How are midlatitude seasonal forecasts affected by stochastic sea ice perturbations?

Reliable Arctic sea ice forecasts are important, not just for Arctic use-cases (such as determining shipping routes), but also for the potential impact that sea ice has on the midlatitude circulation. However, sea ice forecasts are often highly underdispersive, including in the IFS, the model developed and run by the European Centre for Medium-Range Weather Forecasts (ECMWF). We describe here the implementation of a stochastic parameterization scheme to the sea ice component of the IFS, and the impact it has on seasonal forecasts in the northern hemisphere midlatitudes in summer and winter. We show that sea ice ensemble spread is generally enhanced by around 10%, resulting in a more reliable forecast. We also show that the perturbations result in small but robust mean state change in Arctic air temperatures up to at least 850hPa, as a result of robust changes to the mean sea ice. A seeming consequence of this is a large increase in 500hPa geopotential (Z500) winter forecast skill over the Euro-Atlantic sector, which partially projects onto the North Atlantic Oscillation (NAO). We conclude that sea ice stochastic perturbations can be a valuable contribution to increased reliability of seasonal forecasts of the sea ice itself and can impact seasonal forecasts of the atmosphere at high and mid latitudes.