Quasiresonant amplification of planetary waves and recent Northern Hemisphere weather extremes

Abstract

In recent years, the Northern Hemisphere has suffered several devastating regional summer weather extremes, such as the European heat wave in 2003, the Russian heat wave and the Indus river flood in Pakistan in 2010, and the heat wave in the United States in 2011. Here, we propose a common mechanism for the generation of persistent longitudinal planetary-scale high-amplitude patterns of the atmospheric circulation in the Northern Hemisphere midlatitudes. Those patterns-with zonal wave numbers m = 6, 7, or 8-are characteristic of the above extremes. We show that these patterns might result from trapping within midlatitude waveguides of free synoptic waves with zonal wave numbers k ≈ m. Usually, the quasistationary dynamical response with the above wave numbers m to climatological mean thermal and orographic forcing is weak. Such midlatitude waveguides, however, may favor a strong magnification of that response through quasiresonance. Hemisphere (NH) (1-4). Its remarkable feature was a persistent "blocked" circulation pattern over Europe (1-4). Anomalous heat reigned for much of the summer over a large part of Europe, reaching the highest temperature anomalies in Switzerland, northwestern France, and southern Germany. Schär and colleagues (2004) (3) proposed that the observed climatic warming trend (i) shifted the probability distribution of summer temperatures toward warmer values and (ii) widened this probability distribution so that extreme values become much more likely, possibly as the result of a positive feedback between temperature and soil dryness. We note that a shift and widening of the probability distribution due to global warming almost certainly will lead to a marked increase in the frequency of extreme warmseason events (5-7). However, even when the warming trend found in the data is fully taken into account, and if an increase in SD of 50% is assumed, the extreme temperatures and duration of the summer 2003 heat wave still would be highly unlikely. For example, a return period of 100 y was estimated by Luterbacher et al. (2) for the European region. In the meantime, several other unusually strong regional summer extremes already have occurred in recent years that make it questionable that only a purely stochastic mechanism of extremes is at work (8). The global observations attest that these extremes, such as the Russian heat wave in 2010 and the record heat wave in the United States in 2011, persisted over nearly the whole summer-which is not inherent in ordinary blockings with a characteristic e-folding time of about 5-7 d-and were in fact of hemispheric scale: a stable anomalous atmospheric circulation pattern enveloped the whole NH (4, 9-14).Here, based on daily National Centers for Environmental Prediction-National Center for Atmospheric Research (NCEP-NCAR) (15) reanalysis data, we highlight that the NH midlatitude quasistationary meridional velocity during the aforementioned regional summer extremes was characterized by unusual highamplitude wave patterns with zonal wave numbers m = 6, 7, o...

Role of quasiresonant planetary wave dynamics in recent boreal spring-to-autumn extreme events

“…Petoukhov et al (34) showed that these patterns could result from the trapping of free quasistationary barotropic Rossby waves with zonal wave numbers k equal or close to the three integer values indicated above, within the predominantly zonally oriented midlatitude waveguides. Unlike the one considered in Branstator (26) for zonal wave number 5, the formation of these waveguides is based on a specific change in the latitudinal shape (and not the magnitude) of the quasizonal extratropical winds at the equivalent barotropic level (EBL).…”

“…was subsequently applied to explain several important features of some of the recent summer extremes (18)(19)(20)(21)27 Petoukhov et al (34) proposed a common mechanism for generating persistent high-amplitude quasibarotropic planetaryscale wave patterns of the NH midlatitude atmospheric circulation with zonal wave numbers m = 6, 7, and 8 that can explain a number of the major NH summer extremes over the 1980-2011 period (34,35). Petoukhov et al (34) showed that these patterns could result from the trapping of free quasistationary barotropic Rossby waves with zonal wave numbers k equal or close to the three integer values indicated above, within the predominantly zonally oriented midlatitude waveguides.…”

“…The midlatitude waveguides considered in ref. 34 can favor an onset of midlatitude quasiresonant amplification (QRA) of these waves, causing a strong increase in the atmosphere's dynamical response to quasistationary midtroposphere external forcing with zonal wave numbers m = 6, 7, and 8 (Calculation of the Meridional Wave Number). The reason for the change in the shape and positions of atmospheric jet streams is, for now, an issue of debate, with AA as one of the potential candidates (see, e.g., refs.…”

“…34. In Spring to Autumn Weather Extremes, we investigate, within the framework of QRA, severe regional weather extremes that occurred in boreal spring-to-autumn 2012 and 2013 in the NH.…”

Drivers and potential predictability of summer time North Atlantic polar front jet variability

“…The increased frequency and persistence of extreme events have been linked to a weakening of the zonal-mean jet stream and a more wavy flow pattern, through a decrease in the poleward temperature gradient as a consequence of Arctic amplification Vavrus 2012, 2015), although others argue that such linkages are sensitive to methodologies used (Barnes 2013;Screen and Simmonds 2013). A further possible mechanism that has been suggested as leading to extreme and persistent weather patterns is the amplification of quasistationary waves through resonance between forced and free waves in mid-latitude waveguides (Petoukhov et al 2013;Coumou et al 2014), which could also have an origin in recent changes in the Arctic. However, while identifying possible trends and changes in frequency of events, such studies have not addressed questions of interannual variability, and the potential predictability of summertime jet stream configurations, which is currently low in long-range forecast systems (e.g.…”