Externally forced fluctuations in ocean temperature at Greenland glaciers in non-summer months

Enhanced submarine melting of outlet glaciers has been identified as a plausible trigger for part of the accelerated mass loss from the Greenland ice sheet(1-3), which at present accounts for a quarter of global sea level rise(4). However, our understanding of what controls the submarine melt rate is limited and largely informed by brief summer surveys in the fjords where glaciers terminate. Here, we present continuous records of water properties and velocity from September to May in Sermilik Fjord (2011-2012) and Kangerdlugssuaq Fjord (2009-2010), the fjords into which the Helheim and Kangerdlugssuaq glaciers drain. We show that water properties, including heat content, vary significantly over timescales of three to ten days in both fjords. This variability results from frequent velocity pulses that originate on the shelf outside the fjord. The pulses drive rapid water exchange with the shelf and renew warm water in the fjord more effectively than any glacial freshwater-driven circulation. Our observations suggest that, during non-summer months, the glacier melt rate varies substantially and depends on externally forced ocean flows that rapidly transport changes on the shelf towards the glaciers' margins.