Predictability of the downward migration of the Northern Annular Mode: A case study for January 2003

The dynamics and predictability of a downward migration event of the negative Northern Annular Mode (NAM) anomaly following a stratospheric sudden warming (SSW) occurring in January 2003 are examined using the operational 1-month ensemble forecast data set provided by the Japan Meteorological Agency. It is found that the predictable period of the tropospheric negative NAM anomaly is at most 6 days, which is much shorter than that of the precedent SSW event. The tropospheric NAM anomaly is caused by the E-P flux convergence associated with zonal wavenumber 2 planetary waves at the tropopause level, of which propagating property is affected by the zonal-mean zonal wind there. Our results suggest that even large stratospheric circulation changes associated with the SSW event have only limited influence on the predictability of the tropospheric circulation.