Seasonality with a chance of retreat: Quantifying the impact of seasonal forcings on the timing of Greenland Ice Sheet outlet glacier terminus retreat

Seasonal glacier dynamics are an important factor in decadal and centennial projections of the ice sheets, and omitting seasonality can impact the timing of initiation of glacier retreat leading to uncertainty in decadal projections of mass change. However, numerical ice sheet model simulations typically prescribe annual climate forcings, neglecting seasonality in glacier behavior. To advance our understanding, it is necessary to quantify the impact of seasonality on decadal and centennial simulations of the ice sheets. We explore the sensitivity of ocean-terminating outlet glaciers on the Greenland Ice Sheet to seasonality in atmospheric and oceanic forcings. We run an ensemble of simulations using the Ice-sheet and Sea-level System Model (ISSM) to investigate the impact of seasonal forcings on four outlet glaciers in central West Greenland. We perform a hindcast simulation (1985-present), allowing us to use Bayesian calibration to select the ensemble members that best match observations, and a forecast (present-2050) in which we apply a variety of climate forcing projections. Our ensemble samples uncertainty in ice sheet model parameters such as ice viscosity, basal sliding, and frontal ablation including both terminus melt and calving. To quantify the impact of seasonality, we perform simulations from 1985 to 2050 with and without seasonality in the prescribed forcings and compare the timing of the initiation of glacier retreat. Our results will help to improve our understanding of how these outlet glaciers will evolve, specifically by investigating interplay between the seasonality in forcings and uncertainty in model parameters. Ultimately, by improving how seasonal forcings are implemented in ice sheet models, studies like ours will help to improve decadal and centennial projections of ice sheet change and sea-level rise.