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

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.

Figures (20)

• Figure 1: Example of the two zones used to define a double jet: $B$ between the latitudinal band 65-78.5$^\circ$N and $A$ between 46.5-60$^\circ$N.
• Figure 2: (a) Histograms and kernel density estimation of the daily double jet index for CESM1.2 (blue) and ERA5 (orange). (b) The deseasonalised and standardized histograms. The vertical lines indicate the thresholds corresponding to the 5% most extreme events chosen to define a double jet in \ref{['fig:vertical_zonalwind_cesm', 'fig:vertical_zonalwind_era']}.
• Figure 3: (a) Vertical profile of the daily summer zonal wind $U$ for the climate model CESM1.2 (1000 years), longitudinally averaged over the Eurasian sector (10$^\circ$W to 180$^\circ$E). We considered the months of June, July and August - (b) Zonal wind $U$ climatology for the CESM1.2 climate model (1000 years) for the months of June, July and August, (c) - (d) same variable as for (a) - (b) but when selecting days corresponding to the 5% most extreme values of the double jet index. There is a clear sign of a second separate jet appearing at high latitudes.
• Figure 4: (a) Vertical profile of the daily summer zonal wind $U$ for the reanalysis dataset ERA5, longitudinally averaged over the Eurasian sector (10$^\circ$W to 180$^\circ$E). We considered the months of June, July and August - (b) Zonal wind $U$ climatology for the reanalysis dataset ERA5 for the months of June, July and August, (c) - (d) same variable as for (a) - (b) but when selecting days corresponding to the 5% most extreme values of the daily double jet index. There is a clear sign of a second separate jet appearing at high latitudes.
• Figure 5: Composite maps for (a) $T_{2m}$ and (c) $Z_{500}$ anomalies for 5% most extreme double jet days for CESM1.2 control run, (b) - (d) same but for ERA5.