(left) When the Quasi-biennial Oscillation (QBO) is from the west, winter (Dec-Jan) polar stratospheric winds are stronger than average (purple), making sudden stratospheric warming less likely. (right) When the QBO is from the east, the polar vortex is weaker than average (green). The panels above the globes show difference from average east-west winds at altitudes from 100 to 10 hPA across each latitude band from the equator (zero) to the North Pole (90). The dashed line indicates the altitude/pressure level used to define the QBO. Positive values mean westerly winds at a given latitude and altitude were stronger than average; negative values mean weaker-than-average westerlies (or stronger-than-average easterlies). Data is from the ERA5 reanalysis, and the anomalies are defined relative to the 1991-2020 average. NOAA Climate.gov image, adapted from analysis by Amy Butler.