AS3.7

The important role of stratospheric feedbacks for the climate system – most notably how the ozone layer responds to anthropogenic forcings, and how that response then feeds back on the climate itself – remains largely unexplored, apart from the effects associated with gases regulated by the Montreal Protocol. This is because, to date, most models participating to CMIP inter-comparisons do not account for the complex interplay between stratospheric composition, dynamics and radiation. Here, we are providing a review of recent work highlighting the importance of such interplay on a broad range of time-scales, encompassing short-term variability to long-term climate change. First, we will show that increasing carbon dioxide levels lead to substantial changes in the ozone layer, and that these changes have a substantial effect on the circulation response to that forcing in both hemispheres (Chiodo & Polvani 2017; 2019). Then, we will review recent work on stratospheric water vapor (SWV) feedbacks under global warming, showing contrasting results concerning the effects on surface climate. Lastly, we will explore the connection between Arctic ozone and surface climate, highlighting the impacts of springtime ozone depletion on surface climate, and the sizable contribution of ozone feedbacks. Such findings demonstrate that stratospheric composition feedbacks play a key role in shaping climate response to anthropogenic forcings and stratosphere-troposphere coupling, both via radiative and dynamical processes. However, the coupling between ozone, SWV and climate is still subject to large uncertainties. We will discuss sources of uncertainty and model limitations, and implications for CMIP6.

How to cite: Chiodo, G. and Friedel, M.: Stratospheric composition feedbacks in a changing climate: a review, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11951, https://doi.org/10.5194/egusphere-egu22-11951, 2022.

The meridional overturning mass circulation in the middle atmosphere, i.e. the Brewer- Dobson circulation (BDC), was first discovered before decades based on the distribution of trace gases and a basic analytical concept of BDC has been derived using the transformed Eulerian mean equations. Since then, BDC is usually defined as consisting of a diffusive part, and an advective, residual mean circulation.

Climate model simulations robustly show that the advective BDC part accelerates in connection to the greenhouse gas induced climate change and this acceleration dominates the middle atmospheric changes in climate model projections. A prominent quantity that is being studied as a proxy for advective BDC changes is the net tropical upwelling across the 70 hPa, which measures the amount of mass advected by residual circulation to the stratosphere and upwards.

Another robust aspect of the changes in greenhouse gas concentrations is the changing structure of the atmosphere across layers. Particularly, it was debated whether the increasing BDC is not driven by the vertical shift of the circulation. In our research, we give a complete and definitive answer to this question. We developed an analytical method that allows us to attribute the changes in tropical upwelling to kinematic causative factors such as increasing residual mean vertical mass flux, vertical shift of the circulation and for the first time, changes in width of the upwelling region and changing curvature of the 70hPa level. Our results demonstrate that this is the complete set of kinematic factors influencing the net upwelling and that all of these factors are important contributions to the net upwelling change.

How to cite: Šácha, P. and Zajíček, R.: Is the Brewer-Dobson circulation increasing or moving upward? A definitive answer., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7565, https://doi.org/10.5194/egusphere-egu22-7565, 2022.

Both theory and climate model results suggest that the Brewer-Dobson circulation should strengthen with increasing greenhouse gas concentrations. Can this be confirmed by observations?

Directly measuring the circulation strength is not possible, so verification of this sensitivity has been limited to inferences from observations of long-lived chemicals. These methods, however, are complex and accumulation of the data required for them is difficult. Meanwhile, ozone observations are available from multiple sources spanning decades, but have only been applied to qualitative study of the stratospheric circulation, until now.

In this work, we present a new quantity - effective upwelling - which can be derived from ozone observations by a simple calculation. We then show that effective upwelling anomalies can be an effective proxy for residual circulation (i.e. TEM) upwelling. We present a comparison of TEM upwelling and effective upwelling calculated from CCMI model data to show the validity of the method, and follow this by presenting seasonal cycles, trends, and variability of effective upwelling as calculated by satellite observations.

How to cite: Charlesworth, E., Diallo, M., Plöger, F., Birner, T., and Jöckel, P.: A Method for Estimating the Evolution of Brewer-Dobson Circulation Upwelling, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8311, https://doi.org/10.5194/egusphere-egu22-8311, 2022.

The Brewer-Dobson Circulation (BDC) determines the distribution of long-lived tracers in the stratosphere; therefore, their changes can be used to diagnose changes in the BDC. We investigate decadal (2005-2018) trends of nitrous oxide (N2O) stratospheric columns (12-40 km) as measured by four Fourier transform infrared (FTIR) ground-based instruments and by the Atmospheric Chemistry Experiment Fourier Transform Spectrometer (ACE-FTS), and compare them with simulations by two models: a chemistry-transport model (CTM) driven by four different reanalyses, and the Whole Atmosphere Chemistry-Climate Model (WACCM). The limited sensitivity of the FTIR instruments can hide negative N2O trends in the mid-stratosphere because of the large increase in the lowermost stratosphere. When applying the ACE-FTS sampling on model datasets, the reanalyses by the European Centre for Medium Range Weather Forecast (ECMWF) compare best with ACE-FTS, but the N2O trends are consistently exaggerated. Model sensitivity tests show that while decadal N2O trends reflect changes in transport, these trends are less significant in the northern extratropics due to the larger variability of transport over timescales shorter than two years in that region. We further investigate the N2O Transformed Eulerian Mean (TEM) budget in three model datasets. The TEM analysis shows that enhanced advection affects the stratospheric N2O trends more than changes in mixing. While no ideal observational dataset currently exists, this model study of N2O trends still provides new insights about the BDC and its changes thanks to relevant sensitivity tests and the TEM analysis.

How to cite: Minganti, D., Chabrillat, S., Errera, Q., Prignon, M., Kinnison, D., Garcia, R., Abalos, M., Alsing, J., Schneider, M., Smale, D., Jones, N., and Mahieu, E.: N2O rate of change as a diagnostic of the Brewer-Dobson Circulation in the stratosphere, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10086, https://doi.org/10.5194/egusphere-egu22-10086, 2022.

Nitrous oxide (N₂O) is the third most important greenhouse gas in the atmosphere and is now considered as the most important depleting source gas of stratospheric ozone (O₃). Its sources are both natural and anthropogenic, mainly as an unintended by-product of human food production activities. Scientifically, a major issue is the identification and quantification of trends in the N₂O concentration from the middle troposphere to the middle stratosphere (MTMS) by in-situ and remote sensing observations due to the paucity of measurements. To address the temporal variability of N₂O, we assembled the first comprehensive dataset for in-situ and remote sensing N₂O concentrations from 1987 to 2018, based on aircraft and balloon measurements in the MTMS. Using statistical methods, we quality-controlled all the measurements to exclude outliers and particular dynamic cases (tropospheric intrusion, stratospheric descent). This allowed us to determine N₂O trends in the MTMS, based on selected observations during the period 1987-2018. This consistent dataset was also used to study the N₂O seasonal cycle in order to investigate the relationship with its emission sources through zonal means and atmospheric dynamic. The results show a long-term (30 years) increase in global N₂O concentration in the MTMS with an average of 0.89 ± 0.09 ppb/yr in the troposphere and 0.95 ± 0.13 ppb/yr in the stratosphere, consistent with 0.80 ppb/yr derived from ground measurements and ACE-FTS satellite measurements.

How to cite: Dudok de Wit, T. and Krysztofiak, G. and the N2O MTMS in situ Team: N2O temporal variability from the middle troposphere to the middle stratosphere based on airborne and balloon-borne observations during the period 1987-2018, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9807, https://doi.org/10.5194/egusphere-egu22-9807, 2022.

Ascending persistent smoke mesoscale anticyclonic vortices have been recently discovered in the mid-latitude stratosphere after several large wildfires. Such vortices can survive up to three months are rising to top altitudes between 20 and 36 km distributing aerosols along their way. They are also associated with a mini ozone hole.  We will survey these observations from active and passive satellite instruments and the reconstruction of vortices by assimilation of the signature left in the ozone and temperature measurements. We will show in particular how the temperature dipole associated with the vortices is retrieved from the GPS-RO occultation and reproduced by the assimilation and describe a remarkable case of superimposition during the crossing of two vortices.  

We will then describe our current understanding of the dynamics and stability of such structures where the radiative heating by solar absorption on the black carbon is the key forcing and where long wave radiative transfer provide a damping and diffusive effect. We will discuss the similarities and differences between simulations representing and full dynamics and the analysis that relies only on the  temperature information. A simplified 1D-model will be used as a tool for interpretation and sensitivity studies.

How to cite: Legras, B., Podglajen, A., Sellitto, P., Lestrelin, H., Reboud, J., Doc, J., and Lapeyre, G.: Ascending smoke vortices in the stratosphere, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5972, https://doi.org/10.5194/egusphere-egu22-5972, 2022.

The Australian bushfires of 2019/20 caused a massive injection of combustion products into the stratosphere that led to a persistent planetary-scale perturbation of all stratospheric climate-relevant variables. This extreme event enabled study of a striking atmospheric phenomenon, the smoke-charged vortex (SCV) – a persistent synoptic-scale anticyclone, which acts to confine the carbon-rich aerosol clouds during their solar-driven rise. This way, highly-concentrated absorbing aerosols are lofted above 30 km, which prolongs their stratospheric residence time and radiative effects.  Here, we use lidar observations at Lauder, New Zealand together with high-resolution radiosonde data and ozone soundings as well as satellite observations (CALIPSO, MLS, TROPOMI) and ERA5 reanalysis to characterize the optical, chemical and thermodynamical properties of a matured 7-km-tall SCV during its transfer over the South Island at 27 km altitude. The gaseous composition of the SCV was characterized by strongly enhanced water vapour and depleted ozone concentrations, leading to a synoptic-scale ozone hole with the total column reduced by up to 20%. The lidar measurements reveal a characteristic bottom-side elongation of the smoke bubble – a tail of aerosols extending over hundreds of kilometers and rotating together with the main body.

 Using long-term ground-based lidar and satellite measurement records, we show that monthly-mean stratospheric aerosol optical depth in early 2020 was highest since the major eruption of Mt. Pinatubo in 1991. With that, the removal of smoke aerosol from the stratosphere took longer than one year.  

How to cite: Khaykin, S., Querel, R., Liley, B., Sakai, T., Uchino, O., Morino, I., Godin-Beekmann, S., Hauchecorne, A., and Legras, B.: Australian smoke-charged vortex observations above New Zealand, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5597, https://doi.org/10.5194/egusphere-egu22-5597, 2022.

The extensive wildfires during December 2019 – January 2020 in South-East Australia released a mass of smoke into the stratosphere comparable to large volcanic eruptions. The smoke was observed throughout the southern hemisphere stratosphere months after the fires. Pyrocumulunimbus clouds (pyCb) are commonly presented as the main mechanism able to transport wildfire smoke across the tropopause into the stratosphere and were assumed as the driving mechanism also in this case. However, the smoke only appeared in the higher stratospheric levels downstream of the fires in the central south Pacific. Furthermore, there is indication that  pyCb were not active when the smoke was first seen in satellite images.  

In this study, using Lagrangian airmass trajectory analysis together with satellite observations we are able to fill the gap and identify the pathway of the smoke, its entry point into the upper levels and the mechanism that allows the smoke to enter the stratosphere. We find that the transitioning tropical cyclone Sarai merged with an extratropical cyclone to form a troposphere-wide cyclonic system, with a deep potential vorticity cutoff above it. Initially, the smoke traveled in the isentropic layer between 340 and 350 °K, just below the tropopause. Having reached the cyclone, the smoke changed direction, circulated around the low and entered the stratosphere through a dip in the tropopause height within the cutoff.

The cyclonic system described in this case study is not uncommon in these regions, possibly underlining the importance of this mechanism for troposphere-to-stratosphere exchange.

How to cite: Magaritz Ronen, L. and Raveh-Rubin, S.: Wildfire Smoke Highlights Troposphere-to-Stratosphere Pathway, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-771, https://doi.org/10.5194/egusphere-egu22-771, 2022.

Anticyclonically-trapped plumes were discovered following the unprecedented 2020 Australian fires, which saw the rise of a 1,000-km diameter, 6-km deep bubble of tropospheric air enriched in combustion products from 19 to 35 km asl over 3 months. Since then, a number of previous occurrences has been reported, notably in the aftermath of the 2017 Canadian fires. Lifted by solar heating from black carbon aerosols, the long-lived anticyclonic plumes are characterized by a joint upward motion of plume material and anticyclonic potential vorticity (PV).  These newly discovered objects raise fundamental questions from a dynamical standpoint. In particular, although the similar evolution of tracers and PV is a well-known property of quasi-adiabatic flows, it has no reason to hold in the presence of diabatic heating. Hence, there is seemingly a contradiction between the observed preservation of the low PV-aerosol-tracer relationship over time and fundamental properties of PV in this diabatically-forced flow.

In this presentation, we propose a conceptual model for the formation and evolution of smoked-charged anticyclones. The mechanisms at play will first be illustrated using idealized numerical simulations with the Weather Research and Forecast (WRF) model where we explore for the first time the flow response to a Lagrangian tracer locally heating the atmosphere. We will then analyze key features of the observed anticyclonic structures reproduced by the model, including the maintenance of the anticyclonic tracer bubble along its ascent, the formation of a tracer front at its top and of a tail at its lower bound, and a very low, almost-vanishing PV within the vortex. Finally, we will discuss some implications of our findings, in particular regarding the dynamical conditions favoring the formation and maintenance of such structures.

How to cite: Podglajen, A., Legras, B., Lapeyre, G., and Plougonven, R.: Dynamics of ascending smoke-charged anticyclones , EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5854, https://doi.org/10.5194/egusphere-egu22-5854, 2022.

With their frequent abundance in the tropopause region, cirrus clouds and their potential formation regions, the so-called ice-supersaturated regions (ISSRs), may have a significant impact on the tropopause structure by diabatic processes that result from latent heating, driven by phase transitions and interaction with radiation. This may lead to an alteration of the structure of potiential vorticity(PV), leading to changes of large scale dynamics and the stratosphere-to-troposphere exchange.
One of the most important long-term in-situ data set to study water vapor content at the tropopause level is provided by the European Research Infrastructure 'In-service Aircraft for a Global Overserving System' (IAGOS) (Petzold et al., 2020). Along the flight tracks of commercial passenger aircrafts, atmospheric state parameters and chemical properties of the surrounding air are recorded by compact instrument packages. In general, the cruising altitude of these aircraft ranges between 9 and 13 km, making this data set especially viable for studies of the upper troposphere/lowermost stratosphere (UTLS). However, due to the sparsity of these measurements, IAGOS on its own cannot provide three-dimensional water vapour fields with a high temporal resolution in the UTLS region, which are necessary to gain a deeper understanding of the cirrus cloud life cycle. Instead, we use these measurements to evaluate the quality of the well-known ERA-5 data set with regards to e.g. a seasonal cycle of the vertical distribution of water mixing ratio, the relative humidity and the fraction of ice-supersaturated regions. Additionally, the benefit of the higher resolution of ERA-5 over its predecessor ERA-Interim will be quantified.

How to cite: Brast, N., Reutter, P., and Spichtinger, P.: Evaluation of ERA-5 reanalysis data with respect to the humidity in the UTLS region, using the in-situ data set IAGOS, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8421, https://doi.org/10.5194/egusphere-egu22-8421, 2022.

The role of the Asian Summer Monsoon (ASM) on influencing the chemical composition of the upper troposphere and lower stratosphere (UTLS) will be examined using the recently developed three-dimensional MUlti-Scale Infrastructure for Chemistry & Aerosols (MUSICA) chemistry-climate model. MUSICA uses a Spectral Element dynamical core, with an ASM regional refinement (RR) option where the horizontal resolution is increased from ~1.0 º to ~0.25º and the vertical resolution is ~500m in the UTLS. For this study, the specified dynamics option is applied where the temperature, zonal and meridional winds from the NASA Goddard Earth Observing System version 5 (GEOS5) data assimilation model are used to drive the physical parameterization controlling boundary layer exchanges, advective and convective transport, and the hydrological cycle. MUSICA includes fully interactive chemistry with ~240 chemical species and over 500 chemical reactions along with the representation of sulfate, primary and aged black carbon, primary and secondary organic, sea salt, and dust aerosols. This model study will examine the magnitude and variability of ozone precursors (e.g., VOCs, NOx, and CO) and halogen ozone depleting substances in the ASM UTLS outflow region for the 2017 through 2021 boreal summer seasons. The interannual influences of the ASM chemical emissions on UTLS oxidizing capacity and odd-oxygen loss processes will be quantified. The results from this model study will address the main hypothesis of the Asian summer monsoon Chemical and Climate Impact Project (ACCLIP), namely that the western Pacific is a significant pathway for reactive chemical pollutants and climate-relevant emissions from the ASM to enter the global UTLS. ACCLIP is an NSF/NASA supported airborne mission that will based from Osan, South Korea in July/August 2022.

How to cite: Kinnison, D., Zhang, J., Honomichl, S., Smith, W., Pan, L., Tilmes, S., Zhu, Y., Emmons, L., and Saiz-Lopez, A.: Influence of the Asian Summer Monsoon on the Chemical Composition of the Upper Troposphere and Lower Stratosphere using the MUSICA Regionally Refined Chemistry Climate Model, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6619, https://doi.org/10.5194/egusphere-egu22-6619, 2022.

By analysing the global distribution of the highest 2% of daily CO mixing ratios at 400hPa derived from the MOPITT satellite instrument for 20 years (2000-2019), we detect very regularly regions with very high CO values (i.e. mixing ratios belonging to the globally highest 2%) over the remote northern hemispheric (NH) Pacific. Such events of elevated CO over the upper tropospheric NH-Pacific occur throughout the year, with a surprisingly high regularity and frequency (70% of all days during winter, 80% respectively during spring).

During winter, most of these pollution events are detected over the north-eastern and central NH-Pacific, during spring over the central NH-Pacific and during summer over the western NH-Pacific. We detect most pollution events during spring. To link each individual pollution event detected by the 2% filtering method with a specific CO source region, we performed trajectory calculations using MPTRAC, a lagrangian transport model. To analyse transport pathways and uplift mechanisms we combine MOPITT data, the trajectory calculations and ERA-Interim reanalysis data. It becomes apparent, that air masses from China being lifted along a frontal system into the free troposphere are the major CO source throughout the year. The contribution of other source regions and uplift mechanisms shows a strong seasonal cycle: NE-Asia in relation with upward transport of air masses in the warm conveyor belt of a midlatitude cyclone is a significant CO-source region during winter, spring and summer while India is an important source region mainly during spring and summer and SE-Asia mainly during spring.

How to cite: Smoydzin, L. and Hoor, P.: Contribution of Asian emissions to upper tropospheric CO over theremote Pacific, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11050, https://doi.org/10.5194/egusphere-egu22-11050, 2022.

IAGOS (www.iagos.org) is a European research infrastructure using commercial aircraft to measure the atmospheric composition. In particular, IAGOS provides regular carbon monoxide (CO) data since December 2001. In this study we use eighteen years of available data (from 2002 to 2019) to investigate CO anomalies throughout the entire flight i.e. vertical profiles over airports and upper troposphere/lower stratosphere (UTLS) at cruise altitude.

IAGOS flight track is divided into four distinctive vertical groups: boundary layer, middle troposphere, upper troposphere and lower stratosphere. The entire IAGOS data set has been split in 18 regions according to the geographical variability (e.g. continents over northern mid-latitudes, tropics, etc ...) and the different seasonal cycles of CO. CO anomalies are defined as air masses with CO mixing ratios above the 95th/99th percentile of the regional/seasonal/vertical distribution. This unique data set allows us to look at the variety of CO anomalies between regions and seasons.

Soft-io module which couples emission inventories and Lagrangian modelling along IAGOS flight track is used to quantify in which proportion those anomalies are linked to biomass burning and anthropogenic emissions.

The origin of those events presents high seasonal discrepancies (drought season and cold season) but also inter-annual variabilities. Anomalies coming from anthropogenic sources hit the most heavily on the lower part of the atmosphere of densely populated areas. However, none of the region, whatever the altitude range, are spared by anthropogenic pollution. Anomalies coming from biomass burning present large regional variability caused by weather conditions and biomass differences. We quantified these local and temporal variabilities to better understand processes affecting CO anomalies in the troposphere and UTLS.

How to cite: Lebourgeois, T., Sauvage, B., Wolff, P., Josse, B., Marecal, V., and Thouret, V.: Seasonal and regional characteristics of carbon monoxide anomalies as seen by IAGOS between 2002 and 2019:, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7022, https://doi.org/10.5194/egusphere-egu22-7022, 2022.

The chemical composition of the upper troposphere and lower stratosphere (UTLS) region plays a major role in Earth’s climate. Therefore, it is important to learn more about the transport mechanism of secondary organic aerosols (SOA) and their precursors into the UTLS region. One fast way of transport from the boundary layer to the upper troposphere is the deep convection. Organic vapors and particles, which get dissolved in cloud droplets, can take different pathways if the droplets freeze during the transport to the UTLS. Freezing occurs via different processes, for example riming which describes the freezing of supercooled liquid droplets upon the collision with ice crystals. During the riming the organic compounds could either revolatilise in the mixed zone of clouds or stay in the particles and get washed-out by precipitation or get transported to high altitudes and may revolatilise there if the cloud droplets sublimate. This partitioning between the ice and gas phase is given by the so-called retention coefficients.

Riming experiments in the worldwide unique vertical wind tunnel facility of the Johannes Gutenberg University of Mainz were carried out to derive retention coefficients for pinonoic and pinic acid. Both substances are formed during the monoterpene oxidation and represent SOA constituents.  The simulated conditions were close to those prevailing in the mixed phase zone in convective clouds where riming is the predominant growth mechanism of ice particles. Artificial ice particles were captively floated at their approximate terminal velocity and exposed to a cloud of supercooled droplets containing the substance of interest. The cloud had liquid water contents between 1 and 3 g m^-3 and temperatures ranging from -12 to -2 °C representing dry and wet growth conditions. From the concentrations of the substances before and after riming the retention coefficients for pinonic acid or pinic acid were obtained and compared to retention parameterizations available in literature.

How to cite: Borchers, C., Dörholt, K., Theis, A., Vogel, A. L., and Hoffmann, T.: Retention of secondary organic aerosols during riming experiments, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8352, https://doi.org/10.5194/egusphere-egu22-8352, 2022.

A wide variety of observation data sets are used to assess long-term simulations provided by chemistry-climate models (CCMs) and chemistry-transport models (CTMs). However, the upper troposphere – lower stratosphere (UTLS) is hardly assessed in the models because of uncertainties in remote measurements, a limited area for balloon-borne observations and a limited period for aircraft campaigns. Observations performed in the framework of the IAGOS program (In-service Aircraft for a Global Observing System) combine the advantages of in situ measurements in the UTLS with an almost global-scale area, a ~20-year monitoring period and a high sampling frequency. Few model assessments have been made using the IAGOS database, and none of them involved the whole cruise data set.

Cohen et al. (2021, GMD) proposed a method to project all the IAGOS data onto a model monthly grid, in order to make them ready for assessing global climatologies and seasonal cycles above several well-sampled regions in the North Hemisphere. This work has been extended to a daily resolution for an accurate separation between the upper troposphere and the lower stratosphere, and to other chemical species. In this study, we apply this method to a set of simulations generated by the following CTMs or CCMs: Oslo-CTM3, MOZART3, EMAC, UKESM, and LMDZ-OR-INCA, all involved into the ACACIA European Union program (Advancing the Science for Aviation and Climate) that focuses on the climate impact of the subsonic aviation emissions. The runs are generated following a common protocol, notably regarding the boundary conditions (e.g. emission inventories) and the chemical configurations, the latter including gaseous tropospheric and stratospheric chemistry, and heterogeneous chemistry. The multi-model assessment concerns the 1994 – 2017 period, and focuses on ozone, carbon monoxide, water vapour and reactive nitrogen (NO_y) fields.

How to cite: Cohen, Y., Hauglustaine, D., Bellouin, N., Lund, M. T., Matthes, S., Petzold, A., Rohs, S., Skowron, A., Thouret, V., and Ziereis, H.: A multi-model assessment of atmospheric composition in the UTLS with the IAGOS database, in the frame of the ACACIA EU project, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11424, https://doi.org/10.5194/egusphere-egu22-11424, 2022.

Air mass transport within the summertime Asian monsoon circulation provides a major source of anthropogenic pollution for the upper troposphere and lower stratosphere (UTLS). In our study, we investigate the quasi-horizontal transport of airmasses from the Asian summer monsoon anticyclone (ASMA) into the extratropical lower stratosphere and their chemical evolution. For that reason, we developed a method to identify and track the air masses exported from the monsoon. This method is based on the anomalously low potential vorticity (PV) of these air masses (tropospheric low–PV cutoffs) compared to the lower-stratosphere, and uses trajectory calculations and chemical fields from the Chemical Lagrangian Model of the Stratosphere (CLaMS). The results show evidence for frequent summertime transport from the monsoon anticyclone to mid-latitudes over the North Pacific, even reaching high latitude regions of Siberia and Alaska. Particularly, the most promising region and time for measurements of transported anticyclonic air masses that cross the tropopause, is the North Pacific from July to August. Most of the low–PV cutoffs related to air masses exported from the ASMA have lifetimes shorter than one week (about 90%) and sizes smaller than 1 percent of the northern hemisphere (NH) area. The chemical composition of these air masses is characterised by carbon monoxide, ozone and water vapour mixing ratios at an intermediate range between values typical for the monsoon anticyclone and the lower-stratosphere. The chemical evolution during transport within these low–PV cutoffs shows a gradual change from characteristics of the monsoon anticyclone to characteristics of the lower stratospheric background during about one week, indicating continuous mixing with the background atmosphere.

How to cite: Clemens, J., Ploeger, F., Konopka, P., Portmann, R., Sprenger, M., and Wernli, H.: Characterization of transport from the Asian summer monsoon anticyclone into the UTLS via shedding of low-potential vorticity cutoffs, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1443, https://doi.org/10.5194/egusphere-egu22-1443, 2022.

The extratropical transition layer or ExTL has been recognized about 20 years ago as part of the upper troposphere / lower stratosphere (UTLS) of the extratropics. This region encompasses the tropopause and shows the chemical characteristics of both, the stratosphere and the troposphere. Tracer-tracer correlations show this ambiguous chemical character as the integral effect of numerous different processes contributing to transport and subsequent mixing. The ExTL exhibits a chemical composition which is remarkably distinct from the deeper lowermost stratosphere. The ExTL roughly extends 2 km (or 30K potential temperature) above the local (dynamical) tropopause. Notably the ExTL has been identified with only a weak seasonality (if at all) being a persistent feature at the extratropical tropopause all year round.

Various dynamical processes have been recognized to contribute to the chemical composition of the ExTL such as larger scale processes related to stirring and mixing at the jets as well as smaller scale processes such as overshooting convection, gravity wave induced turbulence and radiatively induced diabatics at the tropopause. The sum of these processes does not only affect the tropopause sharpness (i.e. the tropopause inversion layer TIL) but also contributes to the surprisingly distinct composition of the ExTL. This is a direct result of the short time scales of cross tropopause transport and mixing compared to the lowermost stratosphere beyond the ExTL where longer time scales prevail. However, a dynamical process based explanation for the upper bound of the ExTL is yet missing.

Most recent analysis of ERA5 reanalysis data provides strong indication that vertical shear is a key feature for maintaining the ExTL over the whole year. The results show that transient shear processes are a common feature of the tropopause region with a vertical extent of 2km (or 30 K in potential temperature units) around the tropopause. Here, they constitute a persistent potential cause of dynamical instability, which may lead to turbulence and mixing and thus the observed chemical distinctness and extent of the ExTL.

How to cite: Hoor, P., Kaluza, T., Kunkel, D., Lachnitt, H.-C., Amelie, M., Bense, V., Bozem, H., and Joppe, P.: Revisiting the ExTL: From tracer correlations to dynamical processes, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5886, https://doi.org/10.5194/egusphere-egu22-5886, 2022.

Tropopause folds are documented to be frequent occurrences in the vicinity of the polar and subtropical jets. The rapidly changing nature of the folds and their complex fine scale structure make quantifying the associated cross-tropopause transport a significant challenge. To date, observational data sets do not provide sufficient coverage or resolution to easily overcome this challenge. In addition, ground-based observations are only representative of local processes or extreme events and do not directly inform global behavior. As a result, cross-tropopause transport estimates have relied on global models and reanalyses. However, observational evidence suggests that such models are prone to errors in both the occurrence frequency of tropopause folds and the amount of transport they generate individually. These limitations serve as the basis for our work, and we focus on a new framework to quantify the occurrence frequency of tropopause folds.

Existing literature provides various methods to quantify the occurrence frequency of tropopause folds, with some using Lagrangian parcel trajectories and others using tracer-like quantities and dynamical proxies for transport. Results vary greatly in distribution and in amplitude. Overall, because tropopause folds are associated with jet streams, a central problem lies in tracking said jet streams. Existing jet tracking algorithms tend to be complex, computationally expensive, and rely on a variety of ad hoc parameters and thresholds that are based on current climatologies (such as a minimum wind speed threshold). Consequently, these algorithms produce outputs that are sensitive to arbitrary choices and that are not well suited for climate studies.

We develop a jet tracking algorithm with two central improvements:

1) it includes temporal information about the evolution of features of interest, by using a time-integrated variable that provides information about parcel transport;

2) it minimizes the use of ad hoc parameters by defining jet features qualitatively, i.e., as spatially and temporally coherent local maxima in parcel transport;

By including temporal information, we are able to track dynamically relevant features, which is a substantial improvement over existing algorithms that use instantaneous meteorological fields. We present a comparison of the jet stream features identified by our algorithm versus existing ones. We also use the output of our algorithm as a jet-relative coordinate system, which allows us to identify tropopause folding events in global data sets, and to quantify their occurrence frequency.

How to cite: Rivoire, L., Linz, M., Curbelo, J., and Golja, C.: An improved jet-relative coordinate system for the detection of tropopause folds, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8959, https://doi.org/10.5194/egusphere-egu22-8959, 2022.