Quasi‐stationary waves and their impact on European weather and extreme events

Abstract

Large‐scale, quasi‐stationary atmospheric waves (QSWs) have long been known to be associated with weather extremes such as the European heatwave in 2003. There is much debate in the scientific literature as to whether QSW activity may increase under a changing climate, providing a strong motivation for developing a better understanding of the behaviour and drivers of QSWs. This paper presents the first steps in this regard: the development of a robust objective method for a simple identification and characterization of these waves. A clear connection between QSWs and European weather and extreme events is confirmed for all seasons, indicating that blocking anti‐cyclones are often part of a broader‐scale wave pattern. Investigation of the QSW climatology in the Northern Hemisphere reveals that wave activity is typically strongest in midlatitudes, particularly at the exit of the Atlantic and Pacific storm track, with weaker intensities in summer. In general, the structure of individual QSW events tends to follow the climatological pattern, except in winter where the strongest and most persistent QSWs are typically shifted polewards, indicating a distinct evolution of the “strongest” QSW events. Modes of interannual variability are calculated to better understand their importance and connection to European temperatures and to identify relevant QSW patterns. This analysis highlights that European winter temperatures are strongly associated with the meridional location of QSW activity whereas high European summer temperatures are associated with increases in the overall intensity of midlatitude QSW activity. QSWs are shown to be strongly connected to commonly used indices to describe the large‐scale atmospheric circulation (NAO, AO, Niño 3.4, PNA) but offer a more direct link to understanding their impact on regional weather events. It is therefore hoped that objective identification of QSWs will provide a useful new viewpoint for interpreting large‐scale weather alongside more traditional measures.

The record‐breaking heat wave of June 2019 in Central Europe

“…In fact, similar blocking‐like circulation patterns were also found to be the main synoptic features during the 2003 and 2010 European heat waves (Trigo et al ., ; Dole et al ., ). It implies that the European heat waves are likely to happen during some specific summertime atmospheric circulation regimes (Cassou et al ., 2005; Wolf et al ., ).…”

Northern Hemisphere Stationary Waves in a Changing Climate

“…S1 for additional stationary wave variables Similar arguments have been used to understand the temporal variance of temperature [63,163]. In particular, some studies have suggested that periods with greater meridional displacement of the jet stream are associated with extreme temperature events in both winter [66][67][68][69] and summer [68][69][70][71][72][73], though it is still actively debated whether global warming and the associated Arctic amplification have an influence on the statistics of these events [74][75][76][77][78]. These studies highlight the importance of quasi-stationary waves, Rossby waves that persist for longer than a week but do not necessarily influence the long-term climatology.…”

“…Variability in the amplitude and/or phase of stationary waves, such as that associated with quasi-stationary waves (QSWs), is thought to be associated with extreme midlatitude weather such as winter cold air outbreaks [66][67][68][69], summer heat waves [68][69][70][71][72][73], heavy precipitation [137][138][139][140], and drought [141][142][143]. QSWs are atmospheric Rossby waves which have a phase speed close to zero.…”

“…QSWs are atmospheric Rossby waves which have a phase speed close to zero. QSWs that influence extreme weather are typically those with anomalously high amplitudes that persist for longer than synoptic timescales and are thus detectable in, for example, 15-day low-pass filtered data [69,73] or monthly anomalies [68]. For the purpose of this review, we will consider QSWs to be any wave-like disturbance that persists for longer than two weeks but less than a season (i.e., subseasonal variability).…”

Large‐Scale Atmospheric Control on Non‐Gaussian Tails of Midlatitude Temperature Distributions

“…This behavior can be explained in terms of the Rossby wave dispersion relation: where c is the Rossby wave phase speed, u A is the advecting velocity, k is the zonal wavenumber, and l is the meridional wavenumber. Assuming the maximum variance occurs when the waves are quasi‐stationary (e.g., Wolf et al, ), that maximum will occur when u A ≈ β /( k 2 + l 2 ). The meridional wavenumber is relatively consistent (between 3 and 4) for runs with the same zonal wavenumber; thus, a particular u A maximizes variance.…”