Trapped Jet Stream Waves, Which Drive Summer Weather Extremes And Records, Have Tripled Since 1950s

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1. Study: Increased frequency of planetary wave resonance events over the past half-century

https://doi.org/10.1073/pnas.2504482122

Time series of the boreal summer QRA event counts from 1950 to 2024. The annual QRA counts (unitless) based on ERA5 reanalysis data are represented by the blue line, while the series of QRA fingerprints (unitless)—an anomalous meridional temperature “fingerprint” associated with QRA-favorable conditions—using the three series of observational surface temperatures (GISTEMP, HadCRUT4, and Cowtan and Way datasets) are indicated by the green, orange, and red lines, respectively. The QRA fingerprint series (which are available from 1950 to 2016) are centered and rescaled to dimensionally match the annual QRA count series (Eq. 6 in Materials and Methods), allowing for direct comparison of the two series. The linear trends for each series during the period of overlap are depicted by thick dashed lines, with the respective slopes shown in brackets within the legend. The linear trend is also shown for the full QRA event count series through 2024 (thin dashed line). The trends are statistically significant at the P = 0.01 level using either a two-tailed Student’s t test or a nonparametric Mann-Kendall test (39) and the slopes are statistically indistinguishable. Gray vertical dashed lines denote the years when El Niño becomes mature based on the Oceanic Niño Index (ONI) (Materials and Methods). Strong to very strong events are represented by large triangles, while weak to moderate ones are indicated by small triangles (see also SI Appendix, Table S1).