Rare events algorithm study of extreme double jet summers and their connection to heatwaves over the Northern Hemisphere

TL;DR

This study investigates the occurrence and persistence of extreme double jet states over Eurasia and their connection to heatwaves in the Northern Hemisphere, using advanced simulations and rare event algorithms to improve statistical understanding.

Contribution

It introduces a rare event algorithm applied to climate modeling to better characterize and analyze extreme double jet states and their link to heatwaves.

Findings

Extreme double jet states are associated with high surface temperatures and geopotential height anomalies.

A significant co-occurrence exists between heatwaves and double jet states, increasing with the duration of the jet state.

The geopotential height pattern aligns with known climate modes like NAM and quasi-wave-3.

Abstract

Several large scale circulation patterns have been identified in relation to extreme Northern Hemisphere summer heatwaves. Three main ones are a double jet over Eurasia, a positive phase of the summer northern annular mode, and a quasi-wave-3 geopotential height anomaly. While there is some evidence suggesting these patterns are related to each other, the explicit nature of their relation, as well as the explicit mechanisms by which they are related to extreme heatwaves is still not known. The double jet structure has gained attention recently due to evidence that its persistence has been increasing, possibly explaining the rise in the number of extreme heatwaves over Europe. In this paper we study the occurrence and persistence of double jet states in ERA5 and in stationary simulations with the CESM1.2 model, using an index which measures the degree of jet separation. Additionally, we perform simulations with CESM1.2 coupled to a rare event algorithm in order to improve the statistics of rare summer-long double jet states. We find that extreme double jet states are characterised by three centers of extreme high surface temperature and 500hPa geopotential height anomalies, alongside a strong low pressure over the Arctic. The geopotential height anomaly pattern is consistent with both a positive Northern Annular Mode (NAM) and quasi-wave-3 patterns found in the literature. Moreover, we find a large percentage of co-occurrence of heatwaves at these centers, and a double jet state, with the percentage increasing with the duration of the double jet state.