Assessing the Impact of Ice Velocity Observations on Ice Sheet Mass Loss Projections from the Amundsen Sector

Ice streams in the Amundsen Sea Embayment (ASE) are some of most rapidly thinning in Antarctica, experiencing ocean-driven grounding-line retreat and increased mass loss. Ice streams in this sector are crucial for maintaining the stability of the West Antarctic ice sheet, which contains enough ice to raise global mean sea-level by 5.3 m. Ice sheet models are our main tool to make projections of ice sheet mass loss or volume above floatation (which is equivalent to sea level rise). Standard methodologies consist of constraining model parameter fields using satellite observations, and then simulating model response to climate forcing. However, to date there is no comprehensive assessment of how sensitive these projections are to different satellite products or to spatial variations in ice velocity observations, or changes in the spatial distribution of unknown model parameters.  

Mapping the sensitivity of ice streams to a given set of model inputs can be done using an ensemble of simulations and running the model workflow tens (or hundreds) of thousands of times. Automatic Differentiation (AD) and data assimilation, however, provides an efficient method to perform this analysis and get these sensitivity maps at the scale of the model’s mesh. Here we use the ice sheet model Fenics_ice and its AD capabilities to construct maps of the sensitivity of volume above floatation to changes in the calibrated unknown model parameters, as well as changes in ice velocity observations. Our preliminary results show that the sea level rise forecast is more sensitive to ice velocity changes at the grounding line and to changes to the basal drag coefficient at those same locations.