Seasonal evolution of the quasi-biennial oscillation impact on the Northern Hemisphere polar vortex in winter

Using potential vorticity to define Northern Hemisphere (NH) stratospheric polar vortex strength and position, the influence of the QBO on the polar vortex in winter is analyzed. The results show that the weakened/enhanced NH polar vortex in the lower stratosphere associated with an easterly/westerly QBO (E/WQBO) phase is more noticeable in January and February than in November and December. In addition, the NH polar vortex shows a shift towards the Eurasian continent and away from North America in winter during EQBO phases compared with during WQBO phases, with a greater shift in January and February than in November and December. The weaker QBO impact on the zonal mean of zonal wind and temperature in February found in previous studies may be related to the cancelling effects of opposite-sign anomalies over Eurasia and North America induced by the polar vortex shift associated with QBO events. During EQBO phases the upper stratospheric easterly anomalies in early winter increase the frequency of negative refractive index in the middle and high latitudes in late winter. Less planetary wave 1 is propagated upward and more wave 1 accumulates in the lower stratosphere, leading to the weakening and shift of the polar vortex in late winter. The poleward displacement of the subtropical jet during EQBO phases causes more poleward propagation of transient waves in the lower stratosphere, and thus more Rossby wave breaking events over the Eurasian continent in late winter than during WQBO phases, further weakening and shifting the polar vortex in EQBO phases.