Coupled Ice Sheet-Ocean Modeling of the Filchner-Ronne sector using ISSM and FESOM-2

Coupled ice sheet and ocean models are vital for projecting the dynamics of the Antarctic Ice Sheet and for predicting future sea level rise. The Filchner-Ronne sector of Antarctica contains a number of deep-bedded ice streams and glaciers potentially vulnerable to the Marine Ice Sheet Instability. Previous work has shown that, in a warming climate, a mode switch in circulation could bring intrusions of warm Circumpolar Deep Water (CDW) that would increase basal melt rates near the deep grounding lines of these vulnerable glaciers. Furthermore, the adjacent Weddell Sea is an important site of global deep water formation that is heavily dependent on the export of Ice Shelf Water. Here, we develop a new ice-ocean coupling framework for linking the global Finite volumE Sea ice Ocean Model (FESOM-2) with the Ice-sheet and Sea-level System Model (ISSM), and we apply this framework to model the Filchner-Ronne sector of Antarctica and the adjacent Weddell Sea. We use adaptive mesh resolution for FESOM-2 ranging from 100 km elements in the far field down to 3 km in the Weddell Sea and the sub-ice cavity. Our ice sheet model resolution varies from 10 km down to ~300 m, with basal friction taken from an inversion fit to present-day surface velocities. We use offline coupling with a timestep of 1 year. We develop an adaptive filtering technique for the transmission of melt rates from the ocean model to the ice model that effectively removes numerical artifacts caused by the z-coordinate representation of the ice base in the ocean model while preserving true structure in the melt rate field. For the adaptation of the ocean model to the updated ice geometry, we develop an iterative horizontal-vertical extrapolation procedure for ocean tracers and a minimal smoothing procedure for ocean velocities to ensure that the ocean model can restart in a manner that is both realistic and numerically stable. Using this coupling architecture, we are able to directly restart the ocean model after the geometry change without requiring either a cold start or a spinup period with reduced timesteps and increased viscosity. We then simulate the evolution of the coupled ice-ocean system, including a moving calving front, over the next century under a range of climate forcing scenarios. We find that the projected mode switch to warm conditions in the Filchner-Ronne cavity happens earlier in our coupled model than in previous projections, with warm CDW first entering the Filchner cavity in ~2035 under SSP585 forcing, followed by ice shelf thinning, grounding line retreat, and grounded ice mass loss in the ensuing decades. By comparison, previous projections in strongly warming scenarios showed the CDW entering the cavity in 2050-2075. These results emphasize the rapid changes in the cryosphere and the Southern Ocean that could arise from continued anthropogenic warming, and the importance of coupled modeling for fully understanding the dynamics of the ice-ocean system.