冬季成層圏極渦に内在する予測障壁 --2009・2010年の成層圏突然昇温の予測可能性比較--

Abstract

The predictability of two stratospheric sudden warming (SSW) events occurring in 2009 and 2010 is examined by conducting ensemble forecast using an AGCM. It is found that the predictable period of the vortex splitting SSW in 2009 is about 7 days, much shorter than that of the vortex-displacement SSW in 2010, which is assessed at about 14 days. Stability analysis of the stratospheric circulation using a nondivergent barotropic vorticity equation also reveals that the upper stratospheric circulation is highly unstable to infinitesimal perturbations just before the mature phase of the 2009 SSW event while such unstable modes with extremely large growth rates are absent during the 2010 SSW event. Hence, it is suggested that a predictability barrier inherent in the upper stratospheric circulation, which is characterized by the existence of dynamically unstable modes with large growth rates limits a predictable period of the 2009 SSW event.