Seasonal Winter Forecasts of the Northern Stratosphere and Troposphere: Results from JMA Seasonal Hindcast Experiments

AbstractThis study investigates winter forecasts of the Northern stratosphere and troposphere using seasonal hindcast experiments of the Japan Meteorological Agency. A main focus is placed on the seasonal forecasts of December-January-February (DJF)-mean Northern Annular Mode (NAM) when the forecasts are initialized in late fall. Results demonstrate that the hindcast data have significant skill for both ensemble mean and category forecasts of the NAM but only in the stratosphere. Probability forecasts for DJF Major Stratospheric Sudden Warmings (MSSWs) are also suggested to be significant (higher probabilities for actual MSSW years) near the 90 % confidence level. The forecast skill of the stratospheric NAM changes with the observed phase of the Quasi-Biennial Oscillation (QBO), although the QBO is not simulated but is only included in initial conditions. The skill is higher for the easterly phase characterized by hits of negative NAM states (weaker than normal polar vortex), whereas it is lower for the w...