The occurrence of stratospheric sudden warming (SSW) events is a dominant component of the intraseasonal variability of the wintertime stratospheric circulation in the northern hemisphere. Observational studies have showed that a time-mean state of the winter stratosphere is influenced by the equatorial quasi-biennial oscillation (QBO). The aim of the present study is to investigate the effect of the QBO on the occurrence of SSWs by analyzing daily data.

Taguchi et al. (2001) investigated the dependence of the intraseasonal variability on the amplitude of a sinusoidal surface topography by using a simple global circulation model. The same model with large planetary-wave forcing is used in the present study to investigate QBO effects on the variability assuming an idealized zonal mean zonal momentum forcing in the equatorial stratosphere. Magnitude and phase (westerly or easterly) of the QBO are changed as an experimental parameter in perpetual winter integrations for 12,000 days.

Polar temperature in the upper stratosphere shows that SSWs occur more frequently in the easterly phase, consistent with the observational result. More interestingly, the present experiment shows nonlinear dependence on the QBO forcing; in the westerly phase, the frequency of the occurrence of SSWs increases for stronger westerly forcing (1.5 or 2.0 times of the standard forcing). Statistical analyses with the long data records show that some QBO effects are significant even in the troposphere in spite of the small difference in probability distribution functions.

Composite analyses of SSW events also show the QBO dependence. For short-time response within several days, deeper signature of SSW events are obtained down to the lower stratosphere in the easterly phase. On the other hand, for the long-time response with the order of weeks, stronger wave-induced meridional circulation is obtained in westerly phase. Radiative cooling after a SSW event is largely canceled by the dynamical processes in the lower stratosphere.