A Parameter Sweep Experiment on the Effects of the Equatorial QBO on Stratospheric Sudden Warming Events

Abstract

The effects of the equatorial quasi-biennial oscillation (QBO) on stratospheric sudden warming (SSW) events are investigated by performing long time integrations with a simple global circulation model under a perpetual winter condition. Zonal momentum forcing is imposed to produce a westerly or easterly phase of the QBO in the equatorial stratosphere, and a parameter that determines strength and direction of the forcing is swept for nine values as an experimental parameter. The polar night jet is weaker and polar stratosphere is warmer in the runs with easterly “QBO wind” forcing, in qualitative agreement with the observational result. The polar night jet is strongest for a moderate westerly forcing, while it becomes weaker for too strong westerly forcing. Occurrence of SSW events is irregular, intermittent, and least frequent in the run with the moderate westerly forcing. As a result, frequency distribution of the polar stratospheric temperature shows the largest skewness. The frequency distribution of polar temperature is systematically dependent on the QBO wind forcing not only in the stratosphere but in the troposphere. The dependence is statistically significant even in the troposphere in spite of heavy overlap of the frequency distributions. Composite analysis for a large number of SSW events shows downward influence of the upper-stratospheric warming to the lower levels in two characteristic timescales. Short-time cooling response within several days is extended to the summer hemisphere, while long-time response persisting a couple of weeks is confined within the winter hemisphere and extends down to the level near the tropopause. The response of polar temperature in the lower stratosphere shows systematic dependence on the QBO wind forcing; mean warming rate is smallest for the moderate westerly forcing, and warming is more rapid for the easterly forcings.