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 sheet1–3, which at present accounts for about a quarter of global sea level rise4. 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 2011 to May 2012 for Sermilik Fjord-into which HelheimGlacierdrains-and fromSeptember2009toMay2010 for Kangerdlugssuaq Fjord. 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 from the shelf outside the fjord. The pulses drive rapid water exchange with the shelf and renew warm water in the fjord more e ectively than any glacial freshwater-driven circulation. Our observations suggest that, during non-summermonths, the glaciermelt rate varies substantially and depends on externally forced ocean flows that rapidly transport changes on the shelf towards the glaciers’ margins. The submarine melt rate depends on near-glacier ocean temperature and circulation5. Recent studies assume that both these things are governed by the glacier’s freshwater inputs6–10, and that other drivers, such as tides, air–sea fluxes and shelf-driven exchange, can be neglected. In this prevailing framework, submarine meltwater and subglacial discharge (surface meltwater draining at the glacier’s base) form buoyant plumes, entrain ambient water and drive an overturning circulation that transports shelf waters towards the glacier. Under this assumption, enhanced subglacial discharge increases ocean heat transport, submarine melting and the renewal of near-glacier waters6–8. However, the extent to which the glacier-driven circulation influences the renewal of warm water in these fjords is unclear. Limited velocity data indicates that shelf variability may play an important role in driving fjord flows10–12, although conclusive evidence is absent. Furthermore, most observational studies rely on brief, summer surveys2,10,13–17 that offer limited insight into drivers of summer variability and no information about non-summermonths. Here, we present new insight into fjord dynamics from moored records in Sermilik and Kangerdlugssuaq fjords (Fig. 1a), where Helheim and Kangerdlugssuaq Glaciers (the fifth and third largest outlets of the Greenland ice sheet, respectively18) deposit freshwater as submarine meltwater, subglacial discharge, surface runoff and icebergs. The vertical calving fronts of both glaciers ground ∼600m below sea level at the head of their respective fjords (∼70/100 km long and∼6 kmwide)19, which connect the glaciers to the continental shelf. The predominant water masses of Greenland’s southeast shelf form a two-layer structure within Sermilik Fjord11,20: cold, fresh Polar-origin water (PW) overlying warm, salty Atlanticorigin water (AW), with some modification due to glacial inputs16. A similar water mass structure is found in Kangerdlugssuaq, with an additional dense Atlantic water mass originating in the Nordic Seas19. In both Sermilik and Kangerdlugssuq fjords, the shallowest sills (at 530 and 550m, respectively) are well below the AW/PW interface13,21,22, allowing relatively unimpeded exchange between the fjord and shelf (Fig. 1a,b). The shelf region of southeast Greenland outside both fjords is characterized by frequent, strong, along-shore winds23 and fast ocean currents20. Winds and glacial freshwater discharge—two potential drivers of fjord circulation—exhibit a strong seasonality in this region. From September through May, shelf winds are strong along Greenland’s southeast coast23, and subglacial discharge is negligible as air temperatures drop below freezing24. During summer, winds weaken, and subglacial discharge increases, becoming a larger freshwater source than submarine melt24. This seasonality probably modulates the glacier-driven circulation and submarine melt rate; one modelling study estimated the summer melt rate at Helheim Glacier to be twice the non-summer rate as a result of variations in subglacial discharge7. According to this scaling, nevertheless, 60% of the annual submarine melt would occur in non-summer months, an important but unstudied period. Extensive oceanic water property and velocity records (Methods) were collected during non-summer months in 2011–2012 from Sermilik Fjord and the adjacent shelf (Fig. 1a–d) and in 2009–2010 from Kangerdlugssuaq Fjord (excluding velocity, Fig. 1a,e). In Sermilik Fjord, the records indicate that AW and PW are always present, but their properties and thicknesses vary over timescales of hours to months (Fig. 1d). The upper water column near the AW/PW interface exhibits the largest variability, a result of seasonal trends11—for example PW deepening and cooling in the winter—and higher frequency fluctuations in the interface’s depth, often exceeding 50m over several days (Fig. 1d). Within the AW layer, temperature ranges from 2 C to 5.2 C and exhibits transient fluctuations (typically 0.3–0.7 C) that last several days, as well as more sustained shifts, such as an abrupt cooling in late March. Significant variability exists even at 851m, well below sill depth. The fjord’s persistent stratification and occasional increases in heat content preclude internal mixing (which would reduce stratification and redistribute heat) or surface fluxes (which would