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PUBLISHED MODELS, CITED MEASUREMENTS
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DATA STATUS
525 series loaded · last import on 2026-11-03

Polar vortex and NAO Does a strong polar vortex in early winter herald a positive NAO the following month?

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
37
Clear links
25
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: Anomalies of the stratospheric vortex descend into the troposphere over a few weeks: a strong vortex favours a positive NAO (mild, wet and windy winter in northern Europe), a weak vortex a negative NAO and cold-air outbreaks. These are the “stratospheric harbingers” of Baldwin and Dunkerton (2001), a basis of seasonal forecasts of the European winter.
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Method and sources

Vortex: zonal-mean zonal wind at 60° N and 10 hPa from NASA’s MERRA-2/GEOS analyses, mean of December and January. NAO: CPC monthly index, mean of January to March, a month later. One point per winter since 1979/80; Pearson, effective sample size of Bretherton et al., 3-year block bootstrap; test repeated after removing variations longer than 21 years. Continuous counterpart of the “sudden stratospheric warmings and the European winter” test, which keeps only the breakdowns of the vortex.

Reading this result
The expected link shows, modest: r ≈ 0.33. After the strongest third of vortices, the January–March NAO averages 0.7, against 0.3 after the weakest. The vortex explains only part of the NAO (about 10 % of its variance): the troposphere has its own variability, and part of the link runs both ways. 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
Northern stratosphere: polar vortex, polar-cap temperature and wave heat flux, daily (NASA GSFC, MERRA-2 and GEOS FP)
NASA GSFC Atmospheric Chemistry and Dynamics Laboratory, Annual Meteorological Statistics (Northern Hemisphere), from MERRA-2 (Gelaro, R., et al., 2017, Journal of Climate, 30, 5419–5454) and GEOS FP. · doi:10.1175/JCLI-D-16-0758.1 · source page ↗
Licence : NASA data, no restriction on use (NASA Earth science data policy); cite MERRA-2 (Gelaro et al., 2017). The GEOS forecasts are experimental and given for research · retrieved on Nov 2, 2026 · SHA-256 7df51d62c772…
North Atlantic Oscillation index (NOAA CPC)
NOAA CPC, monthly North Atlantic Oscillation index (rotated PCA of 500 hPa height anomalies, 1950–). · source page ↗
Licence : U.S. Government work, public domain · retrieved on Oct 23, 2026 · SHA-256 1d4c1cdcc5d0…
Baldwin et al. (2001)
Baldwin, M. P., & Dunkerton, T. J. (2001). Stratospheric harbingers of anomalous weather regimes. Science, 294(5542), 581–584. · doi:10.1126/science.1063315
Licence : Published literature; cited

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