Comment on wcd-2022-58

Abstract

Extreme stratospheric polar vortex events, such as sudden stratospheric warmings (SSW) or extremely strong polar vortex (SPV) states, can have a prolonged downward impact influencing surface weather for several weeks to months. SSWs are most often associated with negative North Atlantic Oscillation (NAO) conditions, cold air outbreaks in the Arctic and a southward-shifted extratropical storm track, while SPVs tend to be followed by a positive phase of the NAO, relatively warm conditions in the extratropics and a poleward-shifted storm track. Such changes in the position of the storm track and the associated changes in cyclone frequency over the North Atlantic and Europe can lead to infrastructure damage and health impacts due to cyclone-associated extreme winds and the risk of flooding or heavy snowfall. Skillful predictions of the downward impact of stratospheric polar vortex extremes can therefore improve the predictability of extratropical winter storms on subseasonal timescales. However, there exists a strong inter-event variability in these downward impacts on the tropospheric storm track. Using ECMWF reanalysis data and ECMWF reforecasts from the Subseasonal to Seasonal (S2S) Prediction Project database, we investigate the stratospheric influence on extratropical cyclones, identified with a cyclone detection algorithm. Following SSWs, there is an equatorward shift in cyclone frequency over the North Atlantic in reforecasts, and the opposite response is observed after SPV events, consistent with the response in reanalysis. However, less than 70 % of the reforecasts capture the sign of the cyclone frequency response over the North Atlantic during weeks 1–2 after SSWs, and less than 50 % of the reforecasts capture the response during weeks 3–4. The cyclone forecasts following SPV events are generally more successful. Understanding the role of the stratosphere in subseasonal variability and predictability of storm tracks during winter can provide a key for reliable forecasts of midlatitude storms and their surface impacts.