Forecast Skill and Signal-To-Noise Ratio for Stratospheric Polar Vortex Variations in Northern Hemisphere Winter Seasonal Hindcasts

This study explores seasonal forecast skill and signal-to-noise (SN) ratio for stratospheric polar vortex (SPV) variations during Northern winter using hindcast (HC) data of six systems initialized around early November in the Copernicus Climate Change Service database. Results show high skill for December-January-February (DJF) mean El Nino/Southern Oscillation and Quasi-Biennial Oscillation (QBO) variations, although it is suggested that some systems underestimate the QBO amplitude. The ENSO and QBO variations in the HC data are also suggested to be underdispersive having too high SN ratio due to too strong signal and/or too weak noise (spread). The skill for DJF mean SPV variations is limited, with four systems having marginal skill near the 95% level, whereas the SN ratio is suggested to be realistic. It is also shown that no system has skill (evaluated using the Brier Score) at the 95% level in forecasting if each DJF winter experiences one or more major sudden stratospheric warmings, when a bias of the polar vortex strength in each system is simply considered. A comparison of the ensemble members for each system, when divided into two groups by the strength of the QBO-SPV teleconnection, suggests an increase in the skill for SPV variations for the group of stronger teleconnection. This confirms the idea that the QBO-SPV teleconnection contributes to the skill for SPV variations, and also suggests that the skill for SPV variations can be improved as the HC QBO and QBO-SPV teleconnection are better simulated. Variations in the stratospheric polar vortex (SPV) can influence tropospheric and surface weather conditions, making subseasonal-to-seasonal (S2S) forecasts of SPV important. However, good S2S forecasts for SPV variations remain challenging, for example, due to complex interactions with other climate factors. This study investigates the characteristics, specifically skill and signal-to-noise (SN) ratio, of Northern winter seasonal forecasts for SPV and relevant variations, including El Nino/Southern Oscillation (ENSO) and Quasi-Biennial Oscillation (QBO) in the tropics, known to influence the SPV through teleconnections. The forecasts have high skill for winter-mean ENSO and QBO variations, but their SN ratio is too high due to too strong signal and/or too weak noise. Forecasting winter-mean SPV variations remains challenging, with limited skill compared to ENSO and QBO. A better representation of the QBO and its teleconnection with SPV could improve the skill for SPV forecasts, paving the way for better seasonal forecasts for Northern winter. Hindcasts of six C3S systems initialized around November have high skill for DJF ENSO and QBO variations but are underdispersive for them Some systems have marginal skill for DJF stratospheric polar vortex variations, and signal-to-noise ratio is suggested to be realistic The QBO-polar vortex teleconnection is suggested to contribute to the skill for polar vortex variations