Investigating near-glacier circulation and plume theory with high-resolution fjord surveys in West Greenland

The plumes that emanate from Greenland's glaciers are hotspots of mixing between meltwater and ambient fjord waters. The dynamics of these plumes affect both submarine melting of glacier termini and the dilution of freshwater as it is exported into the ocean. Modeling studies often rely on buoyant plume theory to represent the circulation and mixing at the ocean/ice interface. However, a dearth of measurements in the near-glacier region has left open many questions about glacial plumes, entrainment, and the applicability of idealized plume theory to these environments. Here, we present near-glacier ocean surveys from Kangerdlugssuaq Sermerssua in central West Greenland in three consecutive summers (2013-2015). High-resolution measurements of velocity and water properties were collected by ship, by surface drifters, and by a remotely operated surface vessel - all focusing on the region within 2 km of the glacier terminus. These novel measurements of the 3D circulation capture a persistent near-surface plume, along with its time-evolution over a tidal cycle and between different summers. Concurrent multibeam sonar measurements of the submarine terminus morphology show that the plume emerges from a large undercut subglacial channel outlet. Plume theory, when applied with this fjord's stratification and any flux of subglacial discharge, cannot match the observed plume's volume flux and water properties. The discrepancy between our observations and plume theory suggests that there is enhanced entrainment at depth that is not adequately represented in plume theory. The details of this entrainment have important consequences for submarine melt rates, terminus morphology, and fjord circulation.