Abstract. This study makes use of time series of methane (CH4) data from the Halogen Occultation Experiment (HALOE) to detect whether there were any statistically significant changes of the Brewer–Dobson circulation (BDC) within the stratosphere during 1992–2005. The HALOE CH4 profiles are in terms of mixing ratio versus pressure altitude and are binned into latitude zones within the Southern Hemisphere and the Northern Hemisphere. Their separate time series are then analyzed using multiple linear regression (MLR) techniques. The CH4 trend terms for the Northern Hemisphere are significant and positive at 10° N from 50 to 7 hPa and larger than the tropospheric CH4 trends of about 3% decade−1 from 20 to 7 hPa. At 60° N the trends are clearly negative from 20 to 7 hPa. Their combined trends indicate an acceleration of the BDC in the middle stratosphere of the Northern Hemisphere during those years, most likely due to changes from the effects of wave activity. No similar significant BDC acceleration is found for the Southern Hemisphere. Trends from HALOE H2O are analyzed for consistency. Their mutual trends with CH4 are anti-correlated qualitatively in the middle and upper stratosphere, where CH4 is chemically oxidized to H2O. Conversely, their mutual trends in the lower stratosphere are dominated by their trends upon entry to the tropical stratosphere. Time series residuals for CH4 in the lower mesosphere also exhibit structures that are anti-correlated in some instances with those of the tracer-like species HCl. Their occasional aperiodic structures indicate the effects of transport following episodic, wintertime wave activity. It is concluded that observed multi-year, zonally averaged distributions of CH4 can be used to diagnose major instances of wave-induced transport in the middle atmosphere and to detect changes in the stratospheric BDC.

A three‐dimensional model of dynamics and photochemistry is used to investigate how transfers of mass and chemical constituents between the stratosphere and troposphere are influenced by changes of the residual mean circulation. Under climatological‐mean conditions, the integration recovers a transfer of ozone into the extratropical troposphere that is consistent with the observed increase of tropospheric ozone during winter. Large at middle and high latitude, it reflects the downward transfer of stratospheric air, which is compensated at lower latitude by an upward transfer of tropospheric air. Changes representative of those observed between years modulate these transfers by ∼10%. The structure at low latitude, however, is sensitive to the quasi‐biennial oscillation's residual circulation, which interferes with the upwelling that must return mass to the stratosphere at the same rate that it is rejected at middle and high latitude. These exchanges of mass couple the circulation in the troposphere to that in the stratosphere, implying the involvement of the Hadley circulation.

Mercury (Hg) is a global pollutant with substantial risks to human and ecosystem health. By upward transport in tropical regions, mercury enters into the stratosphere, but the contribution of the stratosphere to global mercury dispersion and deposition remains unknown. We find that between 5 and 50% (passing through the 400-kelvin isentropic surface and tropopause, respectively) of the mercury mass deposited on Earth's surface is chemically processed in the lower stratosphere. Our results show the stratosphere as a unique chemical environment where elemental mercury is efficiently converted to long-lived oxidized species. Subsequent downward transport contributes substantially to the oxidized mercury burden in the troposphere. The results show that the stratosphere facilitates the global dispersion of large amounts of mercury from polluted source regions to Earth's remote environments. We find that stratospheric transport is as important as tropospheric transport in interhemispheric mercury dispersion. Future projections suggest that expected changes in atmospheric circulation will increase the transport of mercury into the stratosphere.

Abstract. The stratospheric Brewer-Dobson circulation (BDC) is an important element of climate as it determines the transport and distributions of key radiatively active atmospheric trace gases, which affect the Earth’s radiation budget and surface climate. Here, we evaluate the inter-annual variability and trends of the BDC in the ERA5 reanalysis and inter-compare with the ERA-Interim reanalysis for the 1979–2018 period. We also assess the modulation of the circulation by the Quasi-Biennial Oscillation (QBO) and the El Niño-Southern Oscillation (ENSO), and the forcings of the circulation by the planetary and gravity wave drag. A comparison of ERA5 and ERA-Interim reanalyses shows a very good agreement in the morphology of the BDC and in its structural modulations by the natural variability related to QBO and ENSO. Despite the good agreement in the spatial structure, there are substantial differences in the strength of the BDC and of the natural variability impacts on the BDC between the two reanalyses, particularly in the upper troposphere and lower stratosphere (UTLS), and in the upper stratosphere. Throughout most regions of the stratosphere, the variability and trends of the advective BDC are stronger in the ERA5 reanalysis due to stronger planetary and gravity wave forcings, except in the UTLS below 20 km where the tropical upwelling is about 40 % weaker due to a weaker gravity wave forcings at the equatorial flank of the subtropical jet. In the extra-tropics, the large-scale downwelling is stronger in ERA5 than in ERA-Interim linked to significant differences in planetary and gravity wave forcings. Analysis of the BDC trend shows a global acceleration of the annual mean residual circulation with an acceleration rate of about 1.5 % per decade at 70 hPa due to the long-term intensification in gravity and planetary wave breaking, consistent with observed and future climate model predicted BDC changes. ACP - Peer review - The advective Brewer–Dobson circulation in the ERA5 reanalysis: climatology, variability, and trends Articles | Volume 21, issue 10 Article Assets Peer review Metrics Related articles Articles | Volume 21, issue 10 https://doi.org/10.5194/acp-21-7515-2021 © Author(s) 2021. This work is distributed under the Creative Commons Attribution 4.0 License. Special issue: The SPARC Reanalysis Intercomparison Project (S-RIP) (ACP/ESSD... https://doi.org/10.5194/acp-21-7515-2021 © Author(s) 2021. This work is distributed under the Creative Commons Attribution 4.0 License. Articles | Volume 21, issue 10 Article Assets Peer review Metrics Related articles Research article | 18 May 2021 Research article | | 18 May 2021 The advective Brewer–Dobson circulation in the ERA5 reanalysis: climatology, variability, and trends Mohamadou Diallo , Manfred Ern , and Felix Ploeger Mohamadou Diallo CORRESPONDING AUTHOR m.diallo@fz-juelich.de https://orcid.org/0000-0003-0225-8120 × Institute of Energy and Climate Research, Stratosphere (IEK-7), Forschungszentrum Jülich, 52 425 Jülich, Germany Manfred Ern https://orcid.org/0000-0002-8565-2125 × Institute of Energy and Climate Research, Stratosphere (IEK-7), Forschungszentrum Jülich, 52 425 Jülich, Germany Felix Ploeger CORRESPONDING AUTHOR f.ploeger@fz-juelich.de × Institute of Energy and Climate Research, Stratosphere (IEK-7), Forschungszentrum Jülich, 52 425 Jülich, Germany Institute for Atmospheric and Environmental Research, University of Wuppertal, Wuppertal, Germany Download Final revised paper (published on 18 May 2021) Preprint (discussion started on 28 Sep 2020) Interactive discussion Status: closed Status: closed AC : Author comment | RC : Referee comment | SC : Short comment | EC : Editor comment - Printer-friendly version - Supplement RC1 : 'Review on acp-2020-881, Diallo et al. 2020 ACPD' , Roland Eichinger, 22 Oct 2020 AC1 : 'Reply on RC1' , Mohamadou Diallo, 08 Feb 2021 RC2 : 'Review of Diallo et al' , Anonymous Referee #2, 04 Dec 2020 AC2 : 'Reply on RC2' , Mohamadou Diallo, 08 Feb 2021 Peer-review completion AR : Author's response | RR : Referee report | ED : Editor decision AR by Mohamadou Diallo on behalf of the Authors (08 Feb 2021) Author's response Author's tracked changes Manuscript ED: Referee Nomination & Report Request started (16 Feb 2021) by Peter Haynes RR by Anonymous Referee #2 (26 Feb 2021) RR by Roland Eichinger (08 Mar 2021) Suggestions for revision or reasons for rejection Review of the revised version of: “The advective Brewer-Dobson circulation in the ERA5 reanalysis: climatology, variability and trends”, ACPD, 2020, by Diallo, M. Hide AR by Mohamadou Diallo on behalf of the Authors (25 Mar 2021) Author's response Author's tracked changes Manuscript ED: Publish as is (14 Apr 2021) by Peter Haynes You seem to have addressed the further comments of Reviewer 1 carefully and I am therefore pleased to accept the paper for publication in ACP. Hide AR by Mohamadou Diallo on behalf of the Authors (15 Apr 2021) Manuscript Download Article (26900 KB) Full-text XML BibTeX EndNote Short summary Despite good agreement in the spatial structure, there are substantial differences in the strength of the Brewer–Dobson circulation (BDC) and its modulations in the UTLS and upper stratosphere.

The stratospheric mean-meridional circulation (MMC) and eddy mixing are compared among six meteorological reanalysis data sets: NCEP-NCAR, NCEP-CFSR, ERA-40, ERA-Interim, JRA-25, and JRA-55 for the period 1979–2012. The reanalysis data sets produced using advanced systems (i.e., NCEP-CFSR, ERA-Interim, and JRA-55) generally reveal a weaker MMC in the Northern Hemisphere (NH) compared with those produced using older systems (i.e., NCEP/NCAR, ERA-40, and JRA-25). The mean mixing strength differs largely among the data products. In the NH lower stratosphere, the contribution of planetary-scale mixing is larger in the new data sets than in the old data sets, whereas that of small-scale mixing is weaker in the new data sets. Conventional data assimilation techniques introduce analysis increments without maintaining physical balance, which may have caused an overly strong MMC and spurious small-scale eddies in the old data sets. At the NH mid-latitudes, only ERA-Interim reveals a weakening MMC trend in the deep branch of the Brewer–Dobson circulation (BDC). The relative importance of the eddy mixing compared with the mean-meridional transport in the subtropical lower stratosphere shows increasing trends in ERA-Interim and JRA-55; this together with the weakened MMC in the deep branch may imply an increasing age-of-air (AoA) in the NH middle stratosphere in ERA-Interim. Overall, discrepancies between the different variables and trends therein as derived from the different reanalyses

Lagrangian trajectories have previously been used to reconstruct water vapor variability in the lower stratosphere, where the sensitivity of surface radiation to changes in the water vapor concentration is strongest, by obtaining temperature histories of air parcels that ascend from the troposphere to the stratosphere through the tropical tropopause layer (TTL). Models and theory predict an acceleration of the Brewer–Dobson circulation (BDC) and deceleration of the Walker circulation with surface warming, and both of these will drive future changes to transport across the TTL. Here, we examine the response of TTL transport during boreal winter to idealized changes in the BDC and Walker circulation by comparing the temperature histories of trajectories computed with ERA5 data to those calculated using the same data but with altered vertical and zonal wind velocities. We find that lower-stratospheric water vapor mixing ratios calculated from trajectories' cold point temperatures can increase by about 1.6 ppmv (about 50 %) when only zonal winds are slowed, while changes to vertical winds have a negligible impact on water vapor concentrations. This change follows from a decrease in zonal sampling of the temperature field by trajectories, which weakens the “cold trap” mechanism of dehydration as TTL transport evolves. As the zonal winds of the TTL decrease, the fraction of air that passes through the cold trap while ascending to the stratosphere will decrease and the coldest avera Models and theory predict an acceleration of the Brewer–Dobson circulation (BDC) and deceleration of the Walker circulation with surface warming, and both of these will drive future changes to transport across the TTL. Here, we examine the response of TTL transport during boreal winter to idealized changes in the BDC and Walker circulation by comparing the temperature histories of trajectories computed with ERA5 data to those calculated using the same data but with altered vertical and zonal wind velocities. Received: 15 Feb 2023 – Discussion started: 27 Feb 2023 – Revised: 31 May 2023 – Accepted: 01 Jun 2023 – Published: 06 Jul 2023 1  Introduction The cold temperatures of the tropical tropopause layer (TTL) dehydrate air as it ascends from the troposphere into the lower stratosphere, resulting in extreme dryness throughout the middle atmosphere. In 1949, observations of low relative humidity above the mid-latitude tropopause led Alan Brewer to deduce the Equator-to-pole overturning circulation of the stratosphere ( Brewer ,  1949 ) , now known as the Brewer–Dobson circulation (BDC). This is crucial for explaining the dehydration of air entering the stratosphere, and it reconciles the incompatibility of the previous “stratospheric fountain” hypothesis with observations of net subsidence in the TTL's coldest regions ( Newell and Gould-Stewart ,  1981 ; Sherwood ,  2000 ) . The TTL winds are driven horizontally by the uppermost flow of the troposphere's Walker circulation and vertically by the shallow branch of the BDC ( Fueglistaler et al. ,  2009 ) . The decelerated zonal wind trajectories' mean cold point temperature in panel (a) rises from 183.4 to 184.2 K, indicating that the average coldest temperature experienced by air parcels ascending into the tropical lower stratosphere would increase if the Walker circulation were to decelerate as theory predicts. (These changes are smaller again for 2007 and 2009: 0.2 and 0.3 K, respectively.) For the observed recent Walker circulation strengthening, the mean cold point temperature for air transiting the TTL may have decreased, as is indicated by the mean cold point temperature of 182.8 K for the trajectories with zonal winds accelerated by 20 % shown in panel (b). References Abalos, M., Calvo, N., Benito-Barca, S., Garny, H., Hardiman, S. C., Lin, P., Andrews, M. B., Butchart, N., Garcia, R., Orbe, C., Saint-Martin, D., Watanabe, S., and Yoshida, K.: The Brewer–Dobson circulation in CMIP6, Atmos. Chem. Phys., 21, 13571–13591, https://doi.org/10.5194/acp-21-13571-2021 , 2021. a , W.: Evidence for a world circulation provided by the measurements of helium and water vapour distribution in the stratosphere, Q. J. Roy. Meteor. Soc., 75, 351–363, https://doi.org/10.1002/qj.49707532603 , 1949. a Butchart, N.: The Brewer-Dobson circulation, Rev. Geophys., 52, 157–184, https://doi.org/10.1002/2013RG000448 , 2014. a Butchart, N., Cionni, I., Eyring, V., Shepherd, T., Waugh, D., Akiyoshi, H., Austin, J., Brühl, C., Chipperfield, M., Cordero, E., Dameris, M., Deckert, R., Dhomse, S., Frith, S. M., Garcia, R. R., Gettelman, A., Giorgetta, M. A., Kinnison, D. E., Li, F., Mancini, E., McLandress, C., Pawson, S., Pitari, G., Plummer, D. A., Rozanov, E., Sassi, F., Scinocca, J. F., Mancini, E., Marchand , M., Michou, M., Morgenstern, O., Pawson, S., Pitari, G., Plummer, D., Pyle, J. A., Rozanov, E., Scinocca, J., Shepherd, T. G., Shibata, K., Smale, D., Teyssèdre, H., and Tian, W.: Multimodel assessment of the upper troposphere and lower stratosphere: Tropics and global trends, J. Geophys. Res., 115, D00M08, https://doi.org/10.1029/2009JD013638 , 2010. a Hardiman, S. C., Butchart, N., and Calvo, N.: The morphology of the Brewer–Dobson circulation and its response to climate change in CMIP5 simulations, Q. J. Roy. Meteor. Soc., 140, 1958–1965, https://doi.org/10.1002/qj.2258 , 2014. a Heede, U. K., Fedorov, A. V., and Burls, N. a , b , c Articles Abstract Introduction Methods Results Summary and discussion Appendix A Code and data availability Author contributions Competing interests Disclaimer Acknowledgements Financial support Review statement References Supplement Download Article (4248 KB) Full-text XML Supplement (24913 KB) BibTeX EndNote Short summary Here, we show how projected changes to tropical circulation will impact the water vapor concentration in the lower stratosphere, which has implications for surface climate and stratospheric chemistry.

In response to global warming, the Brewer–Dobson circulation in the stratosphere is expected to accelerate and the mean transport time of air along this circulation to decrease. This would imply a negative stratospheric age of air trend, i.e. an air parcel would need less time to travel from the tropopause to any point in the stratosphere. Age of air as inferred from tracer observations, however, shows zero to positive trends in the northern mid-latitude stratosphere and zonally asymmetric patterns. Using satellite observations and model calculations we show that the observed latitudinal and vertical patterns of the decadal changes of age of air in the lower to middle stratosphere during the period 2002–2012 are predominantly caused by a southward shift of the circulation pattern by about 5°. After correction for this shift, the observations reveal a hemispherically almost symmetric decrease of age of air in the lower to middle stratosphere up to 800 K of up to −0.25 years over the 2002–2012 period with strongest decrease in the northern tropics. This net change is consistent with long-term trends from model predictions. ACP - Shift of subtropical transport barriers explains observed hemispheric asymmetry of decadal trends of age of air Articles | Volume 17, issue 18 Article Assets Peer review Metrics Related articles Articles | Volume 17, issue 18 https://doi.org/10.5194/acp-17-11177-2017 © Author(s) 2017. This work is distributed under the Creative Commons Attribution 3.0 License. https://doi.org/10.5194/acp-17-11177-2017 © Author(s) 2017. This work is distributed under the Creative Commons Attribution 3.0 License. Articles | Volume 17, issue 18 Article Assets Peer review Metrics Related articles Research article | 21 Sep 2017 Research article | | 21 Sep 2017 Shift of subtropical transport barriers explains observed hemispheric asymmetry of decadal trends of age of air Gabriele P. Stiller , Federico Fierli , Felix Ploeger , Chiara Cagnazzo , Bernd Funke , Florian J. Haenel , Thomas Reddmann , Martin Riese , and Thomas von Clarmann Gabriele P. Stiller CORRESPONDING AUTHOR gabriele.stiller@kit.edu https://orcid.org/0000-0003-2883-6873 × Karlsruhe Institute of Technology, IMK, P.O. Box 3640, 76021 Karlsruhe, Germany Federico Fierli × National Research Council, Institute for Atmospheric Sciences and Climate, Via Fosso del Cavaliere 100, 00199 Rome, Italy Felix Ploeger × Institute for Energy and Climate Research – Stratosphere (IEK-7), Forschungszentrum Jülich, 52425 Jülich, Germany Chiara Cagnazzo https://orcid.org/0000-0002-2054-0448 × National Research Council, Institute for Atmospheric Sciences and Climate, Via Fosso del Cavaliere 100, 00199 Rome, Italy Bernd Funke https://orcid.org/0000-0003-0462-4702 × Instituto de Astrofísica de Andalucía (CSIC), Glorieta de la Astronomía s/n, 18008 Granada, Spain Florian J. Haenel × Karlsruhe Institute of Technology, IMK, P.O. Box 3640, 76021 Karlsruhe, Germany Thomas Reddmann https://orcid.org/0000-0003-1733-7016 × Karlsruhe Institute of Technology, IMK, P.O. Box 3640, 76021 Karlsruhe, Germany Martin Riese https://orcid.org/0000-0001-6398-6493 × Institute for Energy and Climate Research – Stratosphere (IEK-7), Forschungszentrum Jülich, 52425 Jülich, Germany Thomas von Clarmann × Karlsruhe Institute of Technology, IMK, P.O. Box 3640, 76021 Karlsruhe, Germany Abstract. In response to global warming, the Brewer–Dobson circulation in the stratosphere is expected to accelerate and the mean transport time of air along this circulation to decrease. This would imply a negative stratospheric age of air trend, i.e. an air parcel would need less time to travel from the tropopause to any point in the stratosphere. Age of air as inferred from tracer observations, however, shows zero to positive trends in the northern mid-latitude stratosphere and zonally asymmetric patterns. Using satellite observations and model calculations we show that the observed latitudinal and vertical patterns of the decadal changes of age of air in the lower to middle stratosphere during the period 2002–2012 are predominantly caused by a southward shift of the circulation pattern by about 5°. After correction for this shift, the observations reveal a hemispherically almost symmetric decrease of age of air in the lower to middle stratosphere up to 800 K of up to −0.25 years over the 2002–2012 period with strongest decrease in the northern tropics. This net change is consistent with long-term trends from model predictions. Download & links Article (PDF, 4066 KB) Download & links Article (4066 KB) Full-text XML BibTeX EndNote Share How to cite. Stiller, G. P., Fierli, F., Ploeger, F., Cagnazzo, C., Funke, B., Haenel, F. J., Reddmann, T., Riese, M., and von Clarmann, T.: Shift of subtropical transport barriers explains observed hemispheric asymmetry of decadal trends of age of air, Atmos. Chem. Phys., 17, 11177–11192, https://doi.org/10.5194/acp-17-11177-2017, 2017. Received: 23 Dec 2016 – Discussion started: 16 Jan 2017 – Revised: 15 Jun 2017 – Accepted: 04 Aug 2017 – Published: 21 Sep 2017 Download Article (4066 KB) Full-text XML BibTeX EndNote Short summary The discrepancy between modelled and observed 25-year trends of the strength of the stratospheric Brewer–Dobson circulation (BDC) is still not resolved. With our paper we trace the observed hemispheric dipole structure of age of air trends back to natural variability in shorter-term (decadal) time frames.

Abstract The strength and structure of the Brewer–Dobson circulation (BDC) are explored in an idealized general circulation model. It is shown that diabatic forcing of the stratosphere and planetary wave forcing by the troposphere can have comparable effects on tracer transport through the stratosphere, as quantified by the mean age of air and age spectrum. Their impact, however, is mediated through different controls on the mass circulation. Planetary waves are modulated by changing surface topography. Increased wave forcing strengthens the circulation, particularly at lower levels. This is primarily a tropospheric control on the BDC, as the wave forcing is set by stationary waves at the base of the stratosphere. Stratospheric control of the circulation is effected indirectly through the strength of the stratospheric polar vortex. A colder vortex creates a waveguide higher into the stratosphere, raising the breaking level of Rossby waves and deepening the circulation. Ventilation of mass in the stratosphere depends critically on the depth of tropical upwelling, and so mass and tracer transport is comparably sensitive to both tropospheric and stratospheric controls. The two controls on the circulation can lead to separate influences on the lower and upper stratosphere, with implications for the seasonal cycle of tropical upwelling. They allow for independent changes in the “shallow” and “deep” branches of the BDC, which may be important for comparing modeled trends with obser

Abstract. Great attention has been paid to the short-term climate response following large volcanic eruptions, in order to understand effects on zonal winds, the polar vortex, and surface temperature across latitude. In contrast, several works have shown that evidence of volcanic forcing can persist for much longer in the stratosphere's chemical composition, even after the instigating aerosol population has dissipated. Heating by volcanic aerosols accelerates tropical upwelling, and thus drives an acceleration of the Brewer–Dobson Circulation (BDC), and enhances troposphere--stratosphere mass exchange. Even after tropical motion returns to its climatological mean, the anomalous mass exchange remains detectable in the stratosphere for several years. In this work, we use an age-of-air (AoA) tracer to diagnose stratospheric composition changes following the simulated 1991 Mt. Pinatubo eruption. Specifically, we employ simulation ensembles from the E3SMv2 climate model to identify statistically significant effects on zonal-mean AoA. In addition, we use the Residual Circulation Transit Time (RCTT) diagnostic to separate the effects of advective transport and mixing. We find that the Mt. Pinatubo eruption lowers AoA in the middle-to-upper stratosphere globally, primarily due to an accelerated residual meridional circulation. We also observe a localized increase of AoA near 20–100 hPa in the hemisphere opposite the eruption, which we attribute to a dampening of the seasonal BDC cycle by the volcanic aerosols. We suggest that a dampened seasonal BDC cycle is perhaps a generic result of any heating process driven by aerosols that evolve on timescales beyond seasonal in the meridional plane.

Two ensemble simulations of a new Earth system model (ESM) SOCOLv4 (SOlar Climate Ozone Links, version 4) for the period from 2015 to 2099 under moderate (SSP2-4.5) and severe (SSP5-8.5) scenarios of greenhouse gas (GHG) emission growth were analyzed to investigate changes in key dynamical processes relevant for Arctic stratospheric ozone. The model shows a 5–10 K cooling and 5%–20% humidity increase in the Arctic lower–upper stratosphere in March (when the most considerable ozone depletion may occur) between 2080–2099 and 2015–2034. The minimal temperature in the lower polar stratosphere in March, which defines the strength of ozone depletion, appears when the zonal mean meridional heat flux in the lower stratosphere in the preceding January–February is the lowest. In the late 21st century, the strengthening of the zonal mean meridional heat flux with a maximum of up to 20 K m/s (∼25%) in the upper stratosphere close to 70°N in January–February is obtained in the moderate scenario of GHG emission, while only a slight increase in this parameter over 50 N–60 N with the maximum up to 5 K m/s in the upper stratosphere and a decrease with the comparable values over the high latitudes is revealed in the severe GHG emission scenario. Although the model simulations confirm the expected ozone layer recovery, particularly total ozone minimum values inside the Arctic polar cap in March throughout the 21st century are characterized by a positive trend in both scenarios, the large-scale

Stable Water Isotopologues (SWIs) are important diagnostic tracers for understanding processes in the atmosphere and the global hydrological cycle. Using eight years (2002–2009) of retrievals from Odin/SMR (Sub-Millimetre Radiometer), the global climatological features of three SWIs, H216O, HDO and H218O, the isotopic composition δD and δ18O in the stratosphere are analysed for the first time. Spatially, SWIs are found to increase with altitude due to stratospheric methane oxidation. In the tropics, highly depleted SWIs in the lower stratosphere indicate the effect of dehydration when the air comes through the cold tropopause, while, at higher latitudes, more enriched SWIs in the upper stratosphere during summer are produced and transported to the other hemisphere via the Brewer–Dobson circulation. Furthermore, we found that more H216O is produced over summer Northern Hemisphere and more HDO is produced over summer Southern Hemisphere. Temporally, a tape recorder in H216O is observed in the lower tropical stratosphere, in addition to a pronounced downward propagating seasonal signal in SWIs from the upper to the lower stratosphere over the polar regions. These observed features in SWIs are further compared to SWI-enabled model outputs. This helped to identify possible causes of model deficiencies in reproducing main stratospheric features. For instance, choosing a better advection scheme and including methane oxidation process in a specific model immediately capture the main

The stratospheric contribution to tropospheric ozone ( O 3 ) has been a subject of much debate in recent decades but is known to have an important influence. Recent improvements in diagnostic and modelling tools provide new evidence that the stratosphere has a much larger influence than previously thought. This study aims to characterise the seasonal and geographical distribution of tropospheric ozone, its variability, and its changes and provide quantification of the stratospheric influence on these measures. To this end, we evaluate hindcast specified-dynamics chemistry–climate model (CCM) simulations from the European Centre for Medium-Range Weather Forecasts – Hamburg (ECHAM)/Modular Earth Submodel System (MESSy) Atmospheric Chemistry (EMAC) model and the Canadian Middle Atmosphere Model (CMAM), as contributed to the International Global Atmospheric Chemistry – Stratosphere-troposphere Processes And their Role in Climate (IGAC-SPARC) (IGAC–SPARC) Chemistry Climate Model Initiative (CCMI) activity, together with satellite observations from the Ozone Monitoring Instrument (OMI) and ozone-sonde profile measurements from the World Ozone and Ultraviolet Radiation Data Centre (WOUDC) over a period of concurrent data availability (2005–2010). An overall positive, seasonally dependent bias in 1000–450 hPa ( ∼0 –5.5 km) sub-column ozone is found for EMAC, ranging from 2 to 8 Dobson units (DU), whereas CMAM is found to be in closer agreement with the observations, although with sub

endurance, higher-altitude delivery is needed; particles injected into the rising leg of the Brewer-Dobson circulation above the tropical tropopause typically Stratospheric aerosol injection (SAI) is a proposed method of solar geoengineering (or solar radiation modification) to reduce global warming. This would introduce aerosols into the stratosphere to create a cooling effect via global dimming and increased albedo, which occurs naturally from volcanic winter. It has been claimed that stratospheric aerosol injection, at a moderate intensity, could cou Lifespan of aerosols: Tropospheric sulfur aerosols are short lived. Particles delivered into the lower stratosphere in the Arctic will typically remain aloft only for a few weeks or months, as air in this region is predominantly descending. To ensure endurance, higher-altitude delivery is needed; particles injected into the rising leg of the Brewer-Dobson circulation above the tropical tropopause typically remain aloft for several years. Sizing of particles is also crucial to their endurance. Aerosol delivery: There are two proposals for how to create a stratospheric sulfate aerosol cloud, either through the release of a precursor gas (SO2) or the direct release of sulfuric acid (H2SO4) and these face different challenges. If SO2 gas is released it will oxidize to form H2SO4 and then condense to form droplets far from the injection site. Releasing SO2 would not allow control over the size of the particles that are formed but would not require a sophisticated release mechanism. Simulations suggest that as the SO2 release rate is increased there would be diminishing returns on the cooling effect, as larger particles would be formed which have a shorter lifetime and are less effective scatterers of light. If H2SO4 is released directly then the aerosol particles would form very quickly and in principle the particle size could be controlled although the engineering requirements for this are uncertain. Assuming a technology for direct H2SO4 release could be conceived and developed, it would allow control over the particle size to possibly alleviate some of the inefficiencies associated with SO2 release. Strength of cooling: The magnitude of the effect of forcing from aerosols by decreasing insolation received at the surface is not completely certain, as its scientific modelling involves complex calculations due to different confounding factors and parameters such as optical properties, spatial and temporal distribution of emission or injection, albedo, geography, loading, rate of transport of sulfate, global burden, atmospheric chemistry, mixing and reactions with other compounds and aerosols, particle size, relative humidity, and clouds. Along with others, aerosol…

Existenceof Brewer-Dobson Circulation ..........e2%eui9%a8- 314 7.2.7 Brewer-Dobson Circulation and Radiative … middle atmosphere is dominated by the annual cycle, whereas in the tropical lower stratosphere the major signal … 314 7.2.8 Hemispheric Differences in Brewer-Dobson Circulation... 315 }.d: Atmospheric. Waves and Tracer