Cryosphere-Ocean System

Could a dynamical instability in the Antarctic Ice Sheet be triggered within the next 40 years?

The objective is to examine the near-term risk of initiating the dynamic instability and onset of the collapse of the Antarctic Ice Sheet due to rapid melting by warming waters adjacent to the ice-sheet grounding lines. The experiment would be the first fully coupled global simulation to include dynamic ice shelf–ocean interactions for addressing the potential instability associated with grounding line dynamics in marine ice sheets around Antarctica. It will utilize several significant advances in the new ACME model, including the ability to enhance spatial resolution in both the ice sheet and ocean model to resolve grounding-line processes while still maintaining global extent in a coupled system and throughput for decadal simulations.

The simulation will include an eddy-resolving Southern Ocean as well to better represent Circumpolar Deep Water (CDW) and dynamics associated with bringing this water onto the continental shelf under the ice sheet. Including the sea-ice model captures the process of buttressing at the ice shelf–sea-ice boundary. Finally, a fully coupled system is able to simulate changes in atmospheric forcing (e.g., poleward displacement of jets) that could influence the behavior of the Southern Ocean and sea ice.

The specific experiment will be a fully coupled simulation from 1970–2050 to explore whether rapid ice-sheet instability is triggered in this time frame. An ensemble would be desirable to address the likelihood of such an event, though this is not likely to be affordable in our configuration in this timeframe. The model configuration for this experiment will be a modified version of the standard high-resolution ACME configuration described below. The base configuration includes the atmosphere/land on a 0.25° cubed-sphere grid using the ACME-modified CAM5-SE atmosphere model. The subgrid orography modifications will be needed to resolve Antarctic surface mass balance at the ice-sheet margins.

The ocean component will be MPAS-O on a Spherical Centroidal Voronoi Tesselations (SCVT) mesh with 15-km grid spacing at the equator, decreasing to 5 km in the Southern Ocean region. The default mesh will be extended southward to include critical Antarctic embayments and the resolution in these regions will be further enhanced if affordable. The vertical grid will be a hybrid coordinate with 100 vertical levels. The sea-ice component will be MPAS-CICE on the same ocean grid. Finally, we will add an Antarctic Ice Sheet model with resolution of 0.5–1 km near likely grounding-line locations and coarser resolution (~10 km) throughout the interior. For initial conditions, we will follow a similar spin-up procedure as with previous high-resolution simulations, with an ocean/ice state from an ocean/ice reanalysis-forced spin-up. For the ice sheet, an optimized initial condition should be available from the PISCEES project.

This first-of-its-kind coupled simulation will be focused largely on the ocean–ice shelf feedbacks and potential for dynamical instability and rapid SLR. It represents a first step toward a comprehensive SLR and impacts capability needed by the DOE to assess threats to coastal facilities. As work proceeds toward the more comprehensive experiments planned in the 10-year timeframe, we will be incrementally adding additional features. For example, work will begin under this project to develop an initial implementation of icebergs and primitive calving laws to capture the transport and distribution of ice and other material as the ice sheets flow into the ocean.

Work also continues (as part of related projects) on a Greenland Ice Sheet model so that we can capture SLR contributions from both major ice sheets. We will also begin to include isostasy and ice-sheet self-gravity that can have a first-order effect on the regional SLR signature around the coastal U.S. We anticipate all of these effects to be included in a following ACME version. Further releases will begin to include wave models, further focusing of resolution in coastal and storm-track regions, and other capabilities needed to further refine SLR impact at regional scales.