High initial-time sensibility observed for the tropospheric NAM predictability in the stratospheric sudden warming

Very high predictability in the tropospheric NAM to a few months was simulated by ensemble runs of the general circulation model when the initial time was set just before the key day of the major stratospheric sudden warming (SSW) in January 2004 (Kuroda, GRL2008). In this study, we examined how the predictability of the tropospheric NAM varies with the change of the initial time. It is found that higher statistical significance of the NAM-predictability is obtained with advancement of initial time to the key day of the SSW. Such higher predictability is obtained until initial time is set to the key day. However, abrupt large decrease in the predictability is observed after the key day, and higher predictability cannot be obtained afterward. The reason of such abrupt decrease in the predictability after the key day is also discussed.