Tropospheric and stratospheric airmasses are separated by the tropopause. Here we investigate the lapse rate tropopause and the cold point tropopause in the Asian summer monsoon anticyclone (ASMA) based on high-altitude airborne measurements in summer 2017. We find that the lapse rate tropopause, and not the cold point, constitutes a good estimate of the upper boundary of the well mixed tropospheric air for many species. There is slow, diabatic, upward transport in the vicinity of the lapse rate tropopause and above. The cold point is located on average about 1 km above the lapse rate tropopause and is about 3 K colder (pressure lower by about 12 hPa). The cold point is in particular important for water vapour. Above the cold point in the ASMA molar water vapour mixing ratios (including hydration patches) range between similar to 3 and 10 ppm. In the observations, no indication of substantial dehydration above the cold point was found. Ozone mixing ratios increase substantially with altitude; between the lapse rate and the cold point tropopause molar ozone mixing ratios are in the range of 50-200 ppb. For strong convection (flight on 10 August 2017) there is substantial dehydration at the cold point tropopause (indicated by high values of total water, ice particle occurrence, and strong supersaturation). Above the cold point, under such conditions, neither ice particle occurrence, nor enhanced molar mixing ratios of water vapour (above about 6 ppm) are observed.

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 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.

Tropospheric ozone pollution in South Asia is mainly blamed on anthropogenic emissions. However, based on ERA5 reanalysis data, this study highlights the contribution of stratospheric ozone intrusions into the Upper Troposphere and Lower Stratosphere (UTLS) associated with Sudden Stratospheric Warming (SSW) events in enhancing upper tropospheric ozone over the South Asian region. We report an enhancement in ozone in the UTLS by more than 80 % for 2018 and ∼ 30 % within ±6 d of the onset during SSW events concurrent with the westerly phase of Quasi-biennial oscillation (WQBO-SSW) compared to non-SSW years. The equatorward shift (south of 30° N) of the subtropical jet during WQBO-SSW causes lowering of the tropopause and more Rossby-wave breaking in the upper troposphere. This results in higher stratospheric ozone intrusions over the South Asian region. The ozone enhancement during WQBO-SSW events produces an instantaneous radiative forcing at the top of the atmosphere of 0.09±0.05 W m−2 due to UTLS ozone changes and 0.17±0.05 W m−2 from total-column ozone changes over South Asia.

Mountain-wave-induced temperature perturbations can locally enable the formation of polar stratospheric clouds (PSCs). We examine a decade-long (2002–2012) record of ice PSCs derived from MIPAS/Envisat measurements. The points with the smallest temperature difference (ΔTice_min) between the frost point temperature (Tice) and the environmental temperature along the line of sight have been proposed and shown to provide a better estimate of the location of ice PSC observation from MIPAS. The temperature for the ice PSC observations is analyzed based on ERA5. Following this, we investigated the temperature history of the ice PSCs detected above Tice at the observation points along 24 h backward trajectories.We find that 52 % of Arctic and 26 % of Antarctic ice PSCs are detected above Tice, with pronounced clustering over mountainous terrain and in downstream regions. The backward trajectories were calculated by using the MPTRAC model, initialized at the ΔTice_min​ locations. Analysis of the temperature evolution along these trajectories shows that the fraction of ice PSCs at a temperature above Tice along the trajectory decreases, with the strongest decrease within the 6 h before observation. Accounting for temperature fluctuations along the air-mass histories, reduces the fractions of too warm ice PSCs at observation to 33 % in the Arctic and 9 % in the Antarctic.These results demonstrate the substantial role of orographic waves in ice PSC formation and provide observational constraints for chemistry–climate model evaluation. This contribution is based on the published analysis of Zou et al. (2024, Atmos. Chem. Phys., 24, 11759–11774, https://doi.org/10.5194/acp-24-11759-2024) .

It is well established that the drastic ozone loss in the Antarctic stratosphere, commonly known as the ozone hole, is primarily driven by gas-phase and heterogeneous chemical processes. While chemistry transport models generally reproduce observed ozone depletion well, they fail to capture the rapid early-winter decline of hydrogen chloride. We here examine the impact of the heterogeneous reaction between chlorine peroxide and hydrogen chloride forming HOOCl, followed by its photolysis. Incorporating this reaction and an additional hypochlorous acid loss pathway into a chemical mechanism significantly improves model agreement with observed levels of several chlorine compounds in the lower polar vortex stratosphere. This revised mechanism increases simulated ozone partial column depletion by over 15% between early July and mid-September 2011. Laboratory confirmation of these proposed reactions is needed to validate the mechanism.

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.

It is well established that the drastic ozone loss in the Antarctic stratosphere, commonly known as the ozone hole, is caused by gas-phase and heterogeneous processes. Chemistry models generally reproduce observed ozone depletion reasonable well. However, models have been unable to reproduce observations of rapid HCl loss at the beginning of the polar winter. Here we examine the impact of the heterogeneous reaction between Cl2O2 and HCl to form HOOCl and its subsequent photolysis on chlorine compounds. A chemical mechanism with these reactions added is able to clearly better reproduce the observed temporal development of the chlorine compounds HCl, ClONO2, ClO, and HOCl in the polar vortex lower stratosphere. The proposed chemical mechanism does moderately increase the chemical ozone column depletion, about 10\% in the lower stratospheric vortex core in September. Laboratory measurements of the proposed reactions are needed to confirm this mechanism.

The intensive catalytic chemical ozone loss cycles involving inorganic chlorine compounds are known to cause the ozone hole, that regularly forms in the Antarctic polar vortex each winter and spring. One key point of the explanation of the ozone hole are heterogeneous reactions on the surface of Polar Stratospheric Cloud (PSC) particles, which are present in the polar stratosphere owing to the very low stratospheric temperatures.Ozone mixing ratios can reach values of a few ppbv which is below detection limit of ozone sonde observations. Under these extreme conditions, fast chlorine deactivation into the reservoir HCl does occur even though polar stratospheric clouds are still present, that are normally causing chlorine activation.In this study we revisit this issue and investigate the occurring chlorine chemistry in more detail. The explanation of the chemical mechanism of the fast net HCl formation is based on the automatic determination of reaction pathways by the Pathway Analysis Program (PAP) (Lehmann, 2004).The simulations of chemical composition are performed by the model CLaMS in box model mode along an ensemble of about 600 trajectories in the Antarctic spring 2018. The simulated rapid complete chlorine deactivation into HCl in the presence of PSCs is in line with satellite observations by the Microwave Limb Sounder (MLS). ReferenceLehmann, R.: An algorithm for the determination of all significant pathways in chemical reaction systems, J. Atmos. Chem. 47, 45-78 (2004).

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.

Python code used to plot the figures in the manuscript. The ERA5 data that support the findings of this study are openly available from https://cds.climate.copernicus.eu/ (https://doi.org/10.24381/cds.bd0915c6). The analysis was performed using Python (version 3.7), along with the following libraries: NumPy (v1.x), Matplotlib (v3.x), NetCDF4 (v1.x), and Cartopy (v0.x). In addition, standard Python libraries such as datetime and os were used for data handling and time processing.

The definition of the boundary of the Asian summer monsoon anticyclone (ASMA) in the upper troposphere-lower stratosphere (UTLS) (350–410 K) is a known challenge that highly impacts the information about the anticyclone's behavior and affects the results when studying of its interannual variability. We present a novel method based on the absolute vortex moments that defines the ASMA boundary by solving an optimization problem. Here, we address the ASMA's climatology (1980–2023), interannual variability, the variability of the start and end dates and the duration of the anticyclone peak phase calculated with help of the defined novel method by using the ERA5 reanalysis provided by ECMWF. In addition, three individual years – 2017, 2022 and 2023 are highlighted during which aircraft campaigns took place to measure air inside the ASMA or its outflow (StratoClim, ACCLIP, PHILEAS). The interannual analysis is based on the anticyclone's centroid latitude and longitude, excess kurtosis, angle and aspect ratio using four isentropic surfaces: 350, 370, 390 and 410 K. Our findings show that the ASMA area decreases at 370, 390 and 410 K over the period 1980–2023 in contrast to previous studies. Further, we provide evidence of possible bimodality of the ASMA where the spatial distribution shows clustering of high-magnitude values (the Montgomery streamfunction values minus an optimized background value) around two centers on climatological data over 44 years as well as counting the number of days when two anticyclones (or two modes) where found simultaneously.

Abstract. Large and moderate volcanic eruptions significantly impact Earth's atmosphere by releasing sulphur emissions, thereby affecting atmospheric dynamics and QBO. Using the ECHAM6-HAMMOZ model, we show the impact of eruptive volcanoes on the tropical stratosphere and Quasi-biennial oscillation (QBO) from 2001 to 2013. Our simulations with volcanoes, when compared without volcanoes, show that volcanic sulfate aerosols enhance the stratospheric aerosol optical depth (SAOD) two months after the eruption of Rabaul (0.0034); Sarychev (0.0040) and Nabro (0.0097). The enhanced SOAD in the tropics (0.0014) led to a radiative forcing at the top of the atmosphere (TOA) by -0.92±0.34 W m-2 and at the surface by -0.88±0.18 W m-2 in the tropical region. The volcanic aerosol precursors enter the tropical stratosphere, propagating upward and enhancing sulfate aerosol concentrations by 46.95 ng m⁻³ and heating rates by 0.13±0.05×10⁻² K d⁻¹. The QBO estimated from model simulations using the wavelet analysis shows that stratospheric heating caused by the volcanoes reduces the amplitude of the QBO and disrupts its phases, resulting in the prolongation of the easterly phase by ~12 to 20 months and the westerly phase by ~16 to 24 months. The secondary meridional circulation induced by the QBO produces the double-peak structure in the amplitude near the equator, with peaks at 10 hPa and at 50 hPa. Our study points out that moderate and large volcanoes modulate the QBO. Since QBO also modulates tropical convection and weather, we suggest including volcanic eruptions and the QBO in weather prediction models for a better forecast.

Temperature fluctuations induced by mountain waves can play a crucial role in the formation of polar stratospheric clouds (PSCs). In particular, the cold phase of the waves can lower local temperatures sufficiently to trigger PSC formation, even when large-scale background temperatures are too high. To provide new quantitative constraints on the relevance of this effect, this study analyzes a decade (2002-2012) of ice PSC detections obtained from Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) measurements and ERA5 data in the polar winter lower stratosphere. In the MIPAS observations, we find that approximately 52 % of the Arctic ice PSCs and 26 % of the Antarctic ice PSCs are detected at temperatures above the local Tice. Ice PSCs above Tice are concentrated around mountainous regions and their downwind directions. A backward-trajectory analysis is performed to investigate the temperature history of each ice PSC observation. The cumulative fraction of ice PSCs above Tice increases as the trajectory gets closer to the observation point. The most significant change in the fraction of ice PSCs above Tice occurs within the 6 h preceding the observations. At the observation point, the mean fractions of ice PSCs above Tice, taking into account temperature fluctuations along the backward trajectory, are 33 % in the Arctic and 9 % in the Antarctic. The results provide a quantitative assessment of the occurrence of ice PSCs above Tice in connection with orographic waves. Additionally, the observational statistics presented can be utilized for comparison with chemistry climate simulations.