Probabilistic forecasting of the Arctic sea ice edge with contour modeling

Sea ice, or frozen ocean water, annually freezes and melts in the Arctic. The need for accurate forecasts of where sea ice will be located weeks to months in advance has increased as the amount of sea ice reduces due to climate change. Typical sea ice forecasts are made with ensemble models, physics-based deterministic models of sea ice and the surrounding ocean and atmosphere. This paper introduces Mixture Contour Forecasting, a method to forecast sea ice that post-processes output from ensembles and weights them with observed sea ice patterns in recent years. These forecasts are better calibrated than unadjusted dynamic ensemble forecasts and other statistical reference forecasts. To produce these forecasts, a novel statistical technique is introduced that directly models the sea ice edge contour, the boundary around the region that is ice-covered. Most of the computational effort in post-processing can therefore be placed on the sea ice edge contour, which is of particular importance due to its role in maritime planning. Mixture Contour Forecasting and reference methods are evaluated for monthly sea ice forecasts for 2008-2016 at lead times ranging from 0.5-6.5 months using the European Centre for Medium-Range Weather Forecasts ensemble.