Abstract. The Northern Hemisphere wintertime circulation response to the eruptions of Krakatoa and Pinatubo is revisited in large ensembles from eight modeling centers with only one time-varying external forcing: volcanic eruptions. All eight models show a warming of the tropical lower stratosphere. In six of the models, the meridional temperature gradient in the winter stratosphere is enhanced, leading to a strengthened stratospheric polar vortex, a positive phase of the North Atlantic Oscillation, a poleward shift in storm tracks, and warm surface temperatures during the winter over subpolar Eurasia. While this warming over subpolar Eurasia is statistically significant in the multi-model mean and in four of the individual models, at least 34 eruptions are needed before it can be robustly distinguished from the global mean cooling with 5 % confidence. An El Niño response is evident shortly after eruption in these models, which transitions from a Central Pacific morphology in the first winter to an East Pacific morphology in the second, and subsequently to an La Niña response in the third and fourth winters. While these ENSO responses require 48 or more eruptions to emerge from the noise, they nonetheless lead to surface impacts over North America. However, these surface impacts do not resemble those classically associated with ENSO in the first year after eruption, likely because the tropical precipitation response differs as well due to the large-scale reduction in tropical precipitation. There is substantial diversity in the magnitude of these responses across models, likely owing to differences in tropical stratospheric diabatic heating despite the models using the same forcing.

More frequent heatwaves in Europe are posing considerable risks to human health, infrastructure, and ecosystems. However, the contributions of external forcing factors such as well-mixed greenhouse gases (GHGs) and aerosols remain to be better quantified. Here, using model outputs from the Large Ensemble Single Forcing Model Intercomparison Project (LESFMIP), a recent atmospheric reanalysis and a machine learning method-self-organising maps (SOMs), we attribute European heatwave trends during 1940-2020 to various external forcings. The Europe-averaged heatwave trend during 1940-2020 (0.87 days per decade) is well captured by the multi-model mean (MMM) response with GHGs dominating the trend. The positive heatwave trend in GHGs and ozone is offset by the effects of aerosols during 1940-1979, leading to weak negative heatwave trends. In contrast, the increase in GHGs has driven about half (53 +/- 17%; MMM and model-spread) of the strong heatwave trends in 1980-2020 (2.5 days per decade), amplified by the reduction in aerosols (23 +/- 15%). This highlights the increasing risk of more frequent heatwaves in Europe if GHG emissions continue to rise without significant mitigation measures. Analysis of atmospheric circulation by SOMs reveals that four major atmospheric circulation patterns, dominated by a blocking high anomaly, are linked to the most spatially-intense European summer heatwaves. A relatively large increase in the occurrence of blocking-like atmospheric circulation has likely exacerbated heatwave trends in Southern and Eastern Europe in 1980-2020. However, this atmospheric circulation trend is much weaker in the model response, and also seems to be outside the internal variability in most of the models. This may partly explain the underestimated heatwave trends in Southern and Eastern Europe. Constraining and further understanding of the thermodynamic and dynamic response in the LESFMIP models is important for attributing and predicting the multi-annual and decadal variability of climate and weather extremes.

This study uses an ensemble of climate model experiments coordinated by the Quasi-Biennial Oscillation initiative (QBOi) to analyze the Madden-Julian Oscillation (MJO) in the presence of either perpetual El Ni & ntilde;o or La Ni & ntilde;a sea surface temperatures during boreal winter. In addition to the prescribed El Ni & ntilde;o Southern Oscillation (ENSO) conditions, the nine models internally generate QBOs, meaning each may influence the MJO. Objectives of our analyses are to assess the response of the MJO to strong idealized ENSO forcing and look for evidence of a QBO influence on the MJO in a multi-model context. The diagnostics used include wavenumber-frequency spectra of tropical convective and dynamical fields, measures of MJO lifetime, an evaluation of MJO diversity and visualization of MJO vertical structure, as well as an assessment of QBO morphology and the QBO's impact on tropical convection. Kelvin wave spectral power increases in the El Ni & ntilde;o simulations whereas equatorial Rossby waves power is stronger in the La Ni & ntilde;a simulations. All models simulate faster MJO propagation under El Ni & ntilde;o conditions. This change in speed is corroborated by the MJO diversity analysis, which reveals that models better reproduce the observed "fast propagating" and "standing" MJO archetypes given perpetual El Ni & ntilde;o and La Ni & ntilde;a, respectively. Regardless of ENSO, QBO descent into the lower stratosphere is underestimated and we detect little QBO influence on tropical tropopause stability and MJO activity. With little influence from the QBO on the MJO activity in these runs, we can be confident that the aforementioned changes in the MJO indeed arise from the different ENSO boundary conditions.

Abstract. The equatorial resolved and parameterized wave forcing of the quasi-biennial oscillation (QBO) in models participating in phase 2 of the Atmospheric Processes and their Role in Climate (APARC) Quasi-Biennial Oscillation initiative (QBOi) is analyzed. We compare two experiments performed by the multi-model ensemble, NoNudge and ObsQBO, covering the period from 1979 to 2020. In this study, NoNudge designates experiments where the QBO is generated internally without nudging, whereas ObsQBO represents experiments in which the QBO is bias-corrected by nudging stratospheric zonal-mean zonal winds toward ERA5, allowing investigation of modelling uncertainties originating from biases in the background winds. While the NoNudge simulations exhibit QBO biases typically seen in models, including eastward wind bias in the mid-stratosphere and underestimation of the QBO amplitude in the lower stratosphere, these discrepancies are effectively mitigated by nudging in ObsQBO simulations. In the NoNudge experiments, the models reveal weaker Kelvin and gravity wave forcing in the tropical lower stratosphere than ERA5, but these forcings are enhanced in the ObsQBO experiment, suggesting that weak wave forcing of internally generated QBOs results from reduced critical-level filtering of waves due to unrealistically weak vertical wind shear at these levels. However, in ObsQBO experiments, the models still exhibit insufficient Kelvin and gravity wave forcing in the lower stratosphere. This suggests that the weak QBO amplitude in the lower stratosphere may stem from the inherent deficit in wave forcing in the models. One notable feature of the ObsQBO experiments is the excessive eastward wave forcing in the mid-to-upper stratosphere during the easterly QBO phase in the lower tropical stratosphere. The correlation coefficient between the 10 hPa eastward wave forcing and westerly QBO amplitude is 0.8, indicating that excessive westerly QBO is associated with strong eastward wave forcing in the mid-to-upper stratosphere.

Abstract. Observations suggest that the stratospheric Quasi-biennial Oscillation (QBO) modulates the Madden-Julian Oscillation (MJO) in the tropical troposphere, where the MJO is stronger with a smoother eastward propagation in the boreal winter seasons with a QBO easterly (QBOE) than that with a QBO westerly (QBOW) phase. Such connection is not captured by current climate models through their internally generated QBO and MJO. The QBO initiative (QBOi) phase 2 project included climate models from global modeling centers and conducted simulations with the tropical zonal-mean zonal wind in the model stratosphere nudged towards the observations. This paper investigates the potential connection between the nudged QBO and the internally generated MJO in 12 participating climate models. Results show that the stratospheric QBO and its associated impacts on the upper troposphere and lower stratosphere stability around the equator are realistically represented in all models through the nudging although a smaller amplitude is found for the temperature responses. However, there is no significant connection between the QBO and MJO in any of the participating models. Further diagnostics suggest this likely result from the biases in the internally generated MJO by the models where the simulated MJO convective variation is constantly underestimated so that the intense MJO OLR and precipitation anomalies are inadequately induced. However, the QBOi phase 2 models show no systematic bias in the MJO cloud-radiative feedback strength, with individual models spanning the full range from underestimation to overestimation of the observed values. These findings emphasize the importance of accurate representation of the MJO convective system in capturing the QBO-MJO connection by climate models. This paper also underscores the urgency of new theoretical understandings for the observed QBO-MJO connection.

Abstract The Earth’s middle atmosphere spans the deep region from the upper troposphere/lower stratosphere at around 10 km altitude to the mesosphere/lower thermosphere at around 100 km altitude. It is being increasingly recognized for its role in driving extreme surface weather and regional climate change. Climate models predict large ongoing and future changes in the middle atmosphere composition and circulation. However, the observations needed to detect, attribute and understand these changes and their impacts, to test predictions, and thereby to improve our models, are lacking. Here we show the capacity of infrared limb-imaging tomography to provide the needed observations. This evaluation is based on studies performed within a recent satellite mission concept – the Changing-Atmosphere Infrared Tomography Explorer, CAIRT. Observing thermal infrared emissions simultaneously from the middle troposphere at about 4 km up to the lower thermosphere at about 115 km altitude this technique provides observations of temperature and an extensive range of trace gases with unprecedented spatial resolution of about 50 by 50 km horizontally and about 1 km vertically. We show how these observations would (a) help to quantify the changing atmospheric circulation, (b) allow characterization and quantification of the gravity waves that are critical in driving this circulation, (c) reveal how variability in solar radiation and energetic particles propagate downward to affect regional climate at the surface, (d) detect how volcanic eruptions and wildfires impact the middle atmosphere and climate, and (e) resolve how stratosphere-troposphere exchange affects ozone and water vapor in the crucial and climate-relevant tropopause region.

Abstract. We describe the protocol for coordinated experiments in phase-2 of the Quasi-Biennial Oscillation initiative (QBOi) and the participating models. The experiments involve nudging tropical stratospheric zonal-mean zonal winds in the models toward observations, enabling analysis of the impact of QBO biases on simulated teleconnections. Additionally, nudging the tropical winds allows investigation of the origins of QBO biases by examining the behaviour of resolved and parameterized equatorial waves under realistic equatorial wind shear conditions. We document here the scientific rationale and design of the nudging experiments, summarize the QBOi data request, and provide an overview of participating models. An initial evaluation is given of tropical stratospheric winds simulated by the multi-model ensemble for both nudged and free-running cases, and the overall impact of nudging on climatological aspects of the atmospheric circulation is examined.

The North Atlantic sector has been identified as a region where the 11-year solar cycle has small but potentially non-negligible impacts on winter climate, but a debate persists about the robustness of such impacts. This work explores the signatures of the 11-year solar cycle over the North Atlantic in the ERA5 and 20th Century Reanalysis datasets. The results confirm previous studies with a robust positive boreal winter response in mean-sea-level pressure (mslp) in the region of the Azores at lags of three years after solar maximum. The spatial evolution of the response is examined in detail by first decomposing the mslp time series into the dominant modes of North Atlantic winter mslp variability, including the North Atlantic Oscillation (NAO), the East Atlantic (EA) and the Scandinavian patterns, before performing a multilinear regression analysis. We find that the maximum 11-year solar response in the December-January-February (DJF) average does not project directly onto the NAO. However, when the early/late-winter responses are examined separately, a statistically significant NAO response is seen in late winter (January-February) at lag 0-1 years and a statistically significant NAO response is also seen at lag +3 years in early winter (November-December). These results are consistent with predicted responses from previously proposed top-down influences from the stratosphere in late winter followed by the re-emergence of a signal from underlying sea surface temperatures in early winter. However, the NAO response is not the primary contributor to the total DJF response at lag +3 years. A previously unidentified solar-cycle response in the EA pattern is found in late winter at lag +3 years with larger amplitude than the NAO response. The evolution of the DJF mslp response over the Azores region can thus be understood as a summation of the NAO and EA patterns at lag +3 years.

This study investigates Quasi-Biennial Oscillation (QBO) teleconnections and their modulation by the El Niño–Southern Oscillation (ENSO) using a multi-model ensemble from the Atmospheric Processes And their Role in Climate (APARC) QBO initiative (QBOi). Analyzing observed QBO–ENSO teleconnections is challenging because it is difficult to separate the respective influences of QBO and ENSO outside the QBO region due to aliasing in the historical record. To isolate these signals, simulations were conducted with annually repeating prescribed sea-surface temperatures (SSTs) representing idealized El Niño and La Niña conditions (the QBOi EN and LN experiments, respectively), and results are compared with the QBOi control experiment (CTL) under ENSO-neutral conditions. The strength of the Holton-Tan relationship between the phase of the QBO and the strength of the polar vortex seen in observations is reproduced in fewer than three models in CTL and by one model in EN. In LN, three out of nine models reproduce the observed Holton–Tan relationship, but with less than half of the observed amplitude. In the Arctic winter climate, sudden stratospheric warmings (SSWs) occur more frequently in EN than in LN; however, unlike in observations, there is no discernible difference in SSW frequency between QBO westerly (QBO-W) and QBO easterly (QBO-E) phases. The Asia-Pacific subtropical jet (APJ) shifts significantly equatorward during QBO-W compared to QBO-E in observations, but this shift is not robust across models, regardless of ENSO phases. In the tropics, the sign and spatial pattern of the QBO precipitation response vary widely across models and experiments, indicating that any potential QBO signal is strongly modulated by the prevailing ENSO phases. Overall, the QBOi models exhibit unrealistically weak QBO wind amplitudes in the lower stratosphere, which may explain the weak polar vortex and APJ responses, as well as the weak precipitation signals in the tropics. In contrast, the QBO teleconnection with the Walker circulation during boreal summer and autumn shows consistent signals in both observations and most models. Specifically, the QBO-W phase is characterized by upper-level westerly and lower-level easterly anomalies over the Indian Ocean–Maritime Continent relative to QBO-E, although the amplitude and timing of these anomalies remain model-dependent. Notably, the influence of QBO phase on the Walker circulation appears insensitive to the ENSO phase.

The Changing-Atmosphere Infra-Red Tomography Explorer (CAIRT) is currently in Phase A as one of two final candidates for ESA’s Earth Explorer 11. As a Fourier transform infrared limb imager, CAIRT will observe simultaneously from the middle troposphere to the lower thermosphere at high spectral resolution and with unprecedented horizontal and vertical resolution. With this, CAIRT will provide critical information on (a) atmospheric gravity waves, circulation and mixing, (b) coupling with the upper atmosphere, solar variability and space weather and, (c) aerosols and pollutants in the upper troposphere and lower stratosphere. In this presentation we will give an overview of CAIRT’s science goals and the expected mission performance, based on latest results from feasibility studies performed during Phase 0.

Significant changes have occurred during the last few decades across the North Atlantic climate system, including in the atmosphere, ocean, and cryosphere. These large-scale changes play a vital role in shaping regional climate and extreme weather events across the UK and Western Europe. This review synthesizes the characteristics of observed large-scale changes in North Atlantic atmospheric and oceanic circulations during past decades, identifies the drivers and physical processes responsible for these changes, outlines projected changes due to anthropogenic warming, and discusses the predictability of these circulations. On multi-decadal time scales, internal variability, anthropogenic forcings (especially greenhouse gases), and natural forcings (such as solar variability and volcanic eruptions) are identified as key contributors to large-scale variability in North Atlantic atmospheric and oceanic circulations. However, there remain many uncertainties regarding the detailed characteristics of these various influences, and in some cases their relative importance. We therefore conclude that a better understanding of these drivers, and more accurate quantification of their relative roles, are crucial for more reliable decadal predictions and projections of regional climate for the North Atlantic and Europe.

Anthropogenic greenhouse gas emissions significantly impact the middle and upper atmosphere. They cause cooling and thermal shrinking and affect the atmospheric structure. Atmospheric contraction results in changes in key atmospheric features, such as the stratopause height or the peak ionospheric electron density, and also results in reduced thermosphere density. These changes can impact, among others, the lifespan of objects in low Earth orbit, refraction of radio communication and GPS signals, and the peak altitudes of meteoroids entering the Earth's atmosphere. Given this, there is a critical need for observational capabilities to monitor the middle and upper atmosphere. Equally important is the commitment to maintaining and improving long‐term, homogeneous data collection. However, capabilities to observe the middle and upper atmosphere are decreasing rather than improving.

The response of the Quasi‐Biennial Oscillation to changing concentrations of anthropogenic greenhouse gases, aerosols, and ozone, and also to volcanic eruptions and solar variability, is explored using 65,000 years of model output contributed by four modeling centers to the Large Ensemble Single Forcing Model Intercomparison Project (LESFMIP). The large ensemble size (at least 10, and in many cases 50) allows for the isolation of weak signals that are usually hidden by internal variability. Increasing greenhouse gas concentrations lead to weakening of the Quasi‐biennial Oscillation (QBO) and an increased likelihood of a disruption event, with the effect most pronounced in the lower stratosphere. Increasing aerosols lead to a strengthening of the QBO. Explosive volcanic eruptions lead to a weakening, and for some phases also a stalling, of the QBO. Volcanic eruptions can also help trigger a QBO disruption. The ozone forcing used for LESFMIP helps synchronize the QBO phase across ensemble members, and also increases the strength of the QBO. Finally, the solar forcing has the smallest impact on the QBO of the five forcings. The large ensemble sizes also allow for exploring the QBO phases and the time of year most prone to a QBO disruption: disruptions preferentially occur around 8 months after the phase with easterlies near 50 hPa, and are most common in late boreal winter and early spring.

The importance of extreme event attribution rises as climate change causes severe damage to populations resulting from unprecedented events. In February 2019, a planetary wave shifted along the U.S.-Canadian border, simultaneously leading to troughing with anomalous cold events and ridging over Alaska and northern Canada with abnormal warm events. Also, a dry-stabilized anticyclonic circulation over low latitudes induced warm extreme events over Mexico and Florida. Most attribution studies compare the climate model simulations under natural or actual forcing conditions and assess probabilistically from a climatological point of view. However, in this study, we use multiple ensembles from an operational forecast model, promising statistical as well as dynamically constrained attribution assessment, often referred to as the storyline approach to extreme event attribution. In the globally averaged results, increasing CO2 concentrations lead to distinct warming signals at the surface, resulting mainly from diabatic heating. Our study fi nds that CO2-induced warming eventually affects the possibility of extreme events in North America, quantifying the impact of anthropogenic forcing over less than a week's forecast simulation. Our study assesses the validity of the storyline approach conditional on the forecast lead times, which is hindered by rising noise in CO2 signals and the declining performance of the forecast model. The forecast-based storyline approach is valid for at least half of the land area within a 6-day lead time before the target extreme occurrence. Our attribution results highlight the importance of achieving net-zero emissions ahead of schedule to reduce the occurrence of severe heatwaves.

The quasi-biennial oscillation (QBO) was unexpectedly disrupted for only the second time in the historical record during the 2019/2020 boreal winter. As the dominant mode of atmospheric variability in the tropical stratosphere and a significant source of seasonal predictability globally, understanding the drivers behind this unusual behaviour is very important. Here, novel data from Aeolus, the first Doppler wind lidar (DWL) in space, are used to observe the 2019/2020 QBO disruption. Aeolus is the first satellite able to observe winds at high resolution on a global scale, and it is therefore a uniquely capable platform for studying the evolution of the disruption and the broader circulation changes triggered by it. This study therefore contains the first direct wind observations of the QBO from space, and it exploits measurements from a special Aeolus scanning mode, implemented to observe this disruption as it happened. Aeolus observes easterly winds of up to 20 m s−1 in the core of the disruption jet during July 2020. By co-locating with radiosonde measurements from Singapore and the ERA5 reanalysis, comparisons of the observed wind structures in the tropical stratosphere are produced, showing differences in equatorial wave activity during the disruption period. Local zonal wind biases are found in both Aeolus and ERA5 around the tropopause, and the average Aeolus-ERA5 Rayleigh horizontal line-of-sight random error is found to be 7.58 m s−1. The onset of the QBO disruption easterly jet occurs 5 d earlier in Aeolus observations compared with the reanalysis. This discrepancy is linked to Kelvin wave variances that are 3 to 6 m2 s−2 higher in Aeolus compared with ERA5, centred on regions of maximum vertical wind shear in the tropical tropopause layer that are up to twice as sharp. The enhanced lower-stratospheric westerly winds which are known to help disrupt the QBO, perhaps with increasing frequency as the climate changes, are also stronger in Aeolus observations, with important implications for the future predictability of such disruptions. An investigation into differences in the equivalent depth of the most dominant Kelvin waves suggests that slower, shorter-vertical-wavelength waves break more readily in Aeolus observations compared with the reanalysis. This analysis therefore highlights how Aeolus and future DWL satellites can deepen our understanding of the QBO, its disruptions and the tropical upper-troposphere lower-stratosphere region more generally.