The Hunga eruption (20° S) in January 2022 injected a substantial amount of water vapor and aerosols into the stratosphere, primarily impacting the Southern Hemisphere and tropics. Using a combination of satellite observations and in situ measurements with optical particle counters, we show that a significant portion of the aerosol plume was transported into the Northern Hemisphere (NH) mid-latitudes. This cross-hemispheric transport occurred within the tropically controlled transition zone, within the shallow branch of the Brewer–Dobson circulation. By October 2022, enhanced aerosol concentrations were observed up to 50° N, at altitudes between 17–23 km with some dense plumes at around 21–22 km. In situ observations reveal an effective radius of around 330 nm, comparable to what was observed in the Southern Hemisphere (SH). Aerosol extinction coefficients in the mid-latitudes (30–50° N) were approximately doubled over background levels, corresponding to an aerosol optical depth (AOD) increase of (1–2) × 10−3 across the SAGE III/ISS wavelengths. These enhancements led to a modest, but not negligible, shortwave top-of-atmosphere (TOA) radiative forcing of -0.05±0.01 W m−2 between November 2022 and February 2023. Our results show that the moderate aerosol impact of the Hunga eruption in the SH produced non-negligible radiative impacts in the NH, emphasizing the importance of considering both hemispheres when analysing the total impact.

Water vapor in the lowermost stratosphere (LMS) plays a critical role in the climate system, as even small perturbations can significantly affect stratospheric temperatures and the position of the subtropical and eddy-driven jets. Climate models such as the ECHAM MESSy Atmospheric Chemistry (EMAC) model simulate strong wet biases in the LMS, reaching up to 400% compared with satellite observations. The strongest biases are found in the summer hemisphere. We find that 19% of air parcels in the LMS in the EMAC simulation exceed 30 ppmv in water vapor, a feature absent in both observations and independent Lagrangian model simulations. To diagnose the origin of this bias, we perform backward trajectory simulations with the Chemical Lagrangian Model of the Stratosphere (CLaMS) to trace the pathways of LMS air parcels and sample their Lagrangian cold points (LCPs). EMAC-simulated large-scale dehydration near the tropical cold trap is consistent with the sampled LCPs and shows no indication of a moist bias. Hence, the excessive moistening must occur downstream during transport into the LMS rather than at entry into the stratosphere. We further analyze the processes contributing to the LMS model moist bias by interpolating the physical and chemical tendencies from the EMAC model along the trajectories, including convection, vertical diffusion, and methane oxidation, as well as ice water content. For the subset of anomalously moist air parcels (water vapor mixing ratios greater than 30 ppmv), these processes collectively explain at most 30% of the simulated water vapor mixing ratios. Among the model processes, ice sublimation provides the dominant contribution, followed by vertical diffusion and convection, while methane oxidation is negligible. The large unexplained residual strongly suggests that numerical diffusion during transport is the primary driver of the excessive climate model wet bias in the LMS.

During the PHILEAS aircraft campaign, which probed the export of air from the Asian summer monsoon anticyclone (ASMA), observations were carried out from Germany and Alaska during August and September 2023. The flights from Germany sampled the western part of the ASMA and its westward outflow. The flights from Alaska probed filaments that had separated from the ASMA at its eastern flank and were transported across the Pacific. This work integrates PHILEAS aircraft in situ measurements with results from Lagrangian transport simulations. Our results show that air masses within the ASMA and its outflow are characterised by a mixture of different continental and marine sources. In the western part of the ASMA and its westward outflow, enhanced pollutants and greenhouse gases such as CH2Cl2, aerosols, CH4, and H2O were detected, indicating sources in South Asia. Additional marine air masses from the western Pacific, characterised by enhanced CH2Br2 and low O3, were found in filaments separated at the ASMA's eastern flank. Thus, air masses from two different surface regions and with different chemical compositions are mixed in filaments separated at the ASMA's eastern flank. Our findings show that the chemical composition of the ASMA's outflow is highly variable and depends on altitude, surface emissions, mixing of air masses from different source regions, and the interplay between the ASMA and tropical cyclones. Overall, the results provide strong support for the Asian summer monsoon is an important pathway for transporting short-lived ozone-depleting and radiatively active substances into the stratosphere.

Abstract. Locating the boundary of the Asian summer monsoon anticyclone (ASMA) in the upper troposphere-lower stratosphere (UTLS) (350 K–410 K potential temperature) is important for analyzing the transport of pollutants by the ASMA into the stratosphere. The definition of the ASMA boundary highly impacts the information about the anticyclone's behavior and affects the results when studying its spatio-temporal variability in particular regarding inferred trends over the last decades. In this work, we quantify the differences between methodologies based on potential vorticity (PV) and the Montgomery streamfunction (MSF) used to determine the location of the ASMA boundary. In addition, we apply the unsupervised machine learning spectral clustering method which combines both the PV and MSF fields to derive the location of the boundary. By analyzing the centroid position and boundary geometry for July and August 2023, we demonstrate that at 380 K, the PV-based, MSF-based, and spectral clustering methods exhibit good agreement during the Asian summer monsoon season. We find that the MSF-based method tends to incorporate the eastward-shed eddy over the Pacific around 140° E into the main anticyclone, whereas the PV-based method tends to separate it from the main body. Spectral clustering, however, can exhibit either behavior during the season. Finally, we compare the derived boundaries with satellite measurements of carbon monoxide, showing that the PV-based and MSF-based methodologies successfully enclose the core of elevated carbon monoxide levels as effectively as the spectral clustering method.

Abstract. Climate models predict changes in the Brewer-Dobson circulation under a changing climate, which could have profound effects on tracer distributions and the radiative budget. Age-of-Air is an important concept for describing transport in the stratosphere and to understand and quantify global atmospheric circulation patterns such as the Brewer-Dobson circulation. Being an unobservable quantity, it must be inferred from other, directly observable quantities such as long-lived trace gases. It is therefore essential to have accurate ways of determining Age-of-Air through observations. These observations are subject to measurement noise, which is a long-known source of uncertainty when deriving Age-of-Air, as such uncertainties can affect the derived Age-of-Air significantly. We present a novel approach of using neural networks to derive Age-of-Air from long-lived trace gases. Multi-layer perceptrons can be used to predict model Age-of-Air with accuracy as little as a single month. The networks can be optimally trained according to expected measurement uncertainties. An unsupervised autoencoder is presented which is capable of achieving similar predictability almost without relying on model Age-of-Air inputs. This study presents an overview of these new approaches and discusses their capabilities, their accuracies and precisions mostly from a technical perspective regarding input parameters, regularization and predictions outside the training domain. Our approach allows us to derive Age-of-Air with accuracy of up to a single month under considerable measurement noise and over a wide altitude range. This accuracy is even retained when predicting values from a completely different period.

Abstract The Asian summer monsoon establishes a strong connection between near-surface pollution in Southeast Asia and the global atmosphere by linking local emission sources with the large-scale circulation. There is a strong impact on the extratropical lower stratosphere, which is thought to occur mainly via quasi-horizontal export of polluted and moist air from the upper-level Asian monsoon anticyclone (AMA). The recent Probing High Latitude Export of Air from the Asian Summer Monsoon (PHILEAS) campaign focused on investigating this eddy transport and the associated mixing of monsoon-influenced air into the extratropical lower stratosphere through dedicated High Altitude and Long-Range Aircraft (HALO) observations from Oberpfaffenhofen, Germany, and Anchorage, Alaska, in the late summer and early autumn 2023. We summarize the mission’s motivation and objectives, place the Asian monsoon season 2023 into a climatological context, and present some representative observations. The observations during flights from Oberpfaffenhofen demonstrate the significant spatial and temporal AMA variability, which allowed HALO to investigate the displaced lower AMA boundary over the eastern Mediterranean, Israel, and Jordan. The observations during flights from Anchorage highlight the influence of long-range transport of moist and polluted air from the region of the Asian summer monsoon on the composition of the extratropical upper troposphere and lower stratosphere (UTLS), which impacts both ozone chemistry and the climate-relevant radiation budget. Significance Statement This study examines the Asian summer monsoon’s role in linking Southeast Asia’s near-surface atmosphere to the global upper troposphere and lower stratosphere. We find that the Asian summer monsoon plays a key role in transporting moisture, aerosols, and pollutants (including very-short-lived chlorinated compounds) to high-latitude regions. This, in turn, impacts atmospheric chemistry including stratospheric ozone depletion and alters the climate-relevant atmospheric radiation budget. Additionally, long-range transport of aerosols, such as ammonium nitrate, has the potential to influence cloud formation, further affecting climate and weather patterns.

Abstract. We develop and apply a novel method for estimating stratospheric mean age of air (AoA) from trace gases observed by the Atmospheric Chemistry Experiment – Fourier Transform Spectrometer (ACE-FTS). The method combines SF6 with five additional long-lived tracers, N2O, CH4, CFC-11, CFC-12, and HCFC-22, and is evaluated for ACE-FTS dataset versions 3.6 and 5.2. A proof of concept using the Chemical Lagrangian Model of the Stratosphere (CLaMS) shows that the multi-tracer approach reduces the uncertainty of zonal-mean AoA by about 50 % relative to conventional methods based only on SF6 and yields AoA estimates at individual profiles with an average uncertainty of about 0.3 years. Applied to ACE-FTS data, the multi-tracer method produces smoother AoA fields and strongly suppresses noise in sparsely sampled regions compared with the conventional convolution method. Comparison with in situ AoA estimates from SF6 and CO2 generally favors the multi-tracer product, although some discrepancies remain. The method provides a precise and spatially resolved satellite-based AoA product and a promising basis for future studies of long-term changes in stratospheric circulation. Comparison of ERA5 AoA with the new satellite product indicates a slow bias in the reanalysis. The long-term AoA trend pattern over 2004–2021 shows increasing age in the Northern Hemisphere stratosphere between about 18–28 km and decreasing age in the tropics and Southern Hemisphere subtropics, largely consistent with ERA5. In particular, AoA increases in the Northern relative to the Southern Hemisphere.

The dynamical tropopause as a transport barrier between the tropical upper troposphere and extratropical lowermost stratosphere is characterized by steep gradients in potential vorticity (PV) along an isentropic surface. Hence, the latitudinal separation between the dynamical tropopause in the Northern and Southern hemispheres can be used as a metric of upper tropospheric width for assessing climate change impacts. Here, we calculate the PV gradient-based dynamical tropopause from different meteorological reanalyses (ERA5, ERA-Interim, JRA-55, MERRA-2) and investigate its climatology, variability and long-term trends. Our results show a large seasonal cycle in the dynamical tropopause, with larger PV values and a poleward movement in summer. The climatological tropopause PV values are substantially different between different reanalyses, but the tropopause latitude is similar. Significant inter-annual variability in the dynamical tropopause latitude is related to El Niño Southern Oscillation (ENSO), is much weaker for the Quasi-Biennial Oscillation (QBO), and is robustly represented in reanalyses. In particular, El Niño causes equatorward shifts of the dynamical tropopause, hence a decrease of upper tropospheric width. Long-term trends in the dynamical tropopause exhibit a distinct vertical structure with poleward shifts below 340 K potential temperature, equatorward shifts between 340 K to 370 K and poleward shifts between 370 K to 380 K, implying an expansion of tropospheric width at lower levels, narrowing at upper levels and expansion near the tropical tropopause. Therefore, the dynamical tropopause as a metric for tropospheric width at a given level appears consistent with a widening of the tropics found from other metrics at lower levels, and furthermore shows a concurrent narrowing of the tropical upper troposphere.

Water vapor in the upper troposphere and lower stratosphere plays a crucial role for climate, affecting radiation, chemistry, and atmospheric dynamics. This study applies a simplified Lagrangian method to reconstruct stratospheric water vapor based on satellite observations from the Stratospheric Aerosol and Gas Experiment III on the International Space Station (SAGE III/ISS) and the Aura Microwave Limb Sounder (MLS). The objective is to improve understanding of moisture enhancements in the Asian and North American monsoons and to identify the key factors contributing to reconstruction biases. The performance of Lagrangian reconstructions significantly improves with the size of trajectory ensembles but exhibits a general dry bias. The reconstruction represents the summertime local water vapor maximum well in the Asian monsoon, particularly above the tropopause, but not in the North American monsoon. The main dehydration region diagnosed from trajectories indicates that water vapor in the Asian monsoon is predominantly controlled by local tropopause temperatures. The dry bias in reconstructions below the tropopause over the Asian monsoon shows a positive correlation with convection intensity, particularly in the western part of the monsoon region, suggesting that underestimated moistening from convection may contribute to this bias. Water vapor mixing ratios in the North American monsoon are largely influenced by long-range transport from dehydrated regions over southern Asia and additional local moistening. The limited performance of reconstructions in the North American monsoon is therefore likely linked to underestimation of local convection or uncertainties in long-range transport.

Using a combination of satellite, ground-based and in-situ observations, we quantify and compare the impact of the most recent moderate volcanic eruptions and extreme fire events (volcanic eruptions: Ambae, Vanuatu in July 2018; Raikoke, Russia and Ulawun, New Guinea in June 2019; extreme fire events: Canadian fires 2017 and Australian fires 2019/2020) on the global stratospheric aerosol layer and climate.A particular focus is set on the Hunga Tonga Hunga Ha’apai (Tonga islands) eruption of January 2022, which was exceptional especially in terms of water injection into the stratosphere. However, even the observed peak global average stratospheric aerosol optical depth exceeded that of the strongest stratospheric aerosol events of the last decade by a factor of more than 2.Since the eruption, we performed multiple measurement campaigns with Optical Particle Counters (POPS and LOAC) to study the aerosol optical properties of the freshly injected plume and its long-term evolution in terms of microphysical properties. The fresh plume consisted mostly of small (

The lowermost stratosphere (LMS) plays an important role in stratosphere–troposphere coupling and the Earth's radiation balance. This study investigates the effects of long-term changes in the tropopause and the lower-stratospheric isentropic structure on the mass of the LMS. We compare five modern reanalyses: ERA5, ERA-Interim, MERRA-2, JRA-55 and JRA-3Q. The focus is on changes after 1998, which marks the anticipated beginning of stratospheric ozone recovery. The trend analysis is performed with a dynamic linear regression model (DLM), capable of modeling non-linear trends. According to our study, isentropic pressure in the lower stratosphere (here 380–430 K) shows negative trends in the tropics and positive trends in the extratropics. In the Northern Hemisphere (NH), we find that the extratropical tropopause is rising, accompanied by decreasing pressure at an average rate of −1 hPa per decade. Additionally, our results indicate that the tropical tropopause in the NH has expanded poleward by 0.5° latitude between 1998–2019. In the Southern Hemisphere (SH) extratropics, the lapse rate tropopause shows a downward tendency of up to +2 hPa per decade after 1998, consistent across all reanalyses except JRA-3Q. The tropical tropopause and the cold point is rising, accompanied by decreasing pressure at a rate of ca. −0.5 hPa per decade in all reanalyses. The sign of the tropical tropopause potential temperature trends, however, differs across the reanalyses. This can be attributed to contrasting (absolute) temperature trends in the tropical tropopause region, such as at the 100 hPa pressure level. Consistent with the upward and poleward trend of the NH tropopause, the mass of the LMS decreases by 2 %–3 % for 1998–2019 if a fixed isentrope (380 K) is chosen as the upper LMS boundary. In ERA5, as well as MERRA-2 and ERA-Interim, this mass decline disappears if dynamical upper LMS boundaries are used that take the upward trends of the tropical tropopause into account.

The Asian monsoon anticyclone (AMA) during northern summer is a major contributor to the transport of tropospheric air masses, rich in water vapour, aerosol precursors and surface emissions , into the UTLS. During previous HALO missions TACTS/ESMVal and WISE a significant impact of the monsoon export on the background composition of the lowermost stratosphere (LMS) could be observed. Recent observations during the research missions StratoClim and ACCLIP show evidence for a strong contribution of ammonium nitrate by the AMA to the UTLS aerosol budget and the Asian Tropopause Aerosol Layer (ATAL), likely relevant for cirrus cloud formation. These missions revealed that the northern central Pacific is a key region for the transition of air masses originating from the AMA and emissions from East Asia and China to cross the tropopause. Particularly, over the northern Pacific dynamical and diabatic forcings lead to a subsequent erosion of these eddies and to mixing into the background lower stratosphere. We will present first results from the PHILEAS mission, which took place between August and October 2023 over Anchorage/Alaska and Europe. We found strong perturbations of the gas phase and chemical composition in the UTLS region. These perturbations can be linked to the Asian monsoon and east Asian pollution sources as well as to Canadian wild fires, which occurred prior and during the measurements. Based on selected cases we will present clear evidence for cross tropopause transport and mixing of pollution from East Asian pollution and the AMA over the eastern Mediterranean as well as over the northern Pacific. We will show that these sources affected the aerosol as well as the gas phase composition of the lowermost stratosphere.

Reanalysis datasets are widely used to understand atmospheric processes; however, different reanalyses may give very different results for the same diagnostics. The Atmospheric Processes And their Role in Climate (APARC; formerly SPARC) Reanalysis Intercomparison Project, or S(soon to be A)-RIP (https://s-rip.github.io/), is a coordinated activity to compare key diagnostics among atmospheric reanalyses, identify differences among reanalyses and their underlying causes, provide guidance on appropriate usage of reanalyses in scientific studies, and contribute to future improvements in the reanalysis products via collaborations with reanalysis centers and data users. S-RIP Phase 1 (completed in early 2022) focused primarily on the upper troposphere and above and processes linking these regions to the troposphere and surface. We are broadening our efforts in Phase 2 (S-RIP2), with new directions including studies of the tropospheric circulation, extreme weather events, and their links to the stratosphere, along with evaluation of chemical reanalyses, both those with a stratosphere / upper troposphere focus and those that focus on air quality applications. This presentation will provide a summary of Phase 1 results and discussion of future directions for S-RIP2, emphasizing applications to composition and chemistry studies and capacity building for Early Career Scientists.

The wave-driven Brewer–Dobson circulation plays a crucial role in determining the transport of trace gases and aerosols in the stratosphere. We examine the structure of the circulation based on reanalysis data (ERA5, ERA-Interim, MERRA2, and JRA55) and the Transformed Eulerian Mean and downward control framework, aiming for a dynamical separation of different circulation branches in terms of outflow generated by wave driving. The results show the existence of different circulation regimes, with a deep circulation branch mainly driven by planetary waves with wavenumbers 1–3, and a shallow circulation branch mainly driven by smaller-scale waves with wavenumbers greater than 3. We propose a definition of the separation level between shallow and deep branches as the lowest level where outflow from planetary waves is larger than outflow from smaller-scale waves. We show that this level occurs at approximately 22 km (43 hPa) with a weak annual cycle. This climatological structure is robust in various reanalyses. The variability of the circulation in the deep branch above the separation level is mainly related to planetary waves, while the variability in the shallow branch is related to both smaller-scale and planetary waves. Trends in the circulation over the period 1980–2017 show an upward shift of the deep branch related to planetary waves and a downward shift of the shallow branch related to both planetary and smaller-scale waves. The height of the separation level shows no significant trend. Taking into account differences in wave driving between the branches of the circulation could explain the spread in model inter-comparisons.

The stratospheric overturning meridional circulation is an important element in the global climate system and observationally-based estimates of its strength and changes are important for model validation and process understanding. But such observational constraints are prone to significant uncertainties related to the low circulation velocities and uncertainties in available trace gas measurements. Here, we propose a method to calculate mean age of air, as a measure for the stratospheric circulation, from mixing ratios of multiple measurable trace gas species, like trichlorofluoromethane (CFC-11), dichlorodifluoromethane (CFC-12), chlorodifluoromethane (HCFC-22), methane (CH4), nitrous oxide (N2O) and sulfur hexafluoride (SF6 ). The method is based on the correlations of these trace gases with mean age. The involved methodological error includes uncertainties due to atmospheric variability and non-compactness of the correlation, and additional instrument uncertainties as would be inherent for e.g. satellite instruments. The age calculation method is evaluated, globally and seasonally, in a model environment and compared against the true model mean age. We show that the tracer-age correlations are, in general, sufficiently compact in the age range between about 1 and 4 to 5 years, depending on the given species. Combination of the six chosen species reduces the resulting uncertainty of the derived mean age to below 0.3 years throughout most regions in the lower stratosphere. Even smaller scale, seasonal features in the global age distribution can be reliably diagnosed from the multi tracer-based mean age. Hence, the proposed mean age calculation method shows promise to reduce the error in mean age estimates from satellite trace gas observations.

Many stratospheric trace gases, including O3, HCl, and NOy, have had opposing trends in the Southern Hemisphere (SH) compared to the Northern Hemisphere (NH) during the last 2 decades. Some of this difference is due to hemispherically asymmetric changes in the rate of transport by the Brewer–Dobson circulation (BDC), and some is due to ozone depletion and recovery. The mean age of air (AoA) is a common proxy for the transport rate by the BDC in models; however it cannot be directly measured. We use observations from the Atmospheric Chemistry Experiment Fourier transform spectrometer (ACE-FTS) along with results from the Chemical Lagrangian Model of the Stratosphere (CLaMS) to derive AoA anomalies and AoA trends. The AoA is derived using observations of N2O, CH4, and CFC-12, all long-lived trace gases with tropospheric sources. We also consider CLaMS simulations driven with four different reanalyses (ERA5, ERA-Interim, JRA-55, MERRA-2). We find that, irrespective of which trace gas or reanalysis is used, air in the NH aged by up to 0.3 years per decade relative to the SH over 2004–2017. The maximum hemispheric difference in aging occurs in the middle stratosphere, near 30 hPa (∼ 24 km). We also show that the aging rate in the NH becomes smaller when the analysis is extended to 2021. The observed aging in the NH middle stratosphere contradicts model predictions of a decrease in stratospheric AoA in response to rising atmospheric greenhouse gas levels. However, the smaller aging rate during 2004–2021 compared to 2004–2017 provides some evidence that the NH aging is impacted by decadal variability and the limited length of the observation period.

The Asian summer monsoon anticyclone is a dominant circulation system in the upper troposphere and lower stratosphere (UTLS) in boreal summer (about June–September). An appropriate simulation of the monsoon anticyclone is an important challenge for chemistry climate and chemistry transport models. Here we compare simulations of the ECHAM5/MESSy Chemistry Climate model (EMAC) and the Chemical Lagrangian Model of the Stratosphere (CLaMS) based on the European Centre for Medium-Range Weather Forecasts Reanalysis-Interim (ERA-Interim); EMAC simulations are nudged towards ERA-Interim, whereas transport in CLaMS is driven by ERA-Interim. We employ surface origin tracers for continental South Asia. These surface origin tracers are lifted upward into the Asian summer monsoon anticyclone, both in EMAC and CLaMS. We investigate monsoon conditions for boreal summer 2015. In summer 2015, the entire monsoon, and in particular upward transport in the monsoon anticyclone, was strongly influenced by El Niño. In both models, in 2015, the simulated impact of surface origin tracers on the composition of air in the Asian summer monsoon anticyclone is very weak at 420 K. Further, in both models, a very strong decline with altitude (between ≈ 370–400 K) of surface origin tracers is obvious. The pattern of the Asian monsoon anticyclone in August and early September is represented very similarly in EMAC and CLaMS, with a lower fraction of the surface origin tracer for continental South Asia in CLaMS. The simulated pattern of surface origin tracers in the Asian summer monsoon anticyclone in CLaMS is much less smooth than in EMAC. Finally, we find a strong day-to-day variability in the Asian summer monsoon anticyclone and a confinement of monsoon air at UTLS altitudes (≈370 K to 400 K) similarly in both, EMAC and CLaMS.

Abstract. The wave driven Brewer-Dobson circulation plays a crucial role in determining the transport of trace gases and aerosols in stratosphere. We examine the structure of the circulation based on reanalyses data (ERA5, ERA-Interim, MERRA2, JRA55), using the Transformed Eulerian Mean and downward control framework, aiming for a dynamical separation of different circulation branches in terms of outflow generated by wave driving. The results show the existence of different circulation regimes, with a deep circulation branch mainly driven by large-scale waves with wavenumbers 1–3, and a shallow circulation branch mainly driven by smaller-scale waves with wavenumbers 4–180. We propose a definition of the separation level between a shallow and deep branch as the lowest level where outflow from waves 1–3 is larger than from waves 4–180. We show that this level occurs at approximately 22 km (43 hPa) and exhibits a weak annual cycle. This climatological structure is robust in various reanalyses. The variability of the circulation in the deep branch above the separation level is mainly related to large-scale waves 1–3, while the variability in the shallow branch is related to both smaller-scale and large-scale waves. Trends in the circulation over the period 1980–2017 show an upward shift of the deep branch related to waves 1–3 and a downward shift of the shallow branch related to both large and smaller scale waves. The height of the separation level shows no significant trend. Taking into account differences in wave driving between the branches of the circulation could reduce the spread in model inter-comparisons.