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

Abstract

Abstract. Ice sheet numerical modeling is an important tool to estimate the dynamic contribution of the Antarctic ice sheet 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 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 surface mass balance 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, and the initial ocean conditions applied, suggesting that ongoing efforts to better represent ice shelves in continental-scale models should continue.

Precipitation Ansatz dependent Future Sea Level Contribution by Antarctica based on CMIP5 Model Forcing

“…These regions are the Amundsen Sea Embayment with both Pine Island and Thwaites Glaciers, some outlet glaciers of the East Antarctic Ice Sheet (EAIS) between George V and Wilkens Land, Amery Ice Sheet, and the Northern Antarctica Peninsula. These regions correspond to those, which have been identified across sixteen models, where ocean warming wanes marginal ice (Seroussi et al, 2019a).…”

“…However, PISM's grounding line parameterizations at medium to lower resolution is consistent with higher-order models (Feldmann et al, 2014). It explains that the present-day grounding line position resembles the current state reasonably, and the simulated grounding line retreat follows the bulk of simulations in the last model intercomparison (Seroussi et al, 2019a); hence, we consider our grounding line migration as reasonable. The apparent stability of ice shelves in the runs driven by the precipitation anomalies seems to comply with the safety band of ice shelves (Fürst et al, 2016), so the calving does stay outside of ice-shelf regions essential for providing buttressing for the inflowing grounded ice streams.…”

“…Ocean temperatures from the World Ocean Atlas 2009 (Locarnini et al, 2010) and the multi-year mean surface mass balance (SMB) from the RACMO 2.3/ANT model (Van Wessem et al, 2014) drive PISM during spin-up (Table 2). A similar model setup has taken part in the initMIP-Antarctica exercise under the name AWI_PISM1Eq with an adjusted Eigen-calving proportionality constant of 2•10 18 and no bed deformation (Seroussi et al, 2019a).…”

ISMIP6 Antarctica: a multi-model ensemble of the Antarctic ice sheet evolution over the 21st century

“…The results presented here do not include any weighting of the ice flow models based on their agreement with observations or the number of simulations submitted. As explained in previous studies (Goelzer et al, 2017(Goelzer et al, , 2018Seroussi et al, 2019), the range of initialization techniques adopted by models leads to varying biases. Some models are initialized with a long paleoclimate spin-up, giving limited spurious trends but an initial configuration further from the observed state, whereas models initialized with data assimilation of present-day observations can capture these conditions accurately but often have non-495 physical trend in their evolution.…”

“…ISMIP6 (Ice Sheet Model Intercomparison Project for CMIP6, Nowicki et al, 2016) is the primary effort of CMIP6 (Climate 40 Model Intercomparison Project Phase 6) focusing on ice sheets and was designed to mitigate this gap as well as improve our understanding of ice sheet-climate interactions. In a first stage, ice sheet model initialization experiments (initMIP, Goelzer et al, 2018;Seroussi et al, 2019) focused on the role of initial conditions and model parameters in ice flow simulations.…”