Sensitivity of submarine melting on North East Greenland towards ocean forcing

Abstract

Abstract. The Nioghalvfjerdsbræ (79NG) is a floating ice tongue on Northeast Greenland draining a large part of the Greenland Ice Sheet. A CTD profile from a rift on the ice tongue close to the northern front shows that Atlantic Water (AW) is present in the cavity below, with maximum temperature of approximately 1 °C at 610 m depth. The AW present in the cavity thus has the potential to drive submarine melting along the ice base. Here, we simulate melt rates from the 79NG with a 1D numerical Ice Shelf Water (ISW) plume model. A meltwater plume is initiated at the grounding line depth (600 m) and rises along the ice base as a result of buoyancy contrast to the underlying AW. Ice melts as the plume entrains the warm AW. Maximum simulated melt rates are 50–76 m yr−1 within 10 km of the grounding line. Within a zone of rapid decay between 10 km and 20 km melt rates drop to roughly 6 m yr−1. Further downstream, melt rates are between 15 m yr−1 and 6 m yr−1. The melt-rate sensitivity to variations in AW temperatures is assessed by forcing the model with AW temperatures between 0.1–1.4 °C, as identified from the ECCOv4 ocean state estimate. The melt rates increase linearly with rising AW temperature, ranging from 10 m yr−1 to 21 m yr−1 along the centerline. The corresponding freshwater flux ranges between 11 km3 yr−1 (0.4 mSv) and 30 km3 yr−1 (1.0 mSv), which is 5 % and 12 % of the total freshwater flux from the Greenland Ice Sheet since 1995, respectively. Our results improve the understanding of processes driving submarine melting of marine-terminating glaciers around Greenland, and its sensitivity to changing ocean conditions.

Response to Reviewer#1

“…Although such a retreat is supported by proxies for the mid-Holocene (Bennike and Weidick, 2001;Larsen et al, 2018), its persistence until the present day is clearly unrealistic. Moreover, it is likely that imposing PD submarine melting peaks of 50-75 m a −1 as estimated at the 79N grounding line (Anhaus et al, 2019;Wilson and Heimbach, 2017), which are even higher than the B ref values considered in this work, could cause a further inland retreat. This bias could be related to (1) the low spatial resolution of the model (10 km), which does not allow for a precise treatment of the grounding-line zone and may trigger non-linear effects, enhancing grounding-line retreat farther inland than expected;…”

“…Moreover, the resulting B ref values are in the range of the submarine melt observed at the grounding line of PD Greenland glaciers that have floating ice shelves (Wilson and Heimbach, 2017). Melting at the base of the ice shelves is defined to be 10 % of that calculated at the grounding line which reflects the decrease of melting rate observed towards the ice shelves (Anhaus et al, 2019;Münchow et al, 2014;Rignot and Steffen, 2008;Wilson and Heimbach, 2017). However, this decrease is not parameterised here as a function of the distance from the grounding line.…”

Submarine melt as a potential trigger of the North East Greenland Ice Stream margin retreat during Marine Isotope Stage 3

“…For the sake of simplicity, T clim,ocn − T f can be considered to be spatially (horizontally and vertically) constant, in the way that B ref is defined to scale directly with κ. Here, we prescribe T clim,ocn − T f = 1 K; thus B ref = κ · 1 K. Also, the glacial-interglacial temperature anomaly T LGM,ocn − T PD,ocn is considered here to be spatially uniform and is set to a value of −1 K (Annan and Hargreaves, 2013;MARGO, 2009). With these simplifications, the system of Eq.…”

“…To the best of our knowledge, little evidence on oceanic changes at orbital timescales is available, and whether the best representation of reality would be through oceanic temperatures varying in phase or antiphase with the atmosphere is unclear. However, proxy-based temperature reconstructions indicate glacial-interglacial surface temperature anomalies to be between 0 and −3 K (Annan and Hargreaves, 2013;MARGO, 2009), and the value chosen for T LGM,ocn − T PD,ocn is within this range (−1 K).…”