Probabilistic forecasts of sea ice trajectories in the Arctic: impact of uncertainties in surface wind and ice cohesion

TL;DR

This study evaluates how uncertainties in surface wind and ice cohesion affect probabilistic sea ice trajectory forecasts in the Arctic, emphasizing the importance of ensemble simulations perturbing both factors for accurate predictions.

Contribution

It introduces a method to quantify the impact of wind and ice cohesion uncertainties on sea ice trajectory forecasts using ensemble simulations with the neXtSIM model.

Findings

Wind perturbations dominate forecast uncertainty.

Inhomogeneities in ice cohesion increase trajectory anisotropy.

Ensemble perturbations of both factors provide comprehensive uncertainty estimates.

Abstract

We study the response of the Lagrangian sea ice model neXtSIM to the uncertainty in the sea surface wind and sea ice cohesion. The ice mechanics in neXtSIM is based on a brittle-like rheological framework. The study considers short-term ensemble forecasts of the Arctic sea ice from January to April 2008. Ensembles are generated by perturbing the wind inputs and ice cohesion field both separately and jointly. The resulting uncertainty in the probabilistic forecasts is evaluated statistically based on the analysis of Lagrangian sea ice trajectories as sampled by virtual drifters seeded in the model to cover the Arctic Ocean and using metrics borrowed from the search-and-rescue literature. The comparison among the different ensembles indicates that wind perturbations dominate the forecast uncertainty i.e. the absolute spread of the ensemble, while the inhomogeneities in the ice cohesion field significantly increase the degree of anisotropy in the spread i.e. trajectories drift differently in different directions. We suggest that in order to get a full flavor of uncertainties in a sea ice model with brittle-like rheologies, to predict sea ice drift and trajectories, one should consider using ensemble-based simulations where both wind forcing and sea ice cohesion are perturbed.