Polar Vortex Tracker

The stratospheric polar vortex is a ring of strong westerly winds around 10-50 km up that spins counter-clockwise over the Arctic each winter. It only exists in the cold half of the year (roughly October to April) and is measured most cleanly by the 10 hPa 60°N zonal-mean zonal wind.

A strong vortex locks cold air near the pole and pushes the jet stream poleward, giving Northern Europe mild, wet, +NAO-style winters. A weak or disrupted vortex lets cold polar air spill south and often triggers 4-8 weeks of cold, blocked, -NAO winter weather over Europe and the eastern US.

A major Sudden Stratospheric Warming (SSW) is defined as a reversal of the 10 hPa 60°N zonal-mean zonal wind from westerly to easterly during the extended NH winter. The polar stratosphere can warm by 30-50°C (54-90°F) in a few days as the vortex is split or displaced by upward-propagating planetary waves.

About two-thirds of major SSWs are followed within 2-4 weeks by a sustained -NAO / -AO pattern at the surface, often delivering the coldest spells of a UK winter (e.g. the 2018 “Beast from the East” followed the Feb-2018 SSW).

The Arctic Oscillation index is the daily atmospheric-pressure see-saw between the Arctic and mid-latitudes. It correlates strongly with stratospheric vortex strength, is published daily by NOAA in a clean text format, and updates within 24 hours. Positive AO means a strong vortex and high-pressure mid-latitudes (mild winters); negative AO means a weak vortex and Arctic air spilling south.

A strong polar vortex correlates with a strong polar night jet and a positive NAO/AO surface pattern - mild, wet, westerly winters in the UK and northern Europe. A weak or disrupted vortex (often following a major SSW) tends to drive a negative NAO/AO pattern with blocking high pressure over Scandinavia / Greenland, easterly winds, and Arctic air outbreaks. The stratospheric signal typically leads the surface by 2-4 weeks, which is why SPV monitoring is a powerful sub-seasonal forecast tool.

Measured on this site from each place's seasonal temperature and rainfall since 1950 (with the long-term trend and El Niño / La Niña's share removed), the Arctic Oscillation's clearest fingerprints are in Europe's winters: around 70% of the year-to-year swing in Norway's December-February rainfall (wetter with a positive AO), more than half of late-winter temperature swings in Sweden, Estonia and Latvia (milder with a positive AO), and winter rainfall in the western Balkans - Montenegro, Albania and Bosnia and Herzegovina - which is drier with a positive AO. Russia's late winters and Scotland's winters are also markedly milder in positive-AO years. Every country and region page shows whether the AO measurably matters there, in its Climate Drivers section.

Major SSWs occur on average about 6 times per decade (roughly 0.6 per winter), but their frequency is highly clustered. Some winters have multiple events; some decades have very few. The 2010s and early 2020s have been an active SSW period. Most events occur between mid-January and mid-March.

This is an active research area. Some studies link rapid Arctic warming and sea-ice loss to a weaker, more wavy polar vortex with more frequent SSWs (the Cohen et al. and Francis-Vavrus lines of evidence). Other studies argue the signal is small relative to natural variability. What is clear is that SSW frequency has not declined despite overall stratospheric warming, and SPV variability remains a leading source of NH winter weather extremes.