A Millennium of ENSO Influence on Jet Stream Driven Summer Climate Extremes

Summertime spatially compound climate extremes in the Northern Hemisphere are associated with dominant jet stream Rossby wavenumber patterns, including wavenumber5 (wave5). However, our knowledge of wave5, including its response to anthropogenic warming, is limited by the short length of instrumental records of upper-level fields. To provide a longer-term perspective, we present a 1,000-year reconstruction of a wave5 pattern that modulates summertime compound extremes, constructed by targeting drought anomalies associated with this pattern in three regions. Our results show no major trends in the occurrence of this pattern over the past millennium. We further show that La Niña winters often precede a wave5 event the following summer, evident over centuries. This pattern was exemplified by the La Niña winter of 2022–2023, which was followed by wave5-driven compound heatwaves in July. The imprint of continued anthropogenic warming on ENSO may exacerbate wave5-driven extremes, especially if the tropical Pacific becomes more La Niña-like.