Modeling the Influence of the Weddell Polynya on the Filchner–Ronne Ice Shelf Cavity

Abstract

Open-ocean polynyas in the Weddell Sea of Antarctica are the product of deep convection, which transports Warm Deep Water (WDW) to the surface and melts sea ice or prevents its formation. These polynyas occur only rarely in the observational record but are a near-permanent feature of many climate and ocean simulations. A question not previously considered is the degree to which the Weddell polynya affects the nearby Filchner-Ronne Ice Shelf (FRIS) cavity. Here we assess these effects using regional ocean model simulations of the Weddell Sea and FRIS, where deep convection is imposed with varying area, location, and duration. In these simulations, the idealized Weddell polynyas consistently cause an increase in WDW transport onto the continental shelf as a result of density changes above the shelf break. This leads to saltier, denser source waters for the FRIS cavity, which then experiences stronger circulation and increased ice shelf basal melting. It takes approximately 14 years for melt rates to return to normal after the deep convection ceases. Weddell polynyas similar to those seen in observations have a modest impact on FRIS melt rates, which is within the range of simulated interannual variability. However, polynyas that are larger or closer to the shelf break, such as those seen in many ocean models, trigger a stronger response. These results suggest that ocean models with excessive Weddell Sea convection may not be suitable boundary conditions for regional models of the Antarctic continental shelf and ice shelf cavities.

Two-timescale response of a large Antarctic ice shelf to climate change

“…the MITgcm ocean model (Massachusetts Institute of Technology general circulation model) 27,28 , which includes ice shelf thermodynamics and sea ice 2. the Úa ice flow model 29 , which governs the movement and geometry of the ice shelves and ice sheet 3. a custom-built coupler that handles the exchange of information between the two models MITgcm ocean. The ocean component is based on the configuration from Naughten et al 30 , which uses a Weddell Gyre domain with open boundaries at 30 ∘ E and 61 ∘ S. Horizontal resolution is 0.25 degrees scaled by cosine of latitude, and there are 120 vertical levels with 25 m resolution throughout the FRIS cavity. Bathymetry follows Bedmap2 31 with updates in the Filchner Ice Shelf cavity from Rosier et al 32 and the grounded iceberg A23-A as defined by Schaffer et al 33 .…”

“…The ocean component is based on the configuration from Naughten et al 30 , which uses a Weddell Gyre domain with open boundaries at 30 ∘ E and 61 ∘ S. Horizontal resolution is 0.25 degrees scaled by cosine of latitude, and there are 120 vertical levels with 25 m resolution throughout the FRIS cavity. Bathymetry follows Bedmap2 31 with updates in the Filchner Ice Shelf cavity from Rosier et al 32 and the grounded iceberg A23-A as defined by Schaffer et al 33 .…”

“…To ensure numerical stability on the long timescales used in this study, we do not use the z * coordinate system as in Naughten et al 30 , but instead employ MITgcm’s linear free surface scheme. We use an ice shelf drag coefficient of 0.0025 following new observational evidence 35 .…”

Remote Control of Filchner‐Ronne Ice Shelf Melt Rates by the Antarctic Slope Current

“…Our results suggest that ice‐shelf/ocean modelers should carefully consider their choice of boundary conditions when running limited area models of the region. In some idealized cases, it has been past practice to use climatological boundary conditions for regional modeling studies (e.g., Hazel & Stewart, 2020; Naughten et al., 2019) but consistency between the boundary conditions and the local forcing is important for realistic representation of inter‐annual variability in re‐analysis focused studies (e.g., Hausmann et al., 2020). More generally, Figures 1a–1f show differences in the observed and simulated temperature and salinity structure of the ASC across different products/time periods.…”

“…Recently, Naughten et al. (2019) showed that offshore polynyas in the Weddell Sea can increase the transport of WDW across the shelf through density changes above the shelf break. Hazel and Stewart (2020) suggest that decreasing the meridional winds by 50% can switch FRIS from a cold to warm state.…”