Quasi-resonant Amplification Research Articles

Increased projected changes in quasi-resonant amplification and persistent summer weather extremes in the latest multimodel climate projections

High-amplitude quasi-stationary atmospheric Rossby waves with zonal wave numbers 6–8 associated with the phenomenon of quasi-resonant amplification (QRA) have been linked to persistent summer extreme weather events in the Northern Hemisphere. QRA is not well-resolved in current generation climate models, therefore, necessitating an alternative approach to assessing their behavior. Using a previously-developed fingerprint-based semi-empirical approach, we project future occurrence of QRA events based on a QRA index derived from the zonally averaged surface temperature field, comparing results from CMIP 5 and 6 (Coupled Model Intercomparison Project). There is a general agreement among models, with most simulations projecting substantial increase in QRA index. Larger increases are found among CMIP6-SSP5-8.5 (42 models, 46 realizations), with 85% of models displaying a positive trend, as compared with 60% of CMIP5-RCP8.5 (33 models, 75 realizations), with a reduced spread among CMIP6-SSP5-8.5 models. CMIP6-SSP3-7.0 (23 models, 26 realizations) simulations display qualitatively similar behavior to CMIP6-SSP5-8.5, indicating a substantial increase in QRA events under business-as-usual emissions scenarios, and the results hold regardless of the increase in climate sensitivity in CMIP6. Projected aerosol reductions in CMIP6-SSP3-7.0-lowNTCF (5 models, 16 realizations) lead to halting effect in QRA index and Arctic Amplification during the 1st half of the twenty-first century. Our analysis suggests that anthropogenic warming will likely lead to an even more substantial increase in QRA events (and associated summer weather extremes) than indicated by past analyses.

Underlying physical mechanisms of winter precipitation extremes over India's high mountain region

AbstractExtreme precipitation events (EPEs) are among the most pervasive weather hazards in the western Himalayan region (WHR), posing widespread damage to life, infrastructure, and agriculture. This study investigates the synoptic and large‐scale characteristics linked to winter precipitation extremes over the WHR. EPEs are identified as events surpassing the 95th percentile threshold. A composite analysis is employed using two reanalyses—the fifth‐generation European Centre for Medium‐Range Weather Forecasts Reanalysis (ERA5) and the Indian Monsoon Data Assimilation and Analysis (IMDAA)—to elucidate the synoptic conditions conducive to EPEs. Our findings suggest that EPEs in the WHR are linked to an intensified subtropical westerly jet, characteristically shifted to south than normal. Enhanced kinetic energy in the upper troposphere, attributed to increased baroclinic instability, reinforces moisture convergence and strengthens synoptic‐scale circulation, triggering deep convection and supporting EPEs. Notably, the interplay of pronounced Rossby waves sinking over the WHR and regional orography significantly modulates the intensity of western disturbances (WDs). Employing clustering analysis, we observed that the strongest EPEs are linked to anomalous vorticity in the upper to middle troposphere, together with deep convection via strengthened WDs, suggesting the potential role of large‐scale influences. Using Lagrangian method, we identify that the Arabian Sea is the primary moisture source for EPEs in the WHR. We further delved into the role of large‐scale connections and EPEs through quasi‐resonant amplification (QRA) analysis. The findings unveil distinct QRA fingerprints in meridional temperature gradients along with notably magnified, quasi‐stationary midlatitude planetary waves characterized by zonal wave numbers 6/7/8 (baroclinic waves) contributing to EPEs. Overall, our results highlight the underlying physical mechanisms for winter precipitation extremes, emphasizing QRA's role in amplifying planetary waves and promoting EPEs, underscoring the WHR's vulnerability to evolving climatic conditions.

Role of atmospheric resonance and land–atmosphere feedbacks as a precursor to the June 2021 Pacific Northwest Heat Dome event

We demonstrate an indirect, rather than direct, role of quasi-resonant amplification of planetary waves in a summer weather extreme. We find that there was an interplay between a persistent, amplified large-scale atmospheric circulation state and soil moisture feedbacks as a precursor for the June 2021 Pacific Northwest "Heat Dome" event. An extended resonant planetary wave configuration prior to the event created an antecedent soil moisture deficit that amplified lower atmospheric warming through strong nonlinear soil moisture feedbacks, favoring this unprecedented heat event.

Synoptic dynamics of cold waves over north India: Underlying mechanisms of distinct cold wave conditions

The cold air outbreaks in the northern parts of India exert significant impacts on human health, energy, agriculture and transportation. In this study, we investigate the synoptic dynamics of cold waves and their linkages to large scale circulations for the winter period from 1982 to2020. Cold waves are classified into normal and intense (NCWs and ICWs) based on intensity and we examine the underlying mechanisms of formation and their atmospheric drivers. Notably, the spatial extent of the ICWs is almost double than that of normal ones thereby having the potential to affect a wider population. The NCWs are often influenced by western disturbances, leading to the inflow of cold air from Siberia (a region of shallow high). In contrast, ICWs are mostly linked to the presence of an omega block over the Ural-Siberian region. The downstream portion of the Ural block favours the inflow of cold northerlies into north India, leading to cold air advection and extreme cold wave conditions. The influence of Arctic warming for ICWs is further confirmed through a prominent Quasi-Resonant Amplification (QRA) fingerprint. Furthermore, La Niña condition seems to play a crucial role in triggering ICWs over north India. During La Niña, the prominent low level cyclonic anomaly helps in advecting the cold air from the higher latitudes into the country. The frequency as well as the duration of cold wave events are also found to be higher in La Niña years compared to El Niño and neutral years. The trend analysis of cold wave events over north India reveals a significant decrease in the frequency, duration and intensity during the analysis period due to a combination of various factors such as rising winter minimum temperatures (due to global warming), decreasing number of synoptic winter weather systems and Arctic amplification.

Response of extreme precipitation to increasing extratropical cyclonic vortex frequency

Since the 2000s, extratropical extremes have been more frequent, which are closely related to anomalies of planetary-scale and synoptic-scale systems. This study focuses on a key synoptic system, the extratropical cyclonic vortex (ECV) over land, to investigate its relations with extreme precipitation. It was found that ECVs have been more active post-2000, which has induced more extreme precipitation, and such variation is projected to persist along with increasing temperature within 1.5°C of global warming. An enhanced quasi-stationary vortex (QSV) primarily contributes to the ECV, rather than inactive synoptic-scale transient eddies (STEs). Inactive STEs respond to a decline in baroclinicity due to the tendency of the homogeneous temperature gradient. However, such conditions are helpful to widening the westerly jet belt, favoring strong dynamic processes of quasi-resonant amplification and interaction of STEs with the quasi-stationary wave, and the result favors an increasing frequency and persistence of QSVs, contributing to extreme precipitation.摘要自21世纪以来, 热带外极端降水频次增加. 随着中高纬度的显著增温, 经向温度梯度减弱导致低层大气斜压性减小, 由此产生的气旋型瞬变涡天气系统等活动减弱. 然而, 热力分布导致西风急流带变宽, 经向环流加大, 有助于行星尺度波动相关的涡旋异常增加, 如东北冷涡, 中亚涡, 东欧-地中海涡, 北美涡等, 进而增加了气旋涡影响范围的极端降水频次. 在未来变暖背景和1.5°C增温的目标内, 热带外气旋涡增强会进一步促进极端降水发生.

Connection of Quasi-Resonant Amplification to the Delay in Atmospheric Residence Times Over India

Mean monthly Atmospheric Residence Times (ART), deduced from the global climate models of Coupled Model Intercomparison Project 5 (CMIP5) under RCP4.5 and RCP8.5 emission scenarios over Indian landmass, show a perceptible increase by the end of the 21st century. India, being a tropical country, faces prolonged ART, particularly during the June month of Southwest monsoon season (June to September) which will be an indicative measure of the increased frequency of extreme weather events. Here we show a possible connection of quasi-resonant amplification (QRA) to the recent (August 2018) Kerala heavy rains that resulted in severe floods and claimed more than 400 mortalities. Remarkable delay in residence times over India during June is shown to have an association with QRA evidenced by the higher magnitudes of amplitudes at the wavenumbers six and seven from the 19 global climate models of CMIP5 under the RCP4.5 and RCP8.5 scenarios.

Large‐scale connection to deadly Indian heatwaves

AbstractIn the present study, we analyse large‐scale connections to deadly heatwaves in India in spring. Deadly Indian heatwaves occur in the spring months of April and May, and are increasing in recent years. These events are associated with the amplification of planetary Rossby waves. This amplification is called quasi‐resonant amplification (QRA). An increase of amplitude or resonance occurs when stationary and moving waves coincide in their size. Previous studies stressed QRA's role in summer because persistent extreme weather events such as the 2003 European heatwave, the 2010 Pakistan flood, Russian heatwave, the 2011 Texas and Oklahoma heat waves, and drought occurred in summer. QRA seems to be related to the amplification of Arctic warming in winter and summer, but observations show that Arctic warming occurs in spring also. The novel aspect of our study is that we found strong evidence for QRA in the boreal spring, the season of Indian heatwaves. Amplified Arctic warming or Arctic amplification thought to be responsible for QRA occurs in spring also. Our results confirm that maximum surface temperatures in spring over India show predictability with high statistical significance, about 4 days ahead. Maximum surface temperatures can be used to predict heatwaves, as shown earlier by some authors.

Wind-driven decadal sea surface height and main pycnocline depth changes in the western subarctic North Pacific

The northward shrinkage of the North Pacific western subarctic gyre (WSAG) in the early 2000s is associated with a sea surface height (SSH) elevation and is correlated to sea surface wind stress change. By using a Rossby wave model forced by wind stress, which computes the component variations due to the barotropic and first to fourth baroclinic modes, we estimated decadal changes in SSH and main pycnocline depth in the subpolar region. Realistic decadal SSH elevation and deepening of the main pycnocline depth associated with the northward shrinkage of the western subarctic gyre from the late 1990s to the mid-2000s were reproduced by the model. The sea surface elevation was caused primarily by the barotropic Rossby wave response to the relaxation of the Ekman suction due to the attenuation of the Aleutian Low by frequent La Niña occurrences after the late 1990s in addition to the long-term weakening of the westerly wind. The northward shrinkage of the WSAG was found to be associated with the intensification of an anticyclonic circulation centered around 43–44 ∘ N, 170–175 ∘ E. The westerly wind weakening deepened the main pycnocline in the western subarctic region through the baroclinic Rossby wave mode response to the wind stress change, which mostly accounts the equivalent halocline deepening at station K2 (47 ∘ N, 160 ∘ E). While the first baroclinic mode variation of the water density significantly attenuates during propagation, the higher mode variations, particularly the second and third mode variations, are locally excited through a quasi-resonant amplification mechanism and have profound impacts on the depth of the upper main pycnocline.

Assessing changes in risk of amplified planetary waves in a warming world

AbstractSummer weather extremes are often associated with high‐amplitude atmospheric planetary waves (Petoukhov et al., 2013). Such conditions lead to stationary weather patterns, triggering heat waves and sometimes prolonged intense rainfall. These wave events, referred to as periods of Quasi‐Resonant Amplification (QRA), are relatively rare though and hence provide only a few data points in the meteorological record to analyse. Here, we use atmospheric models coupled to boundary conditions that have evolved slowly (i.e., climate), to supplement measurements. Specifically we assess altered probabilities of resonant episodes by employing a unique massive ensemble of atmosphere‐only climate simulations to populate statistical distributions of event occurrence. We focus on amplified waves during the two most extreme European heat waves on record, in years 2003 and 2015 (Russo et al., 2015). These years are compared with other modelled recent years (1987–2011), and critically against a modelled world without climate change. We find that there are differences in the statistical characteristics of wave event likelihood between years, suggesting a strong dependence on the known and prescribed Sea Surface Temperature (SST) patterns. The differences are larger than those projected to have occurred under climate change since the pre‐industrial period. However, this feature of small differences since pre‐industrial is based on single large ensembles, with members consisting of a range of estimates of SST adjustment from pre‐industrial to present. Such SST changes are from projections by a set of coupled atmosphere–ocean (AOGCM) climate models. When instead an ensemble for pre‐industrial estimates is subdivided into simulations according to which AOGCM the SST changes are based on, we find differences in QRA occurrence. These differences suggest that to reliably estimate changes to extremes associated with altered amplification of planetary waves, and under future raised greenhouse gas concentrations, likely requires reductions in any spread of future modelled SST patterns.

Projected changes in persistent extreme summer weather events: The role of quasi-resonant amplification.

Persistent episodes of extreme weather in the Northern Hemisphere summer have been associated with high-amplitude quasi-stationary atmospheric Rossby waves, with zonal wave numbers 6 to 8 resulting from the phenomenon of quasi-resonant amplification (QRA). A fingerprint for the occurrence of QRA can be defined in terms of the zonally averaged surface temperature field. Examining state-of-the-art [Coupled Model Intercomparison Project Phase 5 (CMIP5)] climate model projections, we find that QRA events are likely to increase by ~50% this century under business-as-usual carbon emissions, but there is considerable variation among climate models. Some predict a near tripling of QRA events by the end of the century, while others predict a potential decrease. Models with amplified Arctic warming yield the most pronounced increase in QRA events. The projections are strongly dependent on assumptions regarding the nature of changes in radiative forcing associated with anthropogenic aerosols over the next century. One implication of our findings is that a reduction in midlatitude aerosol loading could actually lead to Arctic de-amplification this century, ameliorating potential increases in persistent extreme weather events.

Recent enhanced high-summer North Atlantic Jet variability emerges from three-century context

A recent increase in mid-latitude extreme weather events has been linked to Northern Hemisphere polar jet stream anomalies. To put recent trends in a historical perspective, long-term records of jet stream variability are needed. Here we combine two tree-ring records from the British Isles and the northeastern Mediterranean to reconstruct variability in the latitudinal position of the high-summer North Atlantic Jet (NAJ) back to 1725 CE. We find that northward NAJ anomalies have resulted in heatwaves and droughts in northwestern Europe and southward anomalies have promoted wildfires in southeastern Europe. We further find an unprecedented increase in NAJ variance since the 1960s, which co-occurs with enhanced late twentieth century variance in the Central and North Pacific Basin. Our results suggest increased late twentieth century interannual meridional jet stream variability and support more sinuous jet stream patterns and quasi-resonant amplification as potential dynamic pathways for Arctic warming to influence mid-latitude weather.

Summertime Planetary Wave Resonance in the Northern and Southern Hemispheres

Slow-moving planetary waves of high amplitudes are often associated with persistent surface weather conditions. This persistence can lead to extreme weather events with potentially serious implications for society and nature. Quasi-resonant amplification (QRA) of planetary waves has been proposed as a mechanism to generate high-amplitude hemisphere-wide patterns of wavenumbers 6–8 in the Northern Hemisphere (NH) summer. Here this mechanism is studied in both hemispheres. Analyzing 1979–2015 reanalysis data, evidence for QRA in the Southern Hemisphere (SH) is found for wavenumbers 4 and 5. It is shown that the difference in resonating wavenumbers between hemispheres stems from the different magnitude and latitudinal shape of the respective zonal-mean zonal winds. During resonance events a strong and narrow jet is observed in both hemispheres, whereas the emergence of a second zonal mean jet at high latitudes (i.e., a “double jet”) is seen in the NH only. Strong and narrow jets can form stable, zonally oriented waveguides, which are an essential prerequisite for resonance. After investigating the waves’ preferred phase positions during QRA, a bimodal behavior is identified for wavenumbers 4 and 5 in the SH and for wavenumber 6 in the NH, whereas wavenumbers 7 and 8 exhibit a single preferred phase position in the NH. Composite plots of meridional wind and temperature anomalies during periods when the resonating wave is in the preferred phase position indicate those regions that are most likely to experience heat extremes. These regions include western North America, western Europe, and central Eurasia in the NH and Patagonia and Australia in the SH.

Influence of Anthropogenic Climate Change on Planetary Wave Resonance and Extreme Weather Events

Persistent episodes of extreme weather in the Northern Hemisphere summer have been shown to be associated with the presence of high-amplitude quasi-stationary atmospheric Rossby waves within a particular wavelength range (zonal wavenumber 6–8). The underlying mechanistic relationship involves the phenomenon of quasi-resonant amplification (QRA) of synoptic-scale waves with that wavenumber range becoming trapped within an effective mid-latitude atmospheric waveguide. Recent work suggests an increase in recent decades in the occurrence of QRA-favorable conditions and associated extreme weather, possibly linked to amplified Arctic warming and thus a climate change influence. Here, we isolate a specific fingerprint in the zonal mean surface temperature profile that is associated with QRA-favorable conditions. State-of-the-art (“CMIP5”) historical climate model simulations subject to anthropogenic forcing display an increase in the projection of this fingerprint that is mirrored in multiple observational surface temperature datasets. Both the models and observations suggest this signal has only recently emerged from the background noise of natural variability.

Evidence for wave resonance as a key mechanism for generating high-amplitude quasi-stationary waves in boreal summer

Several recent northern hemisphere summer extremes have been linked to persistent high-amplitude wave patterns (e.g. heat waves in Europe 2003, Russia 2010 and in the US 2011, Floods in Pakistan 2010 and Europe 2013). Recently quasi-resonant amplification (QRA) was proposed as a mechanism that, when certain dynamical conditions are fulfilled, can lead to such high-amplitude wave events. Based on these resonance conditions a detection scheme to scan reanalysis data for QRA events in boreal summer months was implemented. With this objective detection scheme we analyzed the occurrence and duration of QRA events and the associated atmospheric flow patterns in 1979–2015 reanalysis data. We detect a total number of 178 events for wave 6, 7 and 8 and find that during roughly one-third of all high amplitude events QRA conditions were met for respective waves. Our analysis reveals a significant shift for quasi-stationary waves 6 and 7 towards high amplitudes during QRA events, lagging first QRA-detection by typically one week. The results provide further evidence for the validity of the QRA hypothesis and its important role in generating high amplitude waves in boreal summer.

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

In boreal spring-to-autumn (May-to-September) 2012 and 2013, the Northern Hemisphere (NH) has experienced a large number of severe midlatitude regional weather extremes. Here we show that a considerable part of these extremes were accompanied by highly magnified quasistationary midlatitude planetary waves with zonal wave numbers m = 6, 7, and 8. We further show that resonance conditions for these planetary waves were, in many cases, present before the onset of high-amplitude wave events, with a lead time up to 2 wk, suggesting that quasiresonant amplification (QRA) of these waves had occurred. Our results support earlier findings of an important role of the QRA mechanism in amplifying planetary waves, favoring recent NH weather extremes.

Reply to Screen and Simmonds: From means to mechanisms

Our recent article on quasiresonant amplification of planetary waves (1) has been addressed in a letter by Screen and Simmonds (2). They report, in particular, that they do not find statistically significant trends in the mean amplitudes of m = 6–8 waves in the 1979–2012 period. This observation does not contradict the statements made in our paper–it actually complements our work and helps to clarify several important issues:

Quasiresonant amplification of planetary waves and recent Northern Hemisphere weather extremes

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.

Compensation of self-focusing distortions in quasiresonant amplification of a light pulse

A theoretical analysis is made of the compensation of a positive nonlinear refractive index of insulators containing impurities by saturation of the refractive index in the anomalous dispersion region. The exact solutions are obtained for self-similar amplification of a laser pulse under conditions of homogeneous broadening of a resonant transition. It is shown that profiling of the time envelope of a steady-state bleaching wave of a saturable absorber together with detuning of the pulse frequency upward from resonance makes it possible to increase the threshold of small-scale self-focusing.