Medium-Range Predictability of the Wintertime Bering Sea Ice Edge using Linear Inverse Modeling

Beginning in autumn, sea ice expands into the southern Bering Sea where it remains until spring. In winter, some commercial stocks, particularly crab thrive in ice-infested areas, necessitating short-lead forecasts of the ice edge for fishers. At time scales of days to weeks, wintertime expansions and retractions of the ice edge are forced by winds through a combination of dynamic ice transport and thermodynamic coupling with the ocean. Previous research has shown that the mean atmospheric flow across the ice in winter and spring is modulated by the planetary wave structure, which can impart persistence in the tendency of winds through the Bering Strait beyond synoptic time scales. Here, we evaluate the predictability of the wind-forced ice edge in winter using a data-adaptive stochastic dynamical methodology, Linear Inverse Modeling. The analysis is based on the statistics intrinsic to the observed ice edge, taken from the NSIDC Sea Ice Index on 18 meridional transects from 181°E to 198°E, which the LIM treats as a linear, multivariate system. Skillful forecasts of the wintertime Bering Sea ice edge relative to its seasonal cycle are found for 5-6 day lead times, increasing to 6-9 days for "forecasts of opportunity" when off-ice northeasterlies prevail. Interruptions to the mean flow associated with southerly advection through the Bering Strait produce retreat in the sea ice that is less predictable. Generally, predictability is higher in the eastern Bering Sea than the west. The large amplitude of the seasonal cycle provides a source for predictability, even at weather scales.