Implementation of the Zoet-Iverson basal sliding law in CISM

There are large uncertainties in model predictions of the Antarctic Ice Sheet (AIS) contribution to future sea level rise. One source of model uncertainty is the description of basal friction. Here, we implement a new basal sliding law, developed by Zoet and Iverson (2020), in an updated version of the Community Ice Sheet Model (CISM). The Zoet-Iverson sliding law combines properties of two previously used sliding laws: power-law behavior in areas with slow-moving ice and coulomb-law behavior in fast-moving ice streams and outlet glaciers. We adress the behavior and performance of the Zoet-Iverson law in CISM using AIS spin-up procedures developed for the Ice Sheet Model Intercomparison Project for CMIP6 (ISMIP6). We invert a non-dimensional coefficient in the Zoet-Iverson law to match modelled and observed thickness for grounded ice. Ocean temperatures are tuned to nudge ice-shelf thickness via the basal melt rates. These tuning processes are Antarctic-wide, but we focus on the Amundsen Sea region. We then advance the model forward to better represent the present-day Thwaites glacier, by inverting for observed ice velocity and by changing the ocean forcing. The main results from this run are the sub-shelf ocean temperature perturbation, thickness, and velocity profile of Thwaites glacier. Results are compared with different sliding laws to demonstrate the effect of the Zoet-Iverson law on the representation of the ongoing retreat. 

Zoet, L.K. & Iverson, N.R. (2020). A slip law for glaciers on deformable beds. In: Science 368 (6486), pages 76-78. DOI: 10.1126/science.aaz1183