10-20 cm in height, occurring at high tide. The intrusions reveal that the glacier bed must be 100-600 m deeper than expected on the south side, which illustrates that our knowledge of bed topography remains limited in this sector. We expect seawater intrusions to cause rapid melt of basal ice and play a major role in the glacier evolution. Funding provided by: University of California, Irvine Crossref Funder Registry ID: http://dx.doi.org/10.13039/100008476 Award Number: Funding provided by: National Aeronautics and Space Administration Crossref Funder Registry ID: http://dx.doi.org/10.13039/100000104 Award Number: 80NSSC21K1620
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