Rare event algorithm study of extreme double jet summers and their connection to heatwaves over Eurasia

Several large-scale circulation patterns have been linked to extreme summer heatwaves in the Northern Hemisphere, notably the double jet structure over Eurasia, the positive phase of the summer Northern Annular Mode (NAM), and quasi-wave-3 geopotential height anomalies. While these patterns are often discussed separately, emerging evidence suggests they may be dynamically related. However, the explicit mechanisms linking these features and their connection to persistent and severe heatwaves remain poorly understood.

Recent studies have highlighted an increase in the persistence of double jet structures, potentially contributing to the rising frequency and intensity of European summer heatwaves. In this work, we examine the occurrence and persistence of double jet states using both the ERA5 reanalysis and a 1000-year run of the Community Earth System Model (CESM). We employ an index to quantify jet separation and we first assess the correct representation of double jet states and their associated teleconnection patterns in CESM. 

To probe rare and persistent events, we couple CESM with a rare event sampling algorithm to identify summer-long extreme double jet states with return times of 100 and 1000 years. These extreme events are characterized by three centers of anomalously high surface temperatures and 500 hPa geopotential height, co-occurring with a strong low-pressure anomaly over the Arctic. The resulting circulation pattern aligns closely with both the positive summer NAM and the quasi-wave-3 patterns identified in previous literature, and remains robust across return periods. Furthermore, we observe a high degree of co-occurrence between these persistent double jet states and heatwaves at the anomaly centers, with co-occurrence rates increasing with the duration of the jet state.