600 years of reconstructed summertime extreme jet stream behavior driven by a wavenumber-5 Rossby wave pattern

In the Northern Hemisphere, the jet stream is an important driver of terrestrial climate, including extreme events such as heat waves. As such, there is interest in understanding the jet stream's response to anthropogenic greenhouse gas forcing. One hypothesis posits that under anthropogenic global warming, the jet stream is becoming more meandering, or wavier, which can be described by the "wavenumbers" of planetary Rossby waves that influence jet stream position. Wavenumbers have been described in studies focused on the instrumental period, which demonstrate that some summertime wavenumber patterns, such as wavenumbers 5 and 7, are associated with a meandering jet stream that drives hot, dry conditions in regions of North America, Europe, and Asia, impacting important food production regions. However, despite interest in current and future jet stream behavior, little is known about its past and natural variability, making it difficult to determine whether these wavenumbers have become more frequent in recent decades.

In this first attempt to reconstruct Northern Hemisphere Rossby waves and associated jet stream behavior using paleoclimate data, we present a 600-year reconstruction of summer extreme wavenumber 5 events (wave-5) based on tree-ring-inferred Palmer Drought Severity Index (PDSI) data compiled in the North American Drought Atlas (NADA) and the Old World Drought Atlas (OWDA). We developed a wave-5 index as our reconstruction target based on the number of weeks during June and July that wave-5 is in its preferred Rossby wave phase to induce simultaneous heat extremes. Our reconstruction targets PDSI anomalies associated with extreme wave-5 events in three regions across the Northern Hemisphere (two in NADA and one in OWDA) using a stepwise linear regression, and explains ~40% of wave-5 variability over the instrumental period (1948-2005). Our preliminary results suggest that the occurrence of extreme wave-5 jet stream positions has not been increasing in recent decades, and that the frequency of these wave-5 positions has occurred with a pronounced, multi-decadal cyclicity for centuries. As such, our results suggest that this aspect of jet stream behavior has remained stable under global warming.