Jet regimes, waviness metrics, and links to extreme weather

Extreme midlatitude weather is often associated with pronounced Rossby waves. This has motivated interest in how the ‘waviness’ of the atmosphere is changing as Earth warms. Multiple summary metrics have been used to assess midlatitude waviness, which include both descriptions of the magnitudes of associated anomalies in geopotential height, and geometric measures of deviations of the jet from a more zonal state.

Recent work illustrated that a) these metrics can respond differently to warming, and that the same metric can respond differently to warming applied in different ways (Geen et al. 2023), and b) that different metrics can link to rather different patterns of extreme temperature (Roocroft et al. 2025). It remains unclear what specific types of characteristic jet structures these various metrics capture, and how these dynamically link to surface weather extremes.

Here, we first explore how different metrics relate to extreme winter weather events (cold, rain and wind) over Europe and North America, and how these relationships compare to known modes of climate variability such as the NAO. Next, to explore underlying jet structures driving these extremes, we apply a Self Organising Maps analysis to 500-hPa geopotential height anomalies. This allows us to map the values taken by different metrics and the likelihoods of extreme events for different jet configurations in a reduced dimensionality space.

 

References

Geen, R., Thomson, S. I., Screen, J. A., Blackport, R., Lewis, N. T., Mudhar, R., ... & Vallis, G. K. (2023). An explanation for the metric dependence of the midlatitude jet‐waviness change in response to polar warming. Geophysical Research Letters, 50(21), e2023GL105132.

Roocroft, E., White, R. H., & Radić, V. (2025). Linking atmospheric waviness to extreme temperatures across the Northern Hemisphere: Comparison of different waviness metrics. Journal of Geophysical Research: Atmospheres, 130(20), e2024JD042631.