Stratospheric connection to Northern Hemisphere wintertime weather: Implications for prediction

The dynamical coupling between the stratospheric and tropospheric circulations yields a statistically significant level of potential predictability for extreme cold events throughout much of the Northern Hemisphere (NH) mid–high latitudes on both month-to-month and winter-to-winter timescales. Pronounced weakenings of the NH wintertime stratospheric polar vortex tend to be followed by episodes of anomalously low surface air temperatures and increased frequency of occurrence of extreme cold events throughout densely populated regions such as eastern North America, northern Europe, and eastern Asia that persist for ∼2 months. Strengthenings of the vortex tend to be followed by surface temperature anomalies in the opposite sense. During midwinter, the quasi-biennial oscillation (QBO) in the equatorial stratosphere has a similar but somewhat weaker impact on NH weather, presumably through its impact on the strength and stability of the stratospheric polar vortex; that is, the easterly phase of the QBO favors an increased incidence of extreme cold events, and vice versa. The signature of the QBO in NH wintertime temperatures is roughly comparable in amplitude to that observed in relation to the El Niño–Southern Oscillation phenomenon.