Predictability of the Wintertime Arctic Oscillation Based on Autumn Circulation: Assessment on Independent Data

A statistical method for a one month lead prediction of the wintertime seasonal mean Arctic Oscillation (AO) index is discussed. Predictors, independently constructed for each forecast based on October geopotential height of the 500 hPa surface, indicate that the strongest impact of October circulation on the wintertime AO originates from a circulation anomaly over the Taymyr Peninsula. Winters of the positive (negative) AO polarity tend to be preceded by an October cyclonic (anticyclonic) Taymyr circulation anomaly causing advection of the cold (warm) air to the central Arctic, which leads to enhancement (weakening) of the circumpolar vortex, and advection of the warm (cold) air to the East Asia with corresponding weakening (enhancement) of the East Asia trough associated with the planetary wave-1 and wave-2. Independent verification assessments based on series of 30 forecasts of the December-February (DJF) AO index (AOI) reveal a high skill of the developed method, with correlation coefficients between the predicted and observed DJF AOI being 0.61-0.66 and mean squared error skill scores being 0.37-0.43.