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Observational monitoring of the stratospheric transport circulation, the Brewer-Dobson-Circulation (BDC), is crucial to estimate any decadal to long-term changes therein, a prerequisite to interpret trends in stratospheric composition and to constrain the consequential impacts on climate. The transport time along the BDC (i.e., the mean age of stratospheric air, AoA) can best be deduced from trace gas measurements of tracers which increase linearly in time and are chemically passive. The gas SF6 is often used to deduce AoA, because it has been increasing monotonically since the 1950s, and previously its chemical sinks in the mesosphere have been assumed to be negligible for AoA estimates. However, recent studies have shown that the chemical sinks of SF6 are stronger than assumed, and become increasingly relevant with rising SF6 concentrations. To adjust biases in AoA that result from the chemical SF6 sinks, we here propose a simple correction scheme for SF6-based AoA estimates accounting for the time-dependent effects of chemical sinks. The correction scheme is based on theoretical considerations with idealized assumptions, resulting in a relation between ideal AoA and apparent AoA which is a function of the tropospheric reference time-series of SF6 and of the AoA-dependent effective lifetime of SF6. The correction method is thoroughly tested within a self-consistent data set from a climate model that includes explicit calculation of chemical SF6 sinks. It is shown within the model that the correction successfully reduces biases in SF6-based AoA to less than 5% for mean ages below 5 years. Tests with using only sub-sampled data for deriving the fit coefficients show that applying the correction scheme even with imperfect knowledge of the sink is far superior to not applying a sink correction. Further, we show that based on currently available measurements, we are not able to constrain the fit parameters of the correction scheme based on observational data alone. However, the model-based correction curve lies within the observational uncertainty, and we thus recommend to use the model-derived fit coefficients until more high-quality measurements will be able to further constrain the correction scheme. The application of the correction scheme to AoA from satellites and in-situ data suggests that it is highly beneficial to reconcile different observational estimates of mean AoA. [ABSTRACT FROM AUTHOR]

Sudden stratospheric warmings (SSWs) are abrupt disturbances to the Northern Hemisphere wintertime stratospheric polar vortex that can lead to pronounced regional changes in surface temperature and precipitation. SSWs also strongly impact the distribution of chemical constituents within the stratosphere, but the implications of these changes for stratosphere-troposphere exchange (STE) and radiative effects in the upper troposphere-lower stratosphere (UTLS) have not been extensively studied. Here we show, based on a specified-dynamics simulations from the EMAC chemistry-climate model, that SSWs lead to a pronounced increase in high-latitude ozone just above the tropopause (>25 % relative to climatology), persisting for up to 50 days for the ~50 % events classified as downward propagating following Hitchcock et al. (2013). This anomalous feature in lowermost stratospheric ozone is verified from ozone-sonde soundings and using the Copernicus Atmospheric Monitoring Service (CAMS) atmospheric composition reanalysis product. A significant dipole anomaly (>±25 %) in water vapour also persists in this region for up to 75 days, with a drying signal above a region of moistening, also evident within the CAMS reanalysis. Resultant enhanced STE leads to a significant 5-10% increase in ozone of stratospheric origin over the Arctic, with a typical time-lag of 50 days. The signal also propagates to mid-latitudes leading to significant enhancements in UTLS ozone, and, of weakening strength, also in free tropospheric and near-surface ozone up to 90 days after the event. In quantifying the potential significance for surface air quality breaches above ozone regulatory standards, a risk enhancement of up to a factor of 2 to 3 is calculated following such events. The chemical composition perturbations in the Arctic UTLS result in radiatively-driven Arctic stratospheric temperature changes of around 2 K. An idealised sensitivity evaluation highlights the changing radiative importance of both ozone and water vapour perturbations with seasonality. Our results imply that SSW-related transport of ozone needs to be accounted for when studying the drivers of surface air quality. Accurate representation of UTLS composition (namely ozone and water vapour), through its effects on local temperatures, may also help improve numerical weather prediction forecasts on sub-seasonal to seasonal timescales. [ABSTRACT FROM AUTHOR]

Sudden stratospheric warmings (SSWs) are abrupt disturbances to the Northern Hemisphere wintertime stratospheric polar vortex that can lead to pronounced regional changes in surface temperature and precipitation. SSWs also strongly impact the distribution of chemical constituents within the stratosphere, but the implications of these changes for stratosphere-troposphere exchange (STE) and radiative effects in the upper troposphere-lower stratosphere (UTLS) have not been extensively studied. Here we show, based on a specified-dynamics simulations from the EMAC chemistry-climate model, that SSWs lead to a pronounced increase in high-latitude ozone just above the tropopause (>25 % relative to climatology), persisting for up to 50 days for the ~50 % events classified as downward propagating following Hitchcock et al. (2013). This anomalous feature in lowermost stratospheric ozone is verified from ozone-sonde soundings and using the Copernicus Atmospheric Monitoring Service (CAMS) atmospheric composition reanalysis product. A significant dipole anomaly (>±25 %) in water vapour also persists in this region for up to 75 days, with a drying signal above a region of moistening, also evident within the CAMS reanalysis. Resultant enhanced STE leads to a significant 5-10% increase in ozone of stratospheric origin over the Arctic, with a typical time-lag of 50 days. The signal also propagates to mid-latitudes leading to significant enhancements in UTLS ozone, and, of weakening strength, also in free tropospheric and near-surface ozone up to 90 days after the event. In quantifying the potential significance for surface air quality breaches above ozone regulatory standards, a risk enhancement of up to a factor of 2 to 3 is calculated following such events. The chemical composition perturbations in the Arctic UTLS result in radiatively-driven Arctic stratospheric temperature changes of around 2 K. An idealised sensitivity evaluation highlights the changing radiative importance of both ozone and water vapour perturbations with seasonality. Our results imply that SSW-related transport of ozone needs to be accounted for when studying the drivers of surface air quality. Accurate representation of UTLS composition (namely ozone and water vapour), through its effects on local temperatures, may also help improve numerical weather prediction forecasts on sub-seasonal to seasonal timescales. [ABSTRACT FROM AUTHOR]

Stratospheric ozone is important for both stratospheric and surface climate. In the lower stratosphere during winter its variability is governed primarily by transport dynamics induced by wave-mean flow interactions. Here, we focus on interannual co-variations between the zonal mean ozone distribution and the strength of the polar vortex during northern hemispheric winter. Specifically, we study co-variability between the seasonal means of the ozone field from modern reanalyses and polar cap-averaged temperature at 100 hPa, which represents a robust and well-defined index for polar vortex strength. We consider variability in both pressure and isentropic coordinates. In the former case, we find that anomalously weak polar vortex years are associated with increased polar ozone amounts, showing two pronounced local maxima: one in the lower to mid-stratosphere and one just above the polar tropopause. In contrast, in isentropic coordinates, only the mid- to lower stratosphere shows increased ozone, while a small negative ozone anomaly appears in the lowermost stratosphere. These differences are related to contributions due to anomalous adiabatic vertical motion, which are implicit in potential temperature coordinates. In general, our analyses of the ozone budget in the extratropical middle stratosphere show that interannual polar ozone variability can be explained by a combination of anomalous diabatic downwelling and quasi-isentropic eddy mixing that are associated with consecutive, counteracting anomalous ozone tendencies on daily time scales. We find that approx. 71 % of the total variability of polar column ozone in the stratosphere is associated with year-by-year variations in polar vortex strength based on ERA5 reanalyses for the winter seasons 1980-2022. MLS observations for 2005-2020 show that around 86 % can be explained by polar vortex co-variability. [ABSTRACT FROM AUTHOR]

Air masses within the Asian monsoon anticyclone (AMA) show anomalous signatures in various trace gases. In this study, we analyze how air masses are transported from the planetary boundary layer (PBL) to the AMA via multiannual trajectory anlyses. While previous studies analyzed the PBL to AMA transport mainly for individual monsoon seasons or particular periods, we focus on the climatological perspective and on the interannual and intraseasonal variability. To this end we employ backward trajectories, which were computed using reanalysis data. Based on these trajectories, we analyze air mass transport from the PBL to the AMA during northern summer (June–August) for 14 summer seasons. Further, we backtrack forward trajectories from a free-running chemistry-climate model (CCM) simulation, which includes parametrized Lagrangian convection. The analysis of this additional model data set helps us to carve out robust or sensitive features of PBL to AMA transport with respect to the employed model. Results from both the trajectory model and the Lagrangian CCM emphasize the robustness of the three-dimensional transport pathways from the PBL to the AMA. Air masses are transported upwards on the eastern side of the AMA and are uplifted within the full AMA domain above. While this is in agreement with previous modelling studies, we refine the picture of the so-called "conduit" (Bergman et al., 2013). The contributions from the Tibetan Plateau (TP; 17% vs. 15%) and the West Pacific (around 12%) are similar in both model results. However, the contributions from the Indian subcontinent and South-East Asia are considerably larger in the Lagrangian CCM data, which might point towards the importance of convective transport for PBL to AMA transport for these regions. The analysis of both model data sets highlights the interannual and intraseasonal variability with respect to PBL source regions of the AMA. Additionally, we analyze the relation of the interannual east–west displacement of the AMA - which we find to be related to the monsoon Hadley index - to the transport behaviour and find that there are differences for "east" and "west years", the main transport characteristics, however, are comparable. Regarding the intraseasonal variability our trajectory model results show that transport from the PBL over the Tibetan Plateau (TP) to the AMA is weak in early June (less than 4% of the AMA air masses), whereas in August TP air masses contribute considerably (roughly 24%). The evolution of the contribution from the TP is supported by data from the Lagrangian CCM and is related to the northward shift of the subtropical jet and the AMA during this period. This result may help to reconcile previous results and further highlights the need of taking the subseasonal (and interannual) variability of the AMA and associated transport into account. [ABSTRACT FROM AUTHOR]

Despite the expected opposite effects of ozone recovery, the stratospheric Brewer-Dobson circulation (BDC) has been found to weaken in the Northern hemisphere (NH) relative to the Southern hemisphere (SH) in recent decades, inducing substantial effects on chemical composition. We investigate hemispheric asymmetries in BDC changes since about 2000 in simulations with the transport model CLaMS driven with different reanalyses (ERA5, ERA-Interim, JRA–55, MERRA–2) and contrast those to a suite of free-running climate model simulations. We find that age of air increases robustly in the NH stratosphere relative to the SH in all reanalyses considered. Related nitrous oxide changes agree well between reanalysis-driven simulations and satellite measurements, providing observational evidence for the hemispheric asymmetry in BDC changes. Residual circulation metrics further show that the composition changes are caused by structural BDC changes related to an upward shift and strengthening of the deep BDC branch, resulting in longer transit times, and a downward shift and weakening shallow branch in the NH relative to the SH. All reanalyses agree on this mechanism. Although climate model simulations show that ozone recovery will lead to overall reduced circulation and age of air trends, the hemispherically asymmetric signal in circulation trends is small compared to internal variability. Therefore, the observed circulation trends over the recent past are not in contradiction to expectations from climate models. Furthermore, the hemispheric asymmetry in BDC trends imprints on the composition of the lower stratosphere and the signal might propagate into the troposphere, potentially affecting composition down to the surface. [ABSTRACT FROM AUTHOR]

Mean age of air (AoA) is a common diagnostic for the strength of the stratospheric overturning circulation in both climate models and observations. AoA climatologies and its trends over the recent decades of model simulations and proxies derived from observations of long-lived tracers do not agree. Satellite observations show much older air than climate models and while most models compute a clear decrease of AoA over the last decades, a thirty-year timeseries from measurements shows a statistically non-significant positive trend. Measurement-based AoA derivations are often based on observations of the trace gas SF6, a fairly long-lived gas with a near-linear increase of emissions during the recent decades. However, SF6 has chemical sinks in the mesosphere, which are not considered in most model studies. In this study, we explicitly compute the chemical SF6 sinks based on chemical processes in the global chemistry-climate model EMAC. We show that good agreement of stratospheric AoA in EMAC and MIPAS is reached through the inclusion of chemical SF6 sinks, as those lead to a strong increase of the stratospheric AoA and thereby to a better agreement with MIPAS satellite observations. Remaining larger differences in high latitudes are addressed and possible reasons are discussed. Subsequently, we demonstrate that also the AoA trends are strongly influenced by the chemical SF6 sinks. Under consideration of the SF6 sinks, the AoA trends over the recent decades reverse sign from negative to positive. We conduct sensitivity simulations which reveal that this sign reversal results neither from trends of the stratospheric circulation strength, nor from changes in the strength of the SF6 sinks. We illustrate that even a constant SF6 destruction rate causes a positive trend in the derived AoA, since the amount of depleted SF6 scales with the increasing SF6 abundance itself. In our simulations, this effect overcompensates the impact of the accelerating stratospheric circulation which naturally decreases AoA. Although various sources of uncertainties cannot be quantified in detail in this study, our results suggest that the inclusion of SF6 depletion in models has the potential to reconcile the AoA trends of models and observations. [ABSTRACT FROM AUTHOR]

The Brewer-Dobson circulation (BDC) is a key feature of the stratosphere that models need to accurately represent in order to improve the representation of surface climate variability. For the first time, the Climate Model Intercomparison Project includes in its phase 6 (CMIP6) a set of diagnostics that allow for careful evaluation of the BDC. Here, the BDC is evaluated against observations and reanalyses using historical simulations. CMIP6 results confirm the well-known inconsistency in BDC trends between observations and models in the middle and upper stratosphere. The increasing CO2 simulations feature a robust acceleration of the BDC but also reveal large uncertainties in the deep branch trends. The very close connection between the shallow branch and surface temperature is highlighted, which is absent in the deep branch. The trends in mean age of air are shown to be more robust throughout the stratosphere than those in the residual circulation. The paper reflects the current knowledge and main uncertainties regarding the BDC. [ABSTRACT FROM AUTHOR]

Recent observations show a significant decrease of lower stratospheric (LS) ozone concentrations in tropical and mid-latitude regions since 1998. By analyzing 31 chemistry climate model (CCM) simulations performed for the Chemistry Climate Model Initiative (CCMI), we find a large spread in the 1998-2018 trend patterns between different CCMs and between different realizations performed with the same CCM. The latter, in particular, indicates that natural variability strongly influences LS ozone trends. However none of the model simulations reproduces the observed ozone trend structure of coherent negative trends in the LS. In contrast to the observations, most models show a dipole trend pattern in the LS with negative trends in the tropics and positive trends in the northern mid-latitudes or vice versa. To investigate the influence of natural variability on the LS ozone trends we analyze the observational trends and the models' trend probability distributions for slightly varied post-ODS (ozone depleting substances) periods. Generally, modeled and observed LS trends remain robust for different post-ODS periods, however observational data show a systematic change towards weaker mid-latitude trends forced by natural variability for certain periods. Moreover we can show that in the tropics the observed trends agree quite well with the models' trend distribution, whereas in the mid-latitudes the observational trend is a rather extreme value of the models' distribution. We further investigate the LS ozone trends for extended periods reaching into the future and find that all models develop a dipole trend pattern in the future, i.e. in almost all models the trends converge to constant values for the entire period 1998-2060. An investigation of interannual ozone variability also reveals a clear dipole pattern in ozone variability in all CCMs and in observational data, however it is more pronounced in the models. Thus, although the LS ozone variability pattern is similar, the probability of overall negative LS ozone trends simultaneously in the tropics and mid-latitudes is higher in observations. To access the dynamical influence on LS ozone trends, the models' tropical upwelling trends are correlated against their LS ozone trends. For the period 1998-2018 tropical ozone trends are negatively correlated (−0.83) and mid-latitude trends are positively correlated (+0.49). However, the correlation in the mid-latitudes is rather weak and not robust for slightly varied time periods, which indicates that other processes like two-way mixing play an important role here, too. [ABSTRACT FROM AUTHOR]

Mean age of air (AoA) is a diagnostic of transport along the stratospheric Brewer-Dobson circulation. While models consistently show negative trends, long-term time series (1975-2016) of AoA derived from observations show non-significant positive trends in mean AoA in the northern hemisphere. This discrepancy between observed and modeled mean AoA trends is still not resolved. There are uncertainties and assumptions required when deriving AoA from trace gas observations. At the same time, AoA from climate models is subject to uncertainties, too. In this paper, we focus on the uncertainties due to the parameter selection in the method that is used to derive mean AoA from SF6 measurements in Engel et al. (2009) and Engel et al. (2017). To correct for the non-linear increase in SF6 concentrations, a quadratic fit to the time-series at the reference location, i.e. the tropical surface, is used. For this derivation, the width of the AoA distribution (age spectrum) has to be assumed. In addition, to choose the number of years the quadratic fit is performed for, the fraction of the age spectrum to be considered has to be assumed. Even though the uncertainty range due to all different aspects has already been taken into account for the total errors on the AoA values, the systematic influence of the parameter selection on AoA trends is described for the first time in the present study. In addition, a method to derive mean AoA is evaluated that applies a convolution to the reference time series. The resulting mean AoA and its trend only depend on an assumption about the ratio of moments. Also in that case, it is found that the larger the ratio of moments, the more the AoA trend gravitates towards the negative. The linear tracer and SF6 AoA is found to agree within 0.3 % in the mean and 6 % in the trend. The different methods and parameter selections were then applied to the balloon borne SF6 and CO2 observations. We found the same systematic changes in mean AoA trend dependent on the specific selection. When applying a parameter choice that is suggested by the model results, the AoA trend is reduced from 0.15 years/decade to 0.07 years/decade. It illustrates that correctly constraining those parameters is crucial for correct mean AoA and trend estimates and still remains a challenge in the real atmosphere. Engel, A., Möbius, T., Bönisch, H., Schmidt, U., Heinz, R., Levin, I., Atlas, E., Aoki, S., Nakazawa, T., Sugawara, S., et al.: Age of stratospheric air unchanged within uncertainties over the past 30 years, Nature Geoscience, 2, 28, 2009. Engel, A., Bönisch, H., Ullrich, M., Sitals, R., Membrive, O., Danis, F., and Crevoisier, C.: Mean age of stratospheric air derived from AirCore observations, Atmospheric Chemistry and Physics, 17, 6825-6838, https://doi.org/10.5194/acp-17-6825-2017, 2017. [ABSTRACT FROM AUTHOR]

Numerous studies have presented evidence that the Asian summer monsoon anticyclone substantially influences the distribution of trace gases – including water vapour – in the upper troposphere and lower stratosphere (e.g. Santee et al., 2017). Stratospheric water vapour in turn, is strongly affecting surface climate (cf. e.g. Solomon et al., 2010). Here, we analyse the characteristics of water vapour transport from the upper troposphere in the Asian monsoon region to the stratosphere employing a multiannual simulation with the chemistry-transport model CLaMS (Chemical Lagrangian Model of the Stratosphere). This simulation is driven by meteorological data from ERA-Interim and features a water vapour tagging that allows us to assess the contributions of different upper tropospheric source regions to the stratospheric water vapour budget. Our results complement the analysis of air mass transport through the Asian monsoon anticyclone by Ploeger et al. (2017). The results show that the transport characteristics for water vapour are mainly determined by the bulk mass transport from the Asian monsoon region. Further, we find that, although the relative contribution from the Asian monsoon region to water vapour in the deep tropics is rather small (average peak contribution of 14 % at 450 K), the Asian monsoon region is very efficient in transporting water vapour to this region (when judged according to its comparatively small spatial extent). With respect to the Northern Hemisphere extratropics, the Asian monsoon region is much more impactful and efficient regarding water vapour transport than e.g. the North American monsoon region (averaged maximum contributions at 400 K of 29 % vs. 6.4 %). [ABSTRACT FROM AUTHOR]

We perform the first multi-model comparison of the impact of nudged meteorology on the stratospheric residual circulation using hindcast simulations from the Chemistry Climate Model Initiative (CCMI). We examine simulations over the period 1980–2009 from 5 models in which the meteorological fields are nudged towards reanalysis data and compare with equivalent free-running simulations from 9 models. We show that nudging meteorology does not constrain the mean strength of the stratospheric residual circulation and that the inter-model spread is similar, or even larger, than in the free-running simulations. The nudged simulations also simulate stronger upwelling in the tropical lower stratosphere compared to the residual circulation estimated directly from the reanalyses they are nudged towards. Downward control calculations reveal substantial differences between the mean lower stratospheric tropical upward mass flux (TUMF) computed from the modeled wave forcing and that calculated directly from the residual circulation. Although the mean circulation is poorly constrained, the nudged simulations show a high degree of consistency in the interannual variability of the TUMF in the lower stratosphere, which is related to the contribution to variability from the resolved wave forcing. We apply a multiple linear regression (MLR) model to separate the drivers of interannual and long-term variations in the simulated TUMF. The MLR model explains up to ~ 75 % of the variance in TUMF in the nudged simulations and reveals a statistically significant positive trend for most models in TUMF over the period 1980–2009. Overall, nudging meteorological fields leads to increased inter-model spread for most of the measures of the mean climatological stratospheric residual circulation assessed in this study. Our findings show that while nudged simulations by construction produce accurate temperatures and realistic representations of fast horizontal transport, this is not necessarily the case for the slower zonal mean vertical transport. Consequently, caution is required when using nudged simulations to interpret long-lived stratospheric tracers that are controlled by the residual circulation. [ABSTRACT FROM AUTHOR]

Climate model simulations show a Brewer-Dobson circulation (BDC) acceleration in the course of climate change. While the mechanisms for the BDC strengthening are well understood, there are still open questions concerning its dynamical driving. Mean age of stratospheric air (AoA) is a useful transport diagnostic for accessing changes of the BDC. Analysing AoA from a subset of Chemistry Climate Model Initiative part 1 climate projection simulations, we find a remarkable agreement between most of the models in simulating the largest negative AoA trends in the extratropical lower to middle stratosphere of both hemispheres (approximately between 20gpkm and 25gpkm and 20°-50°N/S). We show that the occurrence of AoA trend minima in those regions is directly related to the climatological AoA distribution being sensitive to an upward shift of the circulation in response to a climate change. But also other factors like a reduction of aging by mixing (AbM) and residual circulation transit times (RCTTs) contribute to the AoA distribution changes by widening the AoA isolines. Furthermore we analyze the time evolution of AbM and RCTT trends in the extratropics and examine the connection to possible drivers like local residual circulation strength, net tropical upwelling and wave driving. However, after the correction for a vertical shift of pressure levels, we find only seasonally significant trends of residual circulation strength and zonal mean wave forcing (resolved and unresolved) without a clear relation between the trends of the analyzed quantities. This indicates that additional causative factors may influence the AoA, RCTT and AbM trends. Namely, we postulate a possible influence of stratospheric shrinkage on RCTT, AbM and therefore also on AoA trends. In this study, we postulate that the shrinkage of the stratosphere has the potential to influence the RCTT and AbM trends and thereby cause additional AoA changes over time. [ABSTRACT FROM AUTHOR]

Analysis of stratospheric transport from an observational point of view is frequently realized by evaluation of mean age of air values from long-lived trace gases. However, this provides more insight into general transport strength and less into its mechanism. Deriving complete transit time distributions (age spectra) is desirable, but their deduction from direct measurements is difficult and so far primarily achieved by assumptions about dynamics and spectra themselves. This paper introduces a modified version of an inverse method to infer age spectra from mixing ratios of short-lived trace gases. For a full description of transport seasonality the formulation includes an imposed seasonal cycle to gain multimodal spectra. The EMAC model simulation used for a proof of concept features an idealized dataset of 40 radioactive trace gases with different chemical lifetimes as well as 40 chemically inert pulsed trace gases to calculate pulse age spectra. Annual and seasonal mean inverse spectra are compared to pulse spectra including first and second moments as well as the ratio between them to assess the performance on these time scales. Results indicate that the modified inverse age spectra match the annual and seasonal pulse age spectra well on global scale beyond 1.5 years mean age of air. The imposed seasonal cycle emerges as a reliable tool to include transport seasonality in the age spectra. Below 1.5 years mean age of air, tropospheric influence intensifies and breaks the assumption of single entry through the tropical tropopause, leading to inaccurate spectra in particular in the northern hemisphere. The imposed seasonal cycle wrongly prescribes seasonal entry in this lower region and does not lead to a better agreement between inverse and pulse age spectra without further improvement. As the inverse method aims for future implementation on in situ observational data, possible critical factors for this purpose are delineated finally. [ABSTRACT FROM AUTHOR]

Climate models consistently predict an acceleration of the Brewer-Dobson circulation (BDC) due to climate change in the 21st century. However, the strength of this acceleration varies considerably among individual models, which constitutes a notable source of uncertainty for future climate projections. To shed more light upon the magnitude of this uncertainty and on its causes, we analyze the stratospheric mean age of air (AoA) of 10 climate projection simulations from the Chemistry Climate Model Initiative phase 1 (CCMI-I), covering the period between 1960 and 2100. In agreement with previous multi-model studies, we find a large model spread in the magnitude of the AoA trend over the simulation period. Differences between future and past AoA are found to be predominantly due to differences in mixing (reduced aging by mixing and recirculation) rather than differences in residual mean transport. We furthermore analyze the mixing efficiency, a measure of the relative strength of mixing for given residual mean transport, which was previously hypothesized to be a model constant. Here, the mixing efficiency is found to vary not only across models, but also over time in all models. Changes in mixing efficiency are shown to be closely related to changes in AoA and quantified to roughly contribute 10 % to the long-term AoA decrease over the 21st century. Additionally, mixing efficiency variations are shown to considerably enhance model spread in AoA changes. To understand these mixing efficiency variations, we also present a consistent dynamical framework based on diffusive closure, which highlights the role of basic state potential vorticity gradients in controlling mixing efficiency and therefore aging by mixing. [ABSTRACT FROM AUTHOR]

Stratospheric age of air (AoA) is a useful measure of the overall capabilities of a general circulation model (GCM) to simulate stratospheric transport. Previous studies have reported a large spread in the simulation of AoA by GCMs and coupled chemistry-climate models (CCMs). Compared to observational estimates simulated AoA is mostly too low. Here we attempt to untangle the processes that lead to the AoA differences between the models and between models and observations. AoA is influenced by both, mean transport along the residual circulation and two-way mixing; we quantify the effects of these processes using data from the CCM inter-comparison projects CCMVal-2 and CCMI-1. Transport along the residual circulation is measured by the residual circulation transit time (RCTT). We interpret the difference between AoA and RCTT as additional aging by mixing. Aging by mixing thus includes mixing on both the resolved and subgrid scale. We find that the spread in AoA between the models is primarily caused by differences in the effects of mixing, and only to some extent by differences in residual circulation strength. These effects are quantified by the mixing efficiency, a measure of the relative increase of AoA by mixing. The mixing efficiency varies strongly between the models from 0.21 to 0.99. We show that the mixing efficiency is not only controlled by horizontal mixing, but by vertical mixing and vertical diffusion as well. Possible causes for the differences in the models' mixing efficiencies are discussed. Differences in subgrid scale mixing (including differences in advection schemes and model resolutions) likely contribute to the differences in mixing efficiency. However, differences in the relative contribution of resolved versus parametrized wave forcing do not appear to be related to differences in mixing efficiency or AoA. [ABSTRACT FROM AUTHOR]

The annual variation of water vapour exhibits a distinct isolated maximum in the middle and upper stratosphere in the southern tropics and subtropics, peaking typically around 15°S in latitude and close to 3hPa (~40.5km) in altitude. This enhanced annual variation is primarily related to the Brewer-Dobson circulation and hence also visible in other trace gases. So far this feature has not gained much attention in the literature and the present work aims to add more prominence. Using Envisat/MIPAS (Environmental Satellite/Michelson Interferometer for Passive Atmospheric Sounding) observations and ECHAM/MESSy (European Centre for Medium-Range Weather Forecasts Hamburg/Modular Earth Submodel System) Atmospheric Chemistry (EMAC) simulations we provide a dedicated illustration and a full account for the reasons of this enhanced annual variation. [ABSTRACT FROM AUTHOR]

Mean age of air (AoA) is a widely used metric to describe the transport along the Brewer-Dobson circulation. We seek to untangle the effects of different processes on the simulation of AoA, using the chemistry-climate model EMAC and the Lagrangian chemistry transport model CLaMS. Here, the effects of residual transport and two-way mixing on AoA are calculated. To do so, we calculate the residual circulation transit time (RCTT). The difference of AoA and RCTT is defined as aging by mixing. However, as diffusion is also included in this difference, we further use a method to directly calculate aging by mixing on resolved scales. Comparing these two methods of calculating aging by mixing allows for separating the effect of unresolved aging by mixing (which we term "aging by diffusion" in the following) in EMAC and CLaMS. We find that diffusion impacts AoA by making air older, but its contribution plays a minor role (order of 10 %) in all simulations. However, due to the different advection schemes of the two models, aging by diffusion has a larger effect on AoA and mixing efficiency in EMAC, compared to CLaMS. Regarding the trends in AoA, in CLaMS the AoA trend is negative throughout the stratosphere except in the northern hemisphere middle stratosphere, consistent with observations. This slight positive trend is neither reproduced in a free-running nor in a nudged simulation with EMAC - in both simulations the AoA trend is negative throughout the stratosphere. Trends in AoA are mainly driven by the contributions of RCTT and aging by mixing, whereas the contribution of aging by diffusion plays a minor role. [ABSTRACT FROM AUTHOR]

The South Asian High (SAH) is an important component of the summer monsoon system in Asia. In this study we investigate the location and drivers of the SAH at 100 hPa during the boreal summers of 1979 to 2014 on interannual, seasonal and synoptic time scales using six reanalyses. Special focus lies on the bimodality of the SAH, i.e. the two preferred modes of the SAH centre location: the Iranian Plateau to the west and the Tibetan Plateau to the east. We find that only the National Centers for Environmental Prediction - National Center of Atmospheric Research (NCEP/NCAR) reanalysis shows a clear bimodal structure of the SAH centre distribution with respect to daily and pentad (5-day mean) data. Furthermore, the distribution of the SAH centre location is highly variable from year-to-year. As in simple model studies, which connect the SAH to heating in the tropics, we find that the mean seasonal cycle of the SAH and its centre are dominated by the expansion of convection in the South Asian region (70° E-130° E × 15° N-30° N) on the southeastern border of the SAH. A composite analysis of precipitation and OLR data with respect to the location of the SAH centre reveals that a more westward (eastward) location of the SAH is related to stronger (weaker) convection and rainfall over India and stronger (weaker) precipitation over the West Pacific. [ABSTRACT FROM AUTHOR]