A nested high-resolution unstructured grid 3-D ocean-sea ice-ice shelf setup for numerical investigations of the Petermann ice shelf and fjord

Three-dimensional numerical simulation of circulation in fjords hosting marine-terminating ice shelves is challenging because of the complexity of processes involved in such environments. This often requires a comprehensive model setup. The following elements are needed: bathymetry (usually unknown beneath the glacier tongue), ice shelf draft (impacting water column thickness), oceanographic state (including tidal elevation, salinity, temperature and velocity of the water masses), sea ice and atmospheric forcing. Moreover, a high spatial resolution is needed, at least locally, which may be augmented with a coarser and computationally cheaper (nested) model that provides sufficiently realistic conditions at the boundaries. Here, we describe procedures to systematically create such a setup that uses the Finite Volume Community Ocean Model (FVCOM) for the Petermann Fjord, Northwest Greenland. The first simulations are validated against temperature and salinity observations from the Petermann Fjord in September 2019. We provide•

Complete bathymetry, ice-draft and water column thickness datasets of the Petermann Fjord, with an improved representation of the topography underneath the glacier tongue.

•

Boundary conditions for ocean, atmosphere and sea ice derived from a suite of high-resolution regional models that can be used to initialize and run the regional ocean model with realistic geophysical settings.

Development of Ice-Shelf Estuaries Promotes Fractures and Calving

As the global climate warms, increased surface meltwater production on ice shelves may trigger ice-shelf collapse and enhance global sea-level rise. The formation of surface rivers could help prevent ice-shelf collapse if they can efficiently evacuate meltwater. Here, we present observations of the evolution of a surface river into an ice-shelf estuary atop the Petermann Ice Shelf in northwest Greenland, and identify a second estuary at the nearby Ryder Ice Shelf. This surface hydrology process can foster fracturing and enhance calving. At the Petermann estuary, sea ice was observed converging at the river mouth upstream, indicating a flow reversal. Seawater persists in the estuary, after the surrounding icescape is frozen. Along the base of Petermann estuary, linear fractures were initiated at the calving front and propagated upstream along the channel. Similar fractures along estuary channels shaped past large rectilinear calving events at the Petermann and Ryder Ice Shelves. Increased surface melting in a warming world will enhance fluvial incision, promoting estuary development, longitudinal fracturing orthogonal to ice-shelf fronts, and increase rectilinear calving. Estuaries could develop in Antarctica within the next half-century, resulting in increased calving and accelerating both ice loss and global sea-level rise.

Tidal Modulation of Buoyant Flow and Basal Melt Beneath Petermann Gletscher Ice Shelf, Greenland

A set of collocated, in situ oceanographic and glaciological measurements from Petermann Gletscher Ice Shelf, Greenland, provides insights into the dynamics of under-ice flow driving basal melting. At a site 16 km seaward of the grounding line within a longitudinal basal channel, two conductivity-temperature (CT) sensors beneath the ice base and a phase-sensitive radar on the ice surface were used to monitor the coupled ice shelf-ocean system. A 6 month time series spanning 23 August 2015 to 12 February 2016 exhibited two distinct periods of ice-ocean interactions. Between August and December, radar-derived basal melt rates featured fortnightly peaks of ∼15 m yr-1 which preceded the arrival of cold and fresh pulses in the ocean that had high concentrations of subglacial runoff and glacial meltwater. Estimated current speeds reached 0.20 - 0.40 m s-1 during these pulses, consistent with a strengthened meltwater plume from freshwater enrichment. Such signals did not occur between December and February, when ice-ocean interactions instead varied at principal diurnal and semidiurnal tidal frequencies, and lower melt rates and current speeds prevailed. A combination of estimated current speeds and meltwater concentrations from the two CT sensors yields estimates of subglacial runoff and glacial meltwater volume fluxes that vary between 10 and 80 m3 s-1 during the ocean pulses. Area-average upstream ice shelf melt rates from these fluxes are up to 170 m yr-1, revealing that these strengthened plumes had already driven their most intense melting before arriving at the study site.

First-Order Estimates of Coastal Bathymetry in Ilulissat and Naajarsuit Fjords, Greenland, from Remotely Sensed Iceberg Observations

Warm water masses circulating at depth off the coast of Greenland play an important role in controlling rates of mass loss from the Greenland Ice Sheet through feedbacks associated with the melting of marine glacier termini. The ability of these warm waters to reach glacier termini is strongly controlled by fjord bathymetry, which was unmapped for the majority of Greenland’s fjords until recently. In response to the need for bathymetric measurements in previously uncharted areas, we developed two companion methods to infer fjord bathymetry using icebergs as depth sounders. The main premise of our methods centers around the idea that deep-drafted icebergs will become stranded in shallow water such that estimates of iceberg surface elevation can be used to infer draft, and thus water depth, under the assumption of hydrostatic equilibrium. When and where available, surface elevations of icebergs stranded on bathymetric highs were extracted from digital elevation models (DEMs) and converted to estimates of iceberg draft. To expand the spatial coverage of our inferred water depths beyond the DEM footprints, we used the DEMs to construct characteristic depth–width ratios and then inferred depths from satellite imagery-derived iceberg widths. We tested and applied the methods in two fjord systems in western Greenland with partially constrained bathymetry, Ilulissat Isfjord and Naajarsuit Fjord, to demonstrate their utility for inferring bathymetry using remote sensing datasets. Our results show that while the uncertainties associated with the methods are high (up to ±93 m), they provide critical first-order constraints on fjord bathymetry.

Summer surface melt thins Petermann Gletscher Ice Shelf by enhancing channelized basal melt

Increasing ocean and air temperatures have contributed to the removal of floating ice shelves from several Greenland outlet glaciers; however, the specific contribution of these external forcings remains poorly understood. Here we use atmospheric, oceanographic, and glaciological time series data from the ice shelf of Petermann Gletscher, NW Greenland to quantify the forcing of the ocean and atmosphere on the ice shelf at a site ~16 km from the grounding line within a large sub-ice-shelf channel. Basal melt rates here indicate a strong seasonality, rising from a winter mean of 2 m yr^-1 to a maximum of 80 m yr^-1 during the summer melt season. This increase in basal melt rates confirms the direct link between summer atmospheric warming around Greenland and enhanced ocean-forced melting of its remaining ice shelves. We attribute this enhanced melting to increased discharge of subglacial runoff into the ocean at the grounding line, which strengthens under-ice currents and drives a greater ocean heat flux toward the ice base.

The Dynamics of Greenland's Glacial Fjords and Their Role in Climate

Rapid mass loss from the Greenland Ice Sheet has sparked interest in its glacial fjords for two main reasons: Increased submarine melting of glaciers terminating in fjords is a plausible trigger for glacier retreat, and the anomalous freshwater discharged from Greenland is transformed by fjord processes before being released into the large-scale ocean. Knowledge of the fjords' dynamics is thus key to understanding ice sheet variability and its impact on climate. Although Greenland's fjords share some commonalities with other fjords, their deep sills and deeply grounded glaciers, the presence of Atlantic and Polar Waters on the continental shelves outside the fjords' mouths, and the seasonal discharge at depth of large amounts of surface melt make them unique systems that do not fit existing paradigms. Major gaps in understanding include the interaction of the buoyancy-driven circulation (forced by the glacier) and shelf-driven circulation, and the dynamics in the near-ice zone. These must be addressed before appropriate forcing conditions can be supplied to ice sheet and ocean/climate models.