Influence of stratospheric circulation on the predictability of the tropospheric Northern Annular Mode

Abstract

Influence of stratospheric circulation on the predictability of the tropospheric Northern Hemisphere Annular Mode (NAM) in the boreal winter is examined using 5‐year archive of 1‐month ensemble forecast dataset provided by the Japan Meteorological Agency (JMA). It is found that the prediction skill of the 7‐day averaged ensemble‐mean NAM index in the upper troposphere is significantly improved for 5‐ to 13‐day forecast when negatively large NAM indices are observed in the stratosphere around 30 hPa at the initial time of forecast in comparison with stratospheric positive NAM events. The regression analysis also supports the significant relationship between large prediction error of the upper tropospheric NAM index and stratospheric westerly anomalies. The asymmetric response of the forecast skill of the upper tropospheric NAM index to the polarity of the stratospheric NAM anomaly is also discussed in terms of the dependence of the upward propagation of planetary waves on stratospheric zonal wind anomalies.