Quasi-resonant circulation regimes and hemispheric synchronization of extreme weather in boreal summer

The recent decade has seen an exceptional number of high-impact summer extremes in the Northern Hemisphere midlatitudes. Many of these events were associated with anomalous jet stream circulation patterns characterized by persistent high-amplitude quasi-stationary Rossby waves. Two mechanisms have recently been proposed that could provoke such patterns: (i) a weakening of the zonal mean jets and (ii) an amplification of quasi-stationary waves by resonance between free and forced waves in midlatitude waveguides. Based upon spectral analysis of the midtroposphere wind field, we show that the persistent jet stream patterns were, in the first place, due to an amplification of quasi-stationary waves with zonal wave numbers 6–8. However, we also detect a weakening of the zonal mean jet during these events; thus both mechanisms appear to be important. Furthermore, we demonstrate that the anomalous circulation regimes lead to persistent surface weather conditions and therefore to midlatitude synchronization of extreme heat and rainfall events on monthly timescales. The recent cluster of resonance events has resulted in a statistically significant increase in the frequency of high-amplitude quasi-stationary waves of wave numbers 7 and 8 in July and August. We show that this is a robust finding that holds for different pressure levels and reanalysis products. We argue that recent rapid warming in the Arctic and associated changes in the zonal mean zonal wind have created favorable conditions for double jet formation in the extratropics, which promotes the development of resonant flow regimes.Climatic warming over the 20th century has increased the frequency of extreme heat and heavy rainfall events (1–7). On a global scale, the magnitude of this gradual increase can largely be explained by a slowly warming atmosphere, i.e., by thermodynamic arguments only. Thus, the rise in the number of heat extremes can largely be explained by a shift in the mean to warmer values (4, 5, 8). Likewise, upward trends in annual maximum daily rainfall are consistent with the increase in atmospheric moisture associated with warmer air (1, 2).Global warming is also likely to affect large-scale atmospheric circulation patterns, which potentially could alter the frequency of heat and precipitation extremes on seasonal to subseasonal timescales (9–11). In principle, changes in atmospheric dynamics could cause a disproportionate change in the number and/or intensity of extreme weather events (12–14), beyond what is expected from thermodynamics. Moreover, the magnitude of several recent summer extreme weather events in the Northern Hemisphere midlatitudes cannot be explained by a simple shift in the mean (12, 15, 16). These events, which include high-impact extremes like the European heat wave of 2003 (15), the Russian heat wave and the Pakistan flooding in 2010 (17), and heat waves in the United States in recent years (18), were associated with anomalous circulation patterns characterized by persistent, blocking weather conditions (10, 19–22).