The polar vortex, every season

Each line is one winter of the zonal wind at 60°N, 10 hPa — the strength of the stratospheric polar vortex — all racing from July to June together. Most winters spin up to 40–60 m/s by January. Some collapse: the wind reverses to easterly in a sudden stratospheric warming, and the surface often turns colder over the following weeks. This season leads in bold.

ERA5 zonal-mean zonal wind at 60°N (60°S for the south), 10 hPa; envelope 1991–2020; SSWs by the Charlton & Polvani (2007) reversal criterion with split/displacement classification from vortex moments.

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