Ocean Circulation and Variability Beneath Nioghalvfjerdsbrae (79 North Glacier) Ice Tongue

The floating ice tongue of 79 North Glacier, a major outlet glacier of the Northeast Greenland Ice Stream, has thinned by 30% since 1999. Earlier studies have indicated that long-term warming of Atlantic Intermediate Water (AIW) is likely driving increased basal melt, causing the observed thinning. Still, limited ocean measurements in 79 North Fjord beneath the ice tongue have made it difficult to test this hypothesis. Here we use data from an Ice Tethered Mooring (ITM) deployed in a rift in the ice tongue from August 2016 to July 2017 to show that the subannual AIW temperature variability is smaller than the observed interannual variability, supporting the conclusion that AIW has warmed over the period of ice tongue thinning. In July 2017, the AIW at 500m depth in the ice tongue cavity reached a maximum recorded temperature of 1.5 degrees C. Velocity measurements reveal weak tides and a mean overturning circulation, which is likely seasonally enhanced by subglacial runoff discharged at the grounding line. Deep inflow of AIW and shallow export of melt-modified water persist throughout the record, indicating year-round basal melting of the ice tongue. Comparison with a mooring outside of the cavity suggests a rapid exchange between the cavity and continental shelf. Warming observed during 2016-2017 is estimated to drive a 33 20% increase in basal melt rate near the ice tongue terminus and a 14 2% increase near the grounding line if sustained. Plain Language Summary The 79 North Glacier in northeastern Greenland has a floating ice tongue that extends 70km into 79 North Fjord. The floating ice has thinned by 30% since 1999, likely due to warming of the ocean underneath. To evaluate the role of the ocean in driving this change, we deployed sensors that measured ocean temperature, salinity, and currents in the fjord under the ice for 9months between August 2016 and July 2017. We find that the ocean was warmer during this period than in earlier observations and that the change over this period was small compared to changes between years. We also find that warm, salty water flows into the deep part of the fjord year-round, driving melting of the ice tongue. This results in relatively cold and fresh waters being exported at shallower depths. By comparing these observations with a record from outside the fjord, we show that ocean changes on the continental shelf rapidly affect the ocean under the ice tongue. Finally, we observe similarly warm ocean temperatures in summer 2018, suggesting that warming observed in 2016-2017 was sustained for at least 1year, likely driving continued thinning of the ice tongue.