Ocean Circulation Connecting Fram Strait to Glaciers off Northeast Greenland: Mean Flows, Topographic Rossby Waves, and Their Forcing

Abstract

From 2014 through 2016 we instrumented the ~80-km-wide Norske Trough near 78°N latitude that cuts across the 250-km-wide shelf from Fram Strait to the coast. Our measurements resolve a ~10-km-wide bottom-intensified jet that carries 0.27 ± 0.06 Sv (1 Sv ≡ 106 m3 s−1) of warm Atlantic water from Fram Strait toward the glaciers off northeast Greenland. Mean shoreward flows along the steep canyon walls reach 0.1 m s−1 about 50 m above the bottom in 400-m-deep water. The same bottom-intensified vertical structure emerges as the first dominant empirical orthogonal function that explains about 70%–80% of the variance at individual mooring locations. We interpret the current variability as remotely forced wave motions that arrive at our sensor array with periodicities longer than 6 days. Coherent motions with a period near 20 days emerge in our array as a dispersive topographic Rossby wave that propagates its energy along the sloping canyon toward the coast with a group speed of about 63 km day−1. Amplitudes of wave currents reach 0.1 m s−1 in the winter of 2015/16. The wave is likely generated by Ekman pumping over the shelfbreak where sea ice is always mobile. More than 40% of the along-slope ocean current variance near the bottom of the canyon correlates with vertical Ekman pumping velocities 180 km away. In contrast, the impact of local winds on the observed current fluctuations is negligible. Dynamics appear linear and Rossby wave motions merely modulate the mean flow.

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

“…There is noticeable variability in the currents and ocean properties observed at the ITM on time scales of days to weeks, with larger variance at shallower depths (Figures 3a–3d), though this is not associated with a significant spectral peak. Münchow et al (2020) find that topographic Rossby waves generated by offshore Ekman pumping propagate along Norske Trough with periods of 6–20 days. This could drive oscillations in the AIW layer thickness at the sill outside the pinned front, which may result in isopycnal oscillations in the cavity as suggested above, as well as modulating the inflow strength and causing variability in currents observed at the ITM (Schaffer et al, 2020).…”

“…The sensitivity of ice tongue melt to external ocean variability depends on its timing and magnitude as well as the time scales and mechanisms of exchange between the cavity and shelf (e.g., Holland, 2017). A melt‐driven overturning circulation alone would drive a relatively slow exchange with the shelf (Wilson & Straneo, 2015), but recent studies indicate that AIW inflow into the 79NG cavity is hydraulically controlled by a sill outside the pinned front (Schaffer et al, 2020; Zhao et al, 2019) and may be modulated by remotely forced topographic Rossby waves (Münchow et al, 2020). Studies of other Greenland fjords have shown that exchange is enhanced by energetic shelf‐driven flows and discharge of summer surface melt at the grounding line, both of which can accelerate renewal of fjord waters (Jackson et al, 2014; Straneo et al, 2010; Washam et al, 2018).…”

Bathymetry constrains ocean heat supply to Greenland’s largest glacier tongue

“…In particular, the relevance of shelf wind fields 31 that were shown to drive winterenhanced heat fluxes across continental shelves toward other Greenlandic glacial fjords 32,33 need to be investigated. For the continental shelf offshore the 79NG a recent study shows that energetic topographic Rossby waves (at periods shorter than one month) propagate along the inflow pathway of AIW from the shelf edge toward the 79NG 34 . We posit that such wave processes may have the potential of modulating the AIW transports into the cavity by displacing the AIW interface and thus hydraulic control.…”

“…We filtered the volume, advective heat, and salt flux time series (and all related time series discussed below) with a lowpass-filter using a Hann window of two weeks. The cut-off period has been chosen in order to still resolve the effects of topographic Rossby waves that are shown to be relevant for propagation of energy toward the coast with the peak energy in the shelf circulation at 20-day periods 34 Heat and salt storage from moored records. We estimate the heat and salt storage terms 39 (equations (2) and (4)) based on the assumption that the variability at the calving front is representative of the variability over the entire control volume within the subglacial cavity.…”

“…The adjacent bay is covered by fast-ice most of the year, which since 2000 has been breaking up more regularly in summertime than in previous years 22 . Below a surface layer of Polar Water, hydrographic measurements showed the presence of Atlantic Intermediate Water (AIW) in the trough system on the wide continental shelf 23,34 of Northeast Greenland (Fig. 1a, b) and within the cavity below the 80-km-long floating tongue of the 79NG [25][26][27] .…”