Petermann Glacier Ice Shelf in a warming world : Insights from 3-D numerical modelling of ice shelf-ocean interactions at Petermann Fjord, Northwest Greenland

Sammanfattning: The Greenland Ice Sheet (GrIS) is currently the largest contributor to global mean sea level rise, and contemporary mass loss rates are likely lower bounds for the rates to be observed in decades to come. At present, marine outlet glaciers along the northern GrIS margin, with an ice volume estimated at 400 cm mean global sea level rise equivalent, are still largely buttressed by ice shelves. However, thinning and retreat of these ice shelves, combined with perturbations of the outlet glacier’s grounding line (GL) can lead to a loss of backstress and accelerated mass loss via dynamic ice discharge, and likely render the latter the major contributor (as opposed to surface mass balance) to GrIS mass loss towards the end of the century.Here, the focus is on processes that drive basal melting of the Petermann Glacier Ice Shelf (PGIS), northwest Greenland, because contemporary knowledge regarding the full spectrum of mechanisms that dictate basal melting, and how they respond to a warming climate, is incomplete. This often results in poorly constrained oceanic boundary conditions, and consequently, afflicts estimates of GrIS’s contribution to future sea level rise with uncertainty.To address these questions, a non-idealized, nested, three-dimensional ocean-sea ice-ice shelf setup centered on PGIS and Petermann Fjord (PF) was created, based on the Finite Volume Community Ocean Model. With the setup developed and a “standard run” validated against observations from the fjord, the following scientific questions were investigated: How are basal melt rates at PGIS affected by 1. the presence, and likely future absence, of sea ice arches in Nares Strait? 2. subglacial discharge (Qsg), through increased surface runoff from the GrIS and entering PF across the GL? 3. changes in the PGIS cavity geometry in a post future-calving scenario?Our results indicate that climate warming driven transition towards a mobile and thin sea ice cover from a landfast and thick one could result in up to twofold increase in melt. In such a scenario, wind and convectively upwelled warm Atlantic Water enter the PGIS cavity. Further, in summer, under the deeper regions of PGIS, more efficient melting occurs in a more turbulent cavity, without any noticeable increase in thermal driving. We find that the presence of Qsg at the GL, and its subsequent increase in a warming atmosphere, increases melt by more than threefold. Melting also shows strong sensitivity to how Qsg is routed across the GL. Importantly, we uncover that if Qsg increases beyond 100% of present summer mean estimates, PGIS cavity enters a shear-controlled regime. Here, enhanced turbulent heat delivered by the vertical shear of the Qsg intensified current is sufficient to drive substantial increase in melt, even if there is no further increase in ocean heat forcing. Following the loss of the outer regions of PGIS post-calving, we find that wind enhanced fjord-scale currents act in concert with the Qsg at the GL to strengthen the overturning circulation, thereby increasing the basal melt. In particular, we see up to threefold increase in melt in large sections of the basal channels under the deeper PGIS draft near the GL. These results suggest that intensified basal melting of PGIS in a warming climate; in particular, of its dynamically significant and resilient deeper regions, could accelerate mass loss from Petermann Glacier, with major implications for GrIS’s contribution to future sea level rise.