To compare the predictability of two stratospheric sudden warming (SSW) events occurring in 2009 and 2010, ensemble forecast experiments are conducted using an Atmospheric General Circulation Model. It is found that the predictable period of the vortex-splitting SSW in 2009 is approximately 7 days that is much shorter than that of the vortex-displacement SSW in 2010. The latter event is predictable more than 13 days in advance. The ensemble spread in the upper stratosphere for medium-range forecasts is found to be enlarged just prior to the onset of the 2009 SSW event, whereas no such enlargement is seen for the 2010 SSW event.

Stability analysis of the zonally asymmetric basic states specified by the ensemble mean forecast using a nondivergent barotropic vorticity equation reveals that the extremely distorted polar vortex in the upper stratosphere just before the onset of the 2009 SSW event is highly unstable to infinitesimal perturbations, whereas there is no such unstable mode with an extremely large growth rate during the 2010 SSW event. In addition, the most unstable mode during the onset of the 2009 SSW event has a similar horizontal structure to the 1st EOF of the ensemble spread. Thus, it is suggested that a predictability barrier inherent in the upper-stratospheric circulation, characterized by the presence of dynamically unstable modes with large growth rates, limits the predictable period of the 2009 SSW event.