Stratospheric Polar Vortex Tracker
The stratospheric polar vortex is a ring of strong westerly winds high above the Arctic each winter. A strong vortex locks cold air near the pole and gives mild westerly winters in northern Europe; a weak or disrupted vortex - especially after a Sudden Stratospheric Warming - lets Arctic air flood south, often triggering the coldest 2-4 weeks of a UK winter. This page combines the live NOAA daily Arctic Oscillation index (the surface fingerprint of vortex strength), the 75-year monthly AO history, the operational stratosphere diagnostics and the full Free University of Berlin SSW catalogue.
Current State - Summer Regime · Neutral
The stratospheric polar vortex is a winter phenomenon (roughly October to April); in summer the polar stratosphere runs easterly and there is no vortex to track. The daily Arctic Oscillation (AO) index shown here is still valid year-round as a measure of the tropospheric circulation.
AO - latest
-0.35
2026-07-31
30-day rolling
AO mean
Arctic · Polar Vortex
AO State - Summer
Summer Neutral - No clear circulation signal
There is no stratospheric polar vortex right now - it forms in autumn and breaks down each spring, and by high summer the polar stratosphere has flipped to its easterly regime. The daily AO remains a valid year-round index of the tropospheric circulation, and it is currently near zero: neither a poleward-shifted jet nor high-latitude blocking is favoured. Vortex tracking resumes in October.
Most Recent Major Event: 2024-03-04 (final warming) - Early final warming; ended SPV season.
Strength bands: ≥ +0.5σ strong · -0.5 … +0.5σ neutral · ≤ -0.5σ weak · ≤ -1.5σ disrupted. Daily AO is the published surface proxy for 10 hPa zonal-mean zonal wind at 60°N; that proxy relationship holds during the October-April vortex season, while in summer the polar stratosphere runs easterly and the AO reads the tropospheric circulation only. Data: NOAA CPC daily AO + Free University of Berlin SSW catalogue.
History & Events
AO History
Daily Arctic Oscillation is the surface fingerprint of stratospheric vortex strength. Green bands mark sustained +AO (strong vortex, mild Europe); violet bands mark sustained -AO (weak vortex, cold-air outbreaks). Sky-blue strips are Dec-Feb winters; rose dots mark major Sudden Stratospheric Warmings.
The Arctic Oscillation index is published daily by NOAA CPC from a 1000 hPa height EOF analysis. Its 30-day rolling mean tracks the stratospheric polar-vortex strength closely. Rose markers / lines indicate major Sudden Stratospheric Warming events from the Free University of Berlin catalogue - note how nearly every major SSW is followed by a sustained dip in AO over the next 4-8 weeks. Data: NOAA CPC daily AO index · FU Berlin SSW catalogue.
Catalogue of Sudden Stratospheric Warmings
Each entry is a major SSW (10 hPa 60°N zonal-mean zonal wind reversal) or seasonal final warming. About two-thirds of major SSWs are followed within 2-4 weeks by a sustained -NAO / -AO pattern at the surface, often the coldest spells of a UK winter.
- 2024-03-04Final warmingEarly final warming; ended SPV season.
- 2023-02-16Major - displacement-
- 2021-01-05Major - displacementFollowed by Filomena snowstorm, Texas freeze.
- 2019-01-02Major - split-
- 2018-02-12Major - split"Beast from the East" precursor.
- 2013-01-06Major - splitPreceded UK March 2013 cold spell.
- 2010-03-24Major - displacement-
- 2010-02-09Major - displacementFollowed by deep -NAO winter.
- 2009-01-24Major - splitOne of strongest on record.
- 2008-02-22Major - displacement-
- 2007-02-24Major - displacement-
- 2006-01-21Major - displacement-
- 2004-01-05Major - displacement-
- 2003-01-18Major - split-
- 2002-02-17Major - displacementTriggered cold European outbreak.
- 2001-12-30Major - displacement-
- 2001-02-11Major - displacement-
- 2000-03-20Major - splitVortex split during minor warming pulse.
Source: Free University of Berlin SSW catalogue, cross-checked against NOAA CPC stratosphere diagnostics. 18 events on record back to 1958.
Diagnostics & Impacts
Live Stratospheric Diagnostics
NOAA CPC stratosphere images, fetched directly from cpc.ncep.noaa.gov. These are the same panels operational forecasters watch each winter for early SSW signals.
Polar-cap temperature anomaly
Warm (red) anomalies in the stratosphere are the classic SSW fingerprint. When the red signal works downward over 2-4 weeks it influences winter weather at the surface.
Polar-cap geopotential height anomaly
Positive (red) height anomalies aloft = a weak, blocked vortex. Following major SSWs the red signal propagates downward, driving the surface AO/NAO negative.
Polar-cap mean temperature
Raw polar-cap temperature with climatology overlay. Spikes well above the climatological line signal that warm air has displaced the cold vortex core.
Regional Impacts - Strong vs Weak Vortex
The polar vortex modulates Northern-Hemisphere winter weather with a 2-4 week lead time. These are the canonical impacts each phase favours - not deterministic forecasts.
Strong vortex
- UK & N Europe
Mild, wet, windy - Strong polar-night jet supports a powerful Atlantic westerly flow; +NAO winters with frequent storms. - Mediterranean
Drier than average - Storm track sits well to the north; high pressure dominates the basin. - Eastern US & Canada
Mild, less Arctic outbreaks - Cold air is locked near the pole; -ve East Coast cold extremes. - Scandinavia
Mild, less snow - Westerly maritime air dominates; reduced blocking. - Arctic
Cold, sea-ice growth aided - Cold air bottled up near the pole helps wintertime ice growth.
Weak / disrupted vortex
- UK & N Europe
Cold spells, blocking - Weak vortex / SSW often precedes a sustained -NAO pattern: easterly winds, Scandinavian blocking, snow risk. - Eastern US
Arctic outbreaks - Polar vortex lobes shed equatorward; the eastern US frequently sees the coldest spells of the winter. - Mediterranean
Wetter, stormy - Storm track buckles south; cold lows can spin up across the basin. - Scandinavia
Blocking high, cold dry - High pressure parks over Scandinavia / Greenland, deflecting Atlantic storms south. - Arctic
Warming, ice anomaly - Stratospheric warming propagates downward; Arctic surface temperatures spike above climatology.
Impact patterns from Baldwin & Dunkerton (2001), Charlton & Polvani (2007), Mitchell et al. (2013) and the WMO sub-seasonal forecast verification literature. The downward propagation timescale is 2-4 weeks for ~two-thirds of major SSWs; about one-third have weak or no surface signal.
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FAQs
Stratospheric Polar Vortex: Common Questions
What is the stratospheric polar vortex (SPV)?
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.
What is a Sudden Stratospheric Warming (SSW)?
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).
Why use the Arctic Oscillation (AO) as a proxy for vortex strength?
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.
How does the SPV influence UK and European winters?
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.
How often do SSWs happen?
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.
Is the polar vortex changing with global warming?
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.