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

Abstract

Mass loss from the Greenland ice sheet has increased over the past two decades, currently accounting for 25% of global sea-level rise. This is due to increased surface melt driven by atmospheric warming and the retreat and acceleration of marine-terminating glaciers forced by oceanic heat transport. We use ship-based profiles, bathymetric data and moored time series from 2016 to 2017 of temperature, salinity and water velocity collected in front of the floating tongue of the 79 North Glacier in Northeast Greenland. These observations indicate that a year-round bottom-intensified inflow of warm Atlantic Water through a narrow channel is constrained by a sill. The associated heat transport leads to a mean melt rate of 10.4 ± 3.1 m yr–1 on the bottom of the floating glacier tongue. The interface height between warm Atlantic Water and colder overlying water above the sill controls the ocean heat transport’s temporal variability. Historical hydrographic data show that the interface height has risen over the past two decades, implying an increase in the basal melt rate. Additional temperature profiles at the neighbouring Zachariæ Isstrøm suggest that ocean heat transport here is similarly controlled by a near-glacier sill. We conclude that near-glacier, sill-controlled ocean heat transport plays a crucial role for glacier stability.

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Fig. 1: Circulation and bathymetry around Northeast Greenland and summer 2016/2017 surveys along the 79NG calving front.
Fig. 2: Oceanic measurements at the 79NG calving front in 2016/2017.
Fig. 3: Temporal variability of the cavity overturning and of the heat for melting the underside of the 79NG.
Fig. 4: Sketch of the cavity circulation and water masses at the 79NG.

Data availability

Processed CTD data from 2016 (https://doi.org/10.1594/PANGAEA.871025) and 2017 (https://doi.org/10.1594/PANGAEA.885358) are available at the World Data Center PANGAEA. CTD raw data files from 2016 and 2017 are available at https://doi.org/10.1594/PANGAEA.871701 and https://doi.org/10.1594/PANGAEA.883366. Raw LADCP data are available at https://doi.org/10.1594/PANGAEA.870995 for 2016; data from 2017 are available at https://doi.org/10.1594/PANGAEA.904021. Raw mooring data are available at https://doi.pangaea.de/10.1594/PANGAEA.904023 (for mooring recoveries in 2017) and at https://doi.org/10.1594/PANGAEA.904022 (for mooring recoveries in 2018). Processed mooring data are found at the World Data Center PANGAEA under https://doi.org/10.1594/PANGAEA.909471 together with a report on data processing (Schaffer, Janin (2019): Report on Mooring processing of PS109/PS114 recoveries (NE Greenland continental shelf), 9 pp, hdl:10013/epic.4cf66b0e-b6c2-4e0c-aef1-a709c493c1dc). Temperature profiles in front of ZI taken in 2016 are available at https://doi.org/10.1594/PANGAEA.870997; 2017 data are available at https://doi.org/10.1594/PANGAEA.904016. A collection of historic and recent CTD profiles carried out on the Northeast Greenland continental shelf24 are available from J.S. (janin.schaffer@awi.de) on request. The full-resolution bathymetry data from multibeam echo soundings in front of the 79NG are available from D.H.R. (d.h.roberts@durham.ac.uk) on request. The interpolated bathymetry grid for Northeast Greenland with a 250 m grid resolution that includes multibeam echo-sounding data collected in 2016 in front of 79NG and depth information collected close to ZI is available at https://doi.org/10.1594/PANGAEA.909628. Maps based on the updated RTopo-2 dataset are available at https://doi.org/10.1594/PANGAEA.905295. Satellite images recorded by Landsat 8 on 28 April 2016 (Fig. 1c,d) and 7 September 2016 (Supplementary Fig. 2) can be downloaded from Earth Explorer (https://earthexplorer.usgs.gov/) courtesy of the US Geological Survey. Ice velocities based on ‘Greenland ice velocity map 2017/2018 from Sentinel-1 [version 1.0]’ and the grounding-line position derived from ERS-1/-2 SAR and Sentinel-1 SAR interferometry are available from ENVEO within the ESA Initiative Greenland Ice Sheet CCI (https://esa-icesheets-greenland-cci.org/).

Code availability

MATLAB routines used for data processing and analysis are available from the corresponding author on request.

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Acknowledgements

We thank the captains and crew of R/V Polarstern, the helicopter crews and the German weather forecasters for their support in making our measurements at 79NG and ZI possible; A. Münchow for supplying moored instrumentation (mooring B) and helpful discussions; M. Monsees, C. Engicht, G. Budéus and R. Graupner for instrument preparation and support; G. Rohardt and A. Wisotzki for processing CTD data; R. Timmermann for discussions and constructive feedback; J. M. Lloyd, C. ÓCofaigh, S. L. Callard, M. Kappelsberger, H. Grob and B. Dorschel for running the multibeam echo-sounding system; and all other participants of R/V Polarstern expeditions PS100 and PS109 for their support. J.S. acknowledges support from the German Federal Ministry for Education and Research (BMBF) within the GROCE project (grant 03F0778A). Support for this study was also provided by the Helmholtz Infrastructure Initiative FRAM and by the Natural Environment Research Council (NERC) for the NEGIS project ‘Greenland in a warmer climate: What controls the advance & retreat of the NE Greenland Ice Stream’ (grant NE/N011228/1). Ship time was provided under grants AWI_PS100_01, AWI_PS109_03 and AWI_PS114_01.

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J.S., T.K. and W.-J.v.A. conceived the study; J.S., T.K., W.-J.v.A. and L.v.A. participated in the collection of oceanographic data; J.S. and W.-J.v.A. processed the mooring data; L.v.A. and W.-J.v.A. processed LADCP data; D.H.R. collected the bathymetric data; J.E.A. processed bathymetric data; J.S. was responsible for data analysis and J.S., T.K. and W.-J.v.A. interpreted the data. J.S. wrote the manuscript and all authors commented at all stages.

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Correspondence to Janin Schaffer.

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Supplementary discussion, methods, Figs. 1–3 and Table 1.

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Schaffer, J., Kanzow, T., von Appen, WJ. et al. Bathymetry constrains ocean heat supply to Greenland’s largest glacier tongue. Nat. Geosci. 13, 227–231 (2020). https://doi.org/10.1038/s41561-019-0529-x

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