Meltwater Triggers for an Antarctic Ice-Shelf Melt Tipping Point
Simulation of intrusion of warm offshore water mass into Filchner-Ronne Ice Shelf cavity as near-shore density blockage (turquoise line) becomes shallower. Ice-shelf basal melt rates subsequently increase by an order of magnitude.
Matthew Hoffman, Los Alamos National Laboratory
The largest ice shelf in Antarctica may be susceptible to a tipping point leading to a manyfold increase in melting from the ocean if warm, deep, offshore water gains access to the ice-shelf base. Using a series of experiments with the Energy Exascale Earth System Model (E3SM), we demonstrate that changes in meltwater from icebergs or nearby ice shelves may be sufficient to trigger this tipping point.
Our results show that simulating ice-shelf basal melting realistically in global Earth system models requires careful treatment of Antarctic meltwater and ocean missing in low resolution ocean models. Earth system model projections of the future of the Antarctic Ice Sheet need to demonstrate skill in representing present-day Southern Ocean density structure to be credible.
Filchner-Ronne Ice Shelf (FRIS), the largest ice shelf in Antarctica, is believed to be susceptible to a tipping point from a low to high ice-shelf basal melt regime in the future. This could occur if ocean density on the continental shelf around Antarctica decreases to the point it can no longer block the intrusion of warm, deep, offshore water. Version 1 of E3SM was prone to erroneously crossing the FRIS melt tipping point under preindustrial conditions due to model biases in ocean density. This work identifies controls on the FRIS instability and how they should be treated in a low-resolution global Earth system model like E3SM. We compared E3SM v1.2 simulations with different treatments of factors that affect ocean density on the continental shelf. Necessary features to avoid the unrealistic density bias were a realistic spatial distribution of iceberg melt and an ocean eddy parameterization that takes into account spatial variations in ocean stratification. Additionally, we found that freshwater from the melting of nearby ice shelves could potentially trigger the FRIS tipping point, suggesting a possible domino effect of increasing melt between ice shelves.