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DATA STATUS
456 series loaded · last import on 2026-10-26

Sudden stratospheric warmings and the European winter When the polar vortex breaks down 30 km up, does northern Europe turn cold?

Correspondences

What is linked, what is not

Each arc joins two phenomena tested on this site. Green and animated: clear link (correlation significant at 5 % after correcting for autocorrelation, or a response outside chance in an event study). Grey: inconclusive test. A statistical link alone does not establish causation.

  • Clear link
  • Inconclusive
Hypotheses tested
32
Clear links
23
correlation p < 0.05 or epochs outside chance
Causation demonstrated
0
a correlation demonstrates none

Arc width from |r| of the stored result; several tests between the same phenomena are drawn as stacked arcs.

Hypothesis tested: A sudden stratospheric warming reverses the polar-night jet (westerly wind at 60° N and 10 hPa) within days. The anomaly often descends into the troposphere over a few weeks, tips the North Atlantic Oscillation (NAO) into its negative phase and favours cold-air outbreaks over northern Eurasia (Baldwin and Dunkerton, 2001; Kolstad et al., 2010; King et al., 2019). About a third of the events have no clear surface effect (Domeisen and Butler, 2020).
Events used
42
NAO, days +5 to +60
-0.17
p = 0.003; random draws: 0.00
Events followed by a negative NAO
67 %
out of 42 warmings (days +5 to +60)
Temperature, Northern Europe
-0.44 °C
days +5 to +60, p = 0.20; cold days ×1.34 (p = 0.03)
Temperature, North-western Europe
-0.32 °C
days +5 to +60, p = 0.12; cold days ×1.19 (p = 0.16)
Temperature, Southern Europe
-0.15 °C
days +5 to +60, p = 0.29; cold days ×1.11 (p = 0.24)
Period analysed
1958-02-08 → 2023-02-16
computed on Oct 26, 2026 · engine v1.0
Daily NAO around stratospheric warmings
Show the data as a table
Mean over 42 major warmings, smoothed over 7 days; day 0 = ERA5 central date. Dashed: 5th and 95th percentiles of the same dates in other years.
Temperature, Northern Europe, around the warmings
Show the data as a table
Mean over 42 major warmings, smoothed over 7 days; day 0 = ERA5 central date. Dashed: 5th and 95th percentiles of the same dates in other years. Moscow
Temperature, North-western Europe, around the warmings
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Mean over 42 major warmings, smoothed over 7 days; day 0 = ERA5 central date. Dashed: 5th and 95th percentiles of the same dates in other years. London, Paris, Berlin
Temperature, Southern Europe, around the warmings
Show the data as a table
Mean over 42 major warmings, smoothed over 7 days; day 0 = ERA5 central date. Dashed: 5th and 95th percentiles of the same dates in other years. Madrid, Rome
The 42 warmings and what followed (mean of days +5 to +60)
Central date (ERA5)Reanalyses in agreementNAONorthern Europe (°C)North-western Europe (°C)Southern Europe (°C)
Feb 8, 1958 4 -0.73 -1.01 -1.30 -0.13
Jan 17, 1960 4 -0.65 -1.46 1.21 1.46
Jan 27, 1963 3 -0.43 -4.68 -2.69 -1.55
Dec 16, 1965 4 -0.66 -1.14 -0.33 1.46
Feb 22, 1966 4 -0.11 3.66 0.53 -0.30
Jan 7, 1968 3 -0.27 -3.61 0.02 0.03
Nov 28, 1968 4 -0.63 -3.13 -1.12 0.38
Mar 13, 1969 3 0.11 -0.89 -0.45 -0.40
Jan 2, 1970 4 -0.14 -1.85 -0.96 0.84
Jan 18, 1971 4 -0.25 -0.72 -0.02 -0.83
Mar 20, 1971 4 0.08 -1.08 0.92 0.13
Jan 31, 1973 4 0.32 2.78 0.29 -1.20
Jan 9, 1977 3 -0.46 -1.00 1.87 1.62
Feb 22, 1979 5 -0.16 -0.93 -0.91 -1.06
Feb 29, 1980 6 0.14 -2.82 -1.25 -0.96
Mar 4, 1981 4 -0.10 -1.86 0.67 0.60
Dec 4, 1981 6 -0.32 -0.59 -2.17 1.76
Feb 24, 1984 6 -0.16 0.24 -1.44 -1.06
Jan 1, 1985 6 -0.59 -4.95 -3.60 -0.94
Jan 23, 1987 6 -0.26 -2.78 -2.90 -0.45
Dec 8, 1987 6 0.42 -1.01 2.69 1.55
Mar 14, 1988 6 -0.11 -0.23 0.23 0.25
Feb 21, 1989 6 0.57 2.79 0.89 0.72
Dec 15, 1998 6 0.11 1.50 1.05 -1.29
Feb 26, 1999 6 -0.24 1.22 0.58 -0.12
Mar 20, 2000 6 0.08 -0.19 1.03 -0.19
Feb 11, 2001 6 -0.49 -0.30 -0.41 1.08
Dec 30, 2001 6 0.30 5.04 2.42 -0.05
Feb 17, 2002 3 0.45 2.54 0.66 0.04
Jan 18, 2003 5 0.19 -1.26 -0.51 -1.15
Jan 5, 2004 5 -0.34 0.52 0.11 -0.81
Jan 21, 2006 5 -0.58 -5.40 -2.95 -1.43
Feb 24, 2007 5 0.34 2.60 1.69 0.31
Feb 22, 2008 5 -0.23 3.24 -0.83 -0.24
Jan 24, 2009 5 0.05 0.74 -0.62 -0.30
Feb 9, 2010 5 -0.56 0.08 -0.56 -0.80
Mar 24, 2010 5 -0.52 2.90 -0.98 -0.62
Jan 6, 2013 5 -0.33 -1.00 -2.34 -1.20
Feb 12, 2018 5 -0.26 -4.37 -1.83 -1.70
Jan 1, 2019 5 0.11 1.71 0.61 -0.29
Jan 5, 2021 4 -0.38 -2.92 -0.39 0.18
Feb 16, 2023 4 -0.50 1.12 -0.42 0.28
Reanalyses in agreement: number of the six reanalyses of the compendium (including those not covering the year) that detect the event. In blue, negative values (negative NAO, colder than normal).

Aligned series (CSV)

Method and sources

Events: ERA5 central dates of the NOAA sudden warming compendium (Butler et al., 2017): first day of easterly zonal-mean wind at 60° N and 10 hPa (Charlton and Polvani, 2007), that is 42 major warmings from 1958 to 2023; the table has not yet been updated beyond. Responses: CPC daily NAO; mean temperature (Tmax + Tmin) / 2 of ERA5 (Open-Meteo) at Stockholm and Moscow (north), London, Paris and Berlin (northwest), Madrid and Rome (south), mean of the anomalies by region. Anomalies: linear trend removed, then the day-of-year climatology smoothed over ±15 days.

Test: mean of days +5 to +60, a window fixed in advance from the literature, against 1,000 draws in which each date is moved to the same calendar day of another year taken at random (the season of the events is kept). One-sided p (lower NAO, colder weather). Cold days: below the 10th percentile of the December–March anomalies of the region, compared with the same proportion in the draws. Curves are smoothed over 7 days; the dashed lines bound 90 % of the random draws.

Computation parameters
{
    "null": {
        "seed": 20261025,
        "draws": 1000,
        "method": "same calendar dates in other years drawn at random"
    },
    "events": "major SSWs, ERA5 central dates of the SSW Compendium (Butler et al. 2017)",
    "window": "days +5 to +60 (fixed in advance); precursor: days −30 to −1",
    "display": "composites smoothed by a 7-day running mean",
    "cold_day": "below the 10th percentile of the December–March anomalies of the region",
    "anomalies": "linear trend removed, then the day-of-year climatology smoothed over ±15 days",
    "temperature": "(Tmax + Tmin) / 2 of ERA5 (Open-Meteo), regional mean of the city anomalies"
}

Inputs : csl.ssw.era5 (42 pts, run #4275) · cpc.nao.daily (28,053 pts, run #4274) · openmeteo.moscow.tmax.daily (28,050 pts, run #4297) · openmeteo.moscow.tmin.daily (28,050 pts, run #4297) · openmeteo.london.tmax.daily (28,050 pts, run #4297) · openmeteo.london.tmin.daily (28,050 pts, run #4297) · openmeteo.paris.tmax.daily (28,050 pts, run #4297) · openmeteo.paris.tmin.daily (28,050 pts, run #4297) · openmeteo.berlin.tmax.daily (28,050 pts, run #4297) · openmeteo.berlin.tmin.daily (28,050 pts, run #4297) · openmeteo.madrid.tmax.daily (28,050 pts, run #4297) · openmeteo.madrid.tmin.daily (28,050 pts, run #4297) · openmeteo.rome.tmax.daily (28,050 pts, run #4297) · openmeteo.rome.tmin.daily (28,050 pts, run #4297)

Reading this result
After a warming, the NAO is clearly lower (two events out of three) and northern Europe colder, with a third more very cold days; the effect fades towards the northwest and vanishes in the south: the expected geography of a negative NAO and a Scandinavian block. But the anomaly is already there before the event, and stronger: the cold in the north and the negative NAO of the previous month partly reflect the tropospheric blocking that sends waves upwards and triggers the warming. The test therefore does not separate the cause (descent from the stratosphere) from the precursor; it shows that warmings occur in winters that are cold in the north and that the cold persists afterwards, which is enough to make them a useful signal for forecasts a few weeks ahead. Correlation is not causation: common factors (seasons, cycles, trends) or coincidence can produce it. The p-values account for autocorrelation (effective n) but not for preprocessing choices; the confidence interval comes from a block bootstrap. The limits of the source data apply.
SOURCES OF THIS PAGE
Major sudden stratospheric warmings, SSW Compendium (NOAA CSL)
Butler, A. H., Sjoberg, J. P., Seidel, D. J., & Rosenlof, K. H. (2017). A sudden stratospheric warming compendium. Earth System Science Data, 9, 63–76. · doi:10.5194/essd-9-63-2017 · source page ↗
Licence : U.S. Government work, public domain; cite Butler et al. (2017) · retrieved on Oct 25, 2026 · SHA-256 a16e7318c43b…
North Atlantic Oscillation, daily index (NOAA CPC, CDAS)
NOAA CPC, daily North Atlantic Oscillation index: projection of the daily 500 hPa height anomalies of the CDAS reanalysis onto the NAO loading pattern, normalised (1950–). · source page ↗
Licence : U.S. Government work, public domain · retrieved on Oct 25, 2026 · SHA-256 9510633e30f3…
Daily ERA5 reanalysis at 22 reference places (Open-Meteo historical weather API)
Zippenfenig, P. (2024). Open-Meteo.com Weather API (1.4.0). Zenodo. Hersbach, H., et al. (2020). The ERA5 global reanalysis. Q. J. R. Meteorol. Soc., 146, 1999–2049. Contains modified Copernicus Climate Change Service information. · doi:10.5281/zenodo.7970649 · source page ↗
Licence : CC BY 4.0 (Open-Meteo data); ERA5: Copernicus licence, credit C3S/ECMWF; free API for non-commercial use within the fair-use limits · retrieved on Oct 26, 2026 · SHA-256 6fa18a1a023a…
Butler et al. (2017)
Butler, A. H., Sjoberg, J. P., Seidel, D. J., & Rosenlof, K. H. (2017). A sudden stratospheric warming compendium. Earth System Science Data, 9, 63–76. · doi:10.5194/essd-9-63-2017
Licence : Published literature; cited
Charlton et al. (2007)
Charlton, A. J., & Polvani, L. M. (2007). A new look at stratospheric sudden warmings. Part I: Climatology and modeling benchmarks. Journal of Climate, 20, 449–469. · doi:10.1175/JCLI3996.1
Licence : Published literature; cited
Baldwin et al. (2001)
Baldwin, M. P., & Dunkerton, T. J. (2001). Stratospheric harbingers of anomalous weather regimes. Science, 294, 581–584. · doi:10.1126/science.1063315
Licence : Published literature; cited
Kolstad et al. (2010)
Kolstad, E. W., Breiteig, T., & Scaife, A. A. (2010). The association between stratospheric weak polar vortex events and cold air outbreaks in the Northern Hemisphere. Quarterly Journal of the Royal Meteorological Society, 136, 886–893. · doi:10.1002/qj.620
Licence : Published literature; cited
King et al. (2019)
King, A. D., Butler, A. H., Jucker, M., Earl, N. O., & Rudeva, I. (2019). Observed relationships between sudden stratospheric warmings and European climate extremes. Journal of Geophysical Research: Atmospheres, 124, 13943–13961. · doi:10.1029/2019JD030480
Licence : Published literature; cited
Domeisen et al. (2020)
Domeisen, D. I. V., & Butler, A. H. (2020). Stratospheric drivers of extreme events at the Earth’s surface. Communications Earth & Environment, 1, 59. · doi:10.1038/s43247-020-00060-z
Licence : Published literature; cited

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