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

Abstract

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 500 m depth in the ice tongue cavity reached a maximum recorded temperature of 1.5°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.

Ocean Variability at Greenland's Largest Glacier Tongue Linked to Continental Shelf Circulation

“…Those results are in line with Lindeman et al. (2020), who found evidence for fast changes in T‐S characteristics associated with isopycnal oscillations induced by such waves.…”

“…At that location, Lindeman et al. (2020) observed a major shift in the circulation pattern end of November, simultaneous to the increase in M2 inflow and M1 speeds observed in this study, indicated for example, by the sign change of the PC1 of the intra‐annual variability (see Section 3.3, Figure 5). Schaffer et al.…”

“…We consider variability with periods smaller than 0.25 days to be high-frequency variability including noise and do not discuss them further. We constructed the filters using a Hanning filter (low-pass filter, cut-off period T = 30 days) and Butterworth filters of fourth-order with zero-phase (band-pass filters with cut-off periods of T = 2, T = 0.25 days) using the Matlab toolbox of J. Lilly (Lilly, 2017). We estimated tidal currents from the unfiltered velocity data using classical harmonic analysis as implemented in the Matlab routine T_Tide written by R. Pawlowicz, B. Beardsley, and S. Lentz (Pawlowicz et al, 2002).…”

Submarine Meltwater From Nioghalvfjerdsbræ (79 North Glacier), Northeast Greenland

“…Warm AIW flows into the ice shelf cavity and induces submarine melting (Schaffer et al., 2020). The maximum AIW temperature below the floating ice tongue that was measured by moored instruments at 500 m depth was 1.5°C in July 2017 (Lindeman et al., 2020).…”

Tidal Modulation of Buoyant Flow and Basal Melt Beneath Petermann Gletscher Ice Shelf, Greenland

“…This type of mooring has, therefore, not been utilized as extensively in Greenland as in Antarctica. Indeed, only four sub-ice-shelf moorings have been deployed in Greenland: one beneath Nioghalvfjerdsrae Ice Shelf in the northeast (Lindeman et al, 2020) and three beneath Petermann Gletscher Ice Shelf (PGIS) in the north (Münchow et al, 2016;Washam et al, 2018Washam et al, , 2019. We here report on ocean time series data from two SBE 37-SM Microcat Conductivity-Temperature (CT) sensors located along one of these mooring lines beneath PGIS within 5 and 25 m of the ice base.…”