Dynamical systems methods to understand projected heatwave intensification

Heatwaves pose well-known health dangers, and carry socio-economic and ecological consequences. Blocking highs typically drive such heatwaves during the European summer. The dynamics, surface impacts, and sensitivity to climate forcing of such events are of great interest, but because analysis of these events is sensitive to methodological details, a multi-faceted approach is needed to derive robust results. Such an analysis is carried out here, for observations and future projections. Heatwaves at meteorological stations, defined in terms of the discomfort index, which combines temperature and humidity, are well-captured in reanalysis. Reanalysis also reveals an expected equivalent-barotropic anticyclonic anomaly, with anomalously slow midtropospheric westerlies, associated with these heatwaves. A strong spatial correspondence to this structure is also found with a dynamical-systems theoretic analysis. The latter extracts the most-persistent patterns of midtropospheric flow in terms of the so-called ‘persistence metric’, ${\theta }^{-1}$. Heatwaves and blocks are far more likely to occur during persistent states. Historic and end-of-21st-century projections capture similar behavior, and the distribution of projected ${\theta }^{-1}$ remains largely unchanged, indicating little change in extreme-event persistence. Neither the frequency nor the duration of persistent blocks changes in end-of-century projections, but heatwave intensity does increase. The conclusion is thus that the projected intensification of heatwaves arises from a thermodynamic mechanism and not a dynamic one. This conclusion depends on removing a multi-year running mean background from the flow for the persistence analysis. Without this high-pass filtering, a projected secular increase in persistence arises as the flow becomes characterized by a regional warming trend pattern.