initMIP-Antarctica: An ice sheet model initialization experiment of ISMIP6

Abstract

Abstract. Ice sheet numerical modeling is the best approach to estimate the dynamic contribution of Antarctica to sea level rise over the coming centuries. The influence of initial conditions on ice sheet model simulations, however, is still unclear. To better understand this influence, an initial state intercomparison exercise (initMIP) has been developed to compare, evaluate, and improve initialization procedures and estimate their impact on century-scale simulations. initMIP is the first set of experiments of the Ice Sheet Model Intercomparison Project for CMIP6 (ISMIP6), which is the primary Coupled Model Intercomparison Project Phase 6 (CMIP6) activity focusing on the Greenland and Antarctic Ice Sheets. Following initMIP-Greenland, initMIP-Antarctica has been designed to explore uncertainties associated with model initialization and spin-up and to evaluate the impact of changes in external forcings. Starting from the state of the Antarctic ice sheet at the end of the initialization procedure, three forward experiments are each run for 100 years: a control run, a run with a surface mass balance (SMB) anomaly, and a run with a basal melting anomaly beneath floating ice. This study presents the results of initMIP-Antarctica from 25 simulations performed by 16 international modeling groups. The submitted results use different initial conditions and initialization methods, as well as ice flow model parameters and reference external forcings. We find a good agreement among model responses to the SMB anomaly, but large variations in responses to the basal melting anomaly. These variations can be attributed to differences in the extent of ice shelves and their upstream tributaries, the numerical treatment of grounding line, as well as the initial ocean conditions applied, suggesting that ongoing efforts to better represent ice shelves in continental-scale models should continue.

Long‐term projections of sea‐level rise from ice sheets

“…Other groups have proposed the inclusion of processes such as ice shelf hydrofracture (Pollard et al, ), or use fully coupled simulations that better capture ice–ocean–atmosphere (Gierz, Lohmann, & Wei, ) or ice‐sheet–sea‐level (Gomez, Latychev, & Pollard, ) feedbacks. Stimulated by AR4 and AR5 of the IPCC, the ice sheet modeling community has invested considerable effort over recent years in building, testing, and assessing model code and performance, and the first results from major, multi‐model, intercomparison exercises using the latest generation of ice and climate models are beginning to appear (Goelzer et al, ; Seroussi et al, ). With a substantial increase in the number and quality of models now available, and an ever‐increasing modeling community using these models, there is a good reason to be optimistic that current uncertainties in future sea‐level projections will be better captured over the coming years.…”

A protocol for calculating basal melt rates in the ISMIP6 Antarctic ice sheet projections

“…We choose a polar stereographic grid, with a resolution of 8 km horizontally (identical to the standard ISMIP6-Antarctica grid, Seroussi et al, 2019) and 60 m vertically, which represents and acceptable compromise for ISMIP6 ice-sheet modelers between accuracy and manageable data volume.…”

“…Given that melt rates have been estimated for more than 60 individual ice shelves (Rignot et al, 2013), we could in theory calibrate the parameterizations with different parameters in each cavity. However, ice-sheet models have evolving cavities, and their present-day ice shelves and ice flows do not necessarily correspond to the observed ones, depending on their initialization method (Seroussi et al, 2019). Furthermore, two initially distinct ice shelves may merge at some future time, leading to melt discontinuities if parameters are set at the scale of individual drainage basins.…”

The role of history and strength of the oceanic forcing in sea-level projections from Antarctica with the Parallel Ice Sheet Model

“…Results from the idealized surface mass balance experiment 'asmb' as described in initMIP Antarctica (Seroussi et al, 2019) 130 are very similar for both initial states with 119 mm SLE of mass gains for the 'historic' configuration and 118 mm SLE for the 'cold-start' configuration after 85 years of simulation with respect to the control simulations, see Table 1. This is close to the response of the different models that participated in initMIP Antarctica which showed mass gains between 125 and 186 mm SLE after 100 years.…”

“…In ISMIP6, a protocol for Antarctic projections was developed and ice-sheet model responses to oceanic and atmospheric forcing from selected CMIP5 models (Barthel et al, 2019) were gathered and compared for the first time. As a first step of ISMIP6, initMIP-Antarctica did test the effect of different model initialisation on idealized experiments (Seroussi et al, 2019). While the response of the ice sheet to surface mass balance forcing was similar among the models, they showed very different responses to basal melt rate changes.…”