ALBERT

Description: Simulations of ice shelf basal melting in future climate scenarios from the IPCC’s Fourth Assessment Report (AR4) have revealed a large uncertainty and the potential of a rapidly increasing basal mass loss particularly for the large cold-water ice shelves in the Ross and Weddell Seas. The large spread in model results was traced back to uncertainties in the freshwater budget on the continental shelf, which is governed by sea ice formation. Differences in sea ice formation, in turn, follow the regional differences between the atmospheric heat fluxes imprinted by the climate models. A more recent suite of BRIOS and FESOM model experiments was performed with output from two members of the newer generation of climate models enganged in the IPCC’s Fifth Assessment Report (AR5). Comparing simulations forced with output from the AR5/CMIP5 models HadGem2 and MPI-ESM, we find that uncertainties arising from inter-model differences in high latitudes have reduced considerably. Projected heat fluxes and thus sea ice formation over the Southern Ocean continental shelves have converged to an ensemble with a much smaller spread than between the AR4 experiments. For most of the ten larger ice shelves in Antarctica, a gradual (but accelerating) increase of basal melt rates during the 21st century is a robust feature throughout the various realisations. Both with HadGem2 and with MPI-ESM forcing, basal melt rates for Filchner-Ronne Ice Shelf in FESOM increase by a factor of two by the end of the 21st century in the RCP85 scenario. For the smaller, warm-water ice shelves, inter-model differences in ice shelf basal mass loss projections are still slightly larger than differences between the scenarios RCP45 and RCP85; compared to AR4 projections, however, the model-dependent spread has been strongly reduced.

Description: In the framework of the EU project Ice2sea we utilize a global finite element sea ice - ice shelf - ocean model (FESOM), focused on the Antarctic marginal seas, to assess projections of ice shelf basal melting in a warmer climate. Ice shelf - ocean interaction is described using a three-equation system with a diagnostic computation of temperature and salinity at the ice-ocean interface. A tetrahedral mesh with a minimum horizontal resolution of 4 minutes and hybrid vertical coordinates is used. Ice shelf draft, cavity geometry, and global ocean bathymetry have been derived from the RTopo-1 data set. The model is forced with the atmospheric output from two climate models: (1) the Hadley Centre Climate Model (HadCM3) and (2) Max Planck Institute’s ECHAM5/MPI-OM coupled climate model. Data from their 20th-century simulations are used to evaluate the modeled present-day ocean state. Sea-ice coverage is largely realistic in both simulations. Modeled ice shelf basal melt rates compare well with observations in both cases, but are consistently smaller for ECHAM5/MPI-OM. Projections for future ice shelf basal melting are computed using atmospheric output for IPCC scenarios E1 and A1B. Trends in sea ice coverage depend on the scenario chosen but are largely consistent between the two forcing models. In contrast to this, variations of ocean heat content and ice shelf basal melting are only moderate in simulations forced with ECHAM5/MPI-OM data, while a substantial shift towards a warmer regime is found in experiments forced with HadCM3 output. A strong sensitivity to salinity distribution at the continental shelf break is found for the Weddell Sea, where in the HadCM3-A1B experiment warm water starts to pulse onto the southern continental shelf during the 21st century. As these pulses reach deep into the Filchner-Ronne Ice Shelf (FRIS) cavity, basal melting increases by a factor of three to six compared to the present value of about 100 Gt/yr. By the middle of the 22nd century, FRIS becomes the largest contributor to total ice shelf basal mass loss in this simulation.

Description: Dense shelf water is an essential ingredient to the formation of Antarctic Bottom Water (AABW). It is formed on the continental shelves surrounding Antarctica, when freezing rates are sufficiently high to push ocean salinity to values of 34.65 and higher. Coastal polynyas, where the ice is driven away from the coastline, maintain the highest freezing rates in Antarctic winter. Since the Weddell Sea is considered the most productive source region of AABW, we investigate the dense water formation on the continental shelves of the southwestern Weddell Sea, with a focus on the role of coastal polynyas, using the Finite Element Sea ice-Ocean Model (FESOM), a primitive-equation, hydrostatic ocean model coupled with a dynamic-thermodynamic sea ice model. The horizontal resolution of the global, unstructured mesh is up to 3 km at the southwestern Weddell Sea coastline; in vertical direction the mesh features 37 depth levels (resolution increases toward the surface). The model was initialized on 01/01/1980 with data from the Polar Hydrographic Climatology and forced with NCEP/NCAR Reanalysis data. The 20-year period 1990-2009 is used for analysis. Our results indicate that in an average year, the polynya freezing rates of 9 cm/d cause a seasonal variation in salinity of 0.3 psu under the Ronne polynya and result in the production of dense shelf water, which leaves the continental shelf (outlined by the 700 m isobath in this study) at a long-term mean volume flux of 5.2 Sv. Some of this water contributes to the formation of Weddell Sea Deep/Bottom Water, but a large fraction is diluted by mixing with ambient water and leaves the Weddell Sea at intermediate levels.

Description: Sub-ice shelf circulation and freezing/melting rates depend critically on an accurate and consistent representation of cavity geometry (i.e. ice-shelf draft and ocean bathymetry). Existing global or pan-Antarctic data sets have turned out to contain various inconsistencies and inaccuracies. The goal of this work is to compile independent regional fields into a global data set. We use the S-2004 global 1-minute bathymetry as the backbone and add an improved version of the BEDMAP topography for an area that roughly coincides with the Antarctic continental shelf. Locations of the merging line have been carefully adjusted in order to get the best out of each data set. High-resolution gridded data for the Amery, Fimbul, Filchner-Ronne, Larsen C and George VI Ice Shelves and for Pine Island Glacier have been carefully merged into the ambient ice and ocean topographies. Multibeam ship survey data for bathymetry in the former Larsen B cavity and the southeastern Bellingshausen Sea have been obtained from the data centers of Alfred Wegener Institute (AWI), British Antarctic Survey (BAS) and Lamont-Doherty Earth Observatory (LDEO), gridded, and again carefully merged into the existing bathymetry map. The resulting global 1-minute data set contains consistent masks for open ocean, grounded ice, floating ice, and bare land surface. The Ice Shelf Cavern Geometry Team: Anne Le Brocq, Tara Deen, Eugene Domack, Pierre Dutrieux, Ben Galton-Fenzi, Dorothea Graffe, Hartmut Hellmer, Angelika Humbert, Daniela Jansen, Adrian Jenkins, Astrid Lambrecht, Keith Makinson, Fred Niederjasper, Frank Nitsche, Ole Anders Nøst, Lars Henrik Smedsrud, and Walter Smith

Description: Projection of the forthcoming Antarctic contribution to sea-level rise is seriously hampered by the poor ability of current ice sheet models to properly compute comprehensive dynamics of the grounding line. This is a a serious limitation as large sectors present a bedrock below sea level and marine ice sheet instability may occur with drastic inland retreat of the grounding line. In order to circumvent this restriction we prescribe the grounding line migration in the global ice sheet model GRISLI. All regions presenting a bedrock lying below sea level are considered as unstable and a range of plausible migration rates from 500 to 3000 m/yr are imposed. The resulting simulations of sea level change are moderated using projections of future ocean warming in individual regions of the ice sheet’s coast. These latter estimates are based on results from the FESOM high-resolution, finite element ocean circulation model forced by sea-surface boundary conditions based on HadCM3 and ECHAM5 simulations under the A1B scenario. The probability distribution of projected sea-level contribution is estimated by incorporating uncertainty in the rates of grounding line retreat, the areas vulnerable to such retreat and the magnitude of ocean warming likely to trigger retreat.

Description: Simulations of ice shelf basal melting for several of the IPCC’s future climate change scenarios have revealed the potential of a rapidly increasing basal mass loss particularly for the large Filchner-Ronner Ice Shelf (FRIS) in the Weddell Sea. Basal melt rates in some of these simulations exceed 15 m/yr near the deep grounding lines in the southernmost part of the cavity; modeled basal mass loss rises to more than 1500 Gt/yr in the warmest and freshest scenario. These findings are consistent between two independent sea ice - ice shelf - ocean models forced with identical atmospheric data sets. However, they assume a steady-state ice shelf geometry. To study ice-ocean interaction in a more consistent way, the ice flow model RIMBAY has been configured in a model domain that comprises the FRIS and the grounded ice in the relevant catchment area up to the ice divides. At the base of the model ice shelf, melt rates from the finite-element sea ice – ice shelf – ocean model FESOM are prescribed. With FESOM’s increasing melt rates modelled for future climate warming scenarios, the ice model projects an accelerated grounding line retreat between the Möller and Institute Ice Streams. We use the ice shelf thickness evolution derived from RIMBAY to investigate the effect of a dynamically varying cavity geometry on simulated basal melt rates. A two-way coupling between the two models will be conducted as a natural next step.

Description: In the framework of the EU project Ice2sea we utilize a global finite element sea ice - ice shelf - ocean model (FESOM), focused on the Antarctic marginal seas, to quantify heat and freshwater fluxes in the Antarctic ice shelf cavities and to assess ice shelf basal melting in a warmer climate. Ice shelf - ocean interaction is described using a three-equation system with a diagnostic computation of temperature and salinity at the ice-ocean interface. A tetrahedral mesh with a minimum horizontal resolution of 4 minutes and hybrid vertical coordinates is used. Ice shelf draft, cavity geometry, and global ocean bathymetry have been derived from the RTopo-1 data set. Additional simulations were carried out with the circumpolar coarse-scale finite-difference model developed as part of the Bremerhaven Regional Ice Ocean Simulations (BRIOS). Simulations for present-day climate were forced with the NCEP reanalysis product and the atmospheric output from 20th century simulations of the Hadley Centre Climate Model (HadCM3). The projections for the period 2000-2199 use the output of HadCM3 simulations for the IPCC scenarios A1B and E1. Results from both models indicate a strong sensitivity of basal melting to increased ocean temperatures for the ice shelves in Amundsen Sea. An even stronger impact is found for warm water starting to pulse onto the southern Weddell Sea continental shelf in the middle of the 21st century, originating from a redirected coastal current. As these pulses propagate far into the Filchner-Ronne Ice Shelf (FRIS) cavity, basal melting increases significantly compared to the present value of about 100 Gt/yr. At the end of the 21st / beginning of the 22nd century both models suggest a stabilization of FRIS basal mass loss on a high level.