European heatwaves: Link to large‐scale circulation patterns and intraseasonal drivers

Abstract

This study examines the European heatwaves' predictability at subseasonal timescales. Land surface feedbacks and tropical convection, due to their variability at intraseasonal timescales, are taken into consideration and their potential role in extending the predictability beyond the medium range (10 days) is explored. A classification of European heatwaves into five heatwave types is used to discriminate the effects of surface feedbacks and of tropical variability among the different heatwave types. The classification is computed in terms of circulation patterns. By inferring the near‐surface temperature through atmospheric circulation, we aim to identify the predictable component of the heatwave events. All five heatwave circulation patterns are characterized by persistent anticyclonic anomalies located over the region with maximum temperatures. We show that soil moisture deficit is not a required precondition for the occurrence of heatwaves over most of Europe. However, heatwave events over southern Europe exhibit some sensitivity to dry conditions. We use a simplified index to describe the dominant mode of tropical convection at intraseasonal timescales. The index, based on precipitation anomalies, represents the evolution of the Boreal Summer Intraseasonal Oscillation (BSISO). We find that episodes with strong BSISO amplitudes characterized by enhanced convection over India, Bay of Bengal and China sea favour the occurrence of heatwave events over Russia. The results highlight the role of tropical intraseasonal variability in enhancing the predictability of some extreme temperature events over Europe.

Pattern-based forecasting enhances the prediction skill of European heatwaves at the sub-seasonal range

“…In Rouges et al (2023), HWs have been identi ed and clustered based on their atmospheric circulation.…”

“…The HWs have been de ned using the daily mean 2mT above the 90th percentile for at least four days over a region of 1000km. This resulted in ve HW types characterised by speci c CPs (Rouges et al, 2023). Figure 1 shows the probability of 2mT exceeding the 90th percentile during each of the patterns.…”

“…For some CPs, such as the Russian (RU), Scandinavian (SC) and Tripole CPs, the probability is up to four times higher. One of the ndings of Rouges et al (2023) was that HWs coincide more with persisting CPs ( ve days or longer). To further investigate this result, the conditional probability of 2mT exceeding the 90th percentile for four days during CPs that last for at least ve days has been computed.…”

“…To forecast the different CPs, the forecasted daily Z500 anomaly eld is projected onto the composite Z500 eld anomaly from ERA5 (see Rouges et al, 2023) of each pattern for each forecast. This allows for the creation of indices which are a measure of the similarity of the atmospheric conditions compared to the different patterns.…”

“…

The prediction of European heatwaves at the subseasonal range is of key importance to mitigate their impact. This study builds on previous work (Rouges et al, 2023) which identi es ve main European heatwave types based on their atmospheric circulation patterns (CPs). These CPs are potential predictors of heatwaves, as these patterns are connected with a high probability of 2-meter temperature exceeding the 90th percentile.

…”