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The stratospheric polar vortex is dynamically coupled to the tropospheric circulation. Therefore, a better mechanistic understanding of this coupled system is important to interpret past and future circulation changes correctly. Previously, idealized simulations with a dry dynamical-core general circulation model and imposed tropical upper-tropospheric warming (TUTW) have shown that a critical warming level exists at which the polar vortex transitions from a weak and variable to a strong and stable regime. Here, we investigate the dynamical mechanism responsible for this regime transition and its influence on the troposphere by performing similar idealized experiments with (REF) and without a polar vortex (NPV). According to the critical-layer control mechanism, the strengthened upper flank of the subtropical jet in response to TUTW leads to an accelerated wave-driven residual circulation in both experiments. For the REF experiment, the stronger residual circulation is associated with changes in the lower-stratospheric thermal structure that are consistent with an equatorward shift of the polar vortex. At a certain threshold of TUTW in the REF experiment, the tropospheric jet and the stratospheric polar vortex form a confined waveguide for planetary-scale waves that presumably favors downward wave coupling events. Consistently, the polar vortex strengthens in combination with an enhanced poleward shift of the tropospheric jet compared to the NPV experiment. Overall, these idealized experiments suggest that a polar vortex strengthening can be caused by greenhouse gas–induced warmings via modifications of the waveguide. This mechanism might also be relevant to understand the polar vortex changes in more complex models.

Gravity waves (GWs) transport energy within the atmosphere both in vertical and horizontal direction. While the former is considered in climate models via parameterizations, the latter is often not modeled in most long time simulations. Especially orographic GWs can propagate horizontally more than 1000 km from their sources, however, and thereby de- or accelerate winds in completely different regions. To remedy this, we present a model that is capable of describing orographic GW sources and the associated GWs and their propagation in the atmosphere. From this, we can approximate general propagation pattern, which in the following can be used in climate models (here EMAC) to improve the orographic GW parameterization. The first simulations show strong redistribution of wave drag from land masses and thereby a closing of the gap of missing wave drag at 60°S.

The tropical circulation is typically not well represented in idealized models used to study jet dynamics. We implement a convective relaxation algorithm into a dry dynamical core model following Schneider and Walker (2006) to improve the representation of the driving mechanism behind the subtropical jet: the tropical meridional overturning.We study the dependence of the general circulation on the vertical stratification set by a convective relaxation scheme.Varying tropospheric lapse rates produces two jet regimes that are characterized by the distance between the subtropical jet and the eddy-driven jet.The separated jet state features distances of more than 12° latitude between subtropical and eddy-driven jet and is dominant in simulations with higher tropospheric static stability.Both jets approximately coincide in the joined jet regime, which is dominant in lower stability simulations.In addition to a steady state analysis, transitions from one regime to the other are induced by changes in convective lapse rate.Regime changes are also observed as events produced by natural variability in some of the model runs.This time-dependent perspective shows that the structure of net Rossby wave dissipation in the upper troposphere, measured by the Eliassen-Palm flux divergence, is crucial in order to understand the regimes.Jet merge and split events are mediated by upper tropospheric momentum flux variability.They are preceded by heat flux variability and are tied to variations in the hemispheric eddy kinetic energy.The results are also interpreted through the concept of criticality, relating meridional and vertical gradients in potential temperature.The analysis highlights the importance of static stability and its changes to mid-latitude jet dynamics.

Sudden stratospheric warming (SSW) events can form a window of forecast opportunity for polar vortex predictions on subseasonal-to-seasonal time scales. Analysing numerical ensemble simulations, we show that negative wind anomalies in the polar stratosphere following SSWs lead to a reduction in upward planetary wave propagation and hence a reduction in the dynamical variability of the polar vortex. Ensembles that predict an SSW show reduced ensemble spread in terms of polar vortex strength for several weeks to follow, as well as a corresponding reduction in forecast errors. The associated increase in predictability is particularly pronounced for strong SSWs and even occurs if not all ensemble members predict a major SSW. The decrease in upward wave fluxes and polar vortex variability following the event then manifests in a delay of the final warming during years that experience an SSW and potentially has further implications for the tropospheric or mesospheric circulation.

The columnar approach of gravity wave (GW) parameterisations in weather and climate models has been identified as a potential reason for dynamical biases in middle atmospheric dynamics. For example, GW momentum flux (GWMF) discrepancies between models and observations at 60° S arising through the lack of horizontal orographic GW propagation is suspected to cause deficiencies in representing the Antarctic polar vortex. However, due to the decomposition of the model domains onto different computing tasks for parallelisation, communication between horizontal grid boxes is computationally extremely expensive, making horizontal propagation of GWs unfeasible for global chemistry-climate simulations. To overcome this issue, we here present a simplified solution approximating horizontal GW propagation through redistribution of the GWMF at one single altitude by means of tailor-made redistribution maps. To generate the global redistribution maps averaged for each grid box, we use a parameterisation describing orography as a set of mountain ridges with specified location, orientation and height combined with a ray-tracing model describing lateral propagation of so-generated mountain waves. In the global chemistry-climate model (CCM) EMAC (ECHAM MESSy Atmospheric Chemistry), these maps then allow us to redistribute the GW momentum flux horizontally at one level obtaining an affordable overhead of computing resources. The results of our simulations show GWMF and drag patterns which are horizontally more spread-out than with the purely columnar approach, GWs now also are present above the ocean and regions without mountains. In this paper, we provide a detailed description of how the redistribution maps are computed and how the GWMF redistribution is implemented in the CCM. Moreover, an analysis shows why 15 km is the ideal altitude for the redistribution. First results with the redistributed orographic GWMF provide clear evidence that the redistributed GW drag in the Southern Hemisphere has the potential to modify and improve Antarctic polar vortex dynamics, thereby paving the way for enhanced credibility of CCM simulations and projections of polar stratospheric ozone.

In the lower stratosphere during winter, variability of stratospheric ozone 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 up to 86 % of the total variability of polar column ozone in the stratosphere is associated with year-by-year variations in polar vortex strength based on MLS observations for the winter seasons 2005-2020., The 28th IUGG General Assembly (IUGG2023) (Berlin 2023)

Ensemble forecasting has become a standard means to obtain information about forecast uncertainties in meteorological centres across the world. The large datasets generated by ensemble prediction systems carry much information that is difficult to analyse manually – here, techniques from the field of artificial intelligence can be beneficial to aid the analysis. Cluster analysis is one commonly used (unsupervised machine learning) approach to automatically determine distinct scenarios in numerical weather forecasting ensembles, both in atmospheric research and operational forecasting. Typically, a cluster analysis focusses on a selected meteorological forecast variable, a specific region, and time (or a time window). The dimensionality of the data is reduced by techniques like principal component analysis, and a clustering algorithm – typically k-means – is applied to the reduced data set. Challenges with such an approach arise through the determined clusters often being sensitive to factors including the selected region, forecast variable, and algorithm parameters, and also through the employed algorithms often appearing as a “black box” to the user. In our work, we attempt to make the clustering process more transparent by providing a visual analysis framework to analyse the sensitivity of generated clusters with respect to various factors. The presented framework is coupled to the open-source meteorological ensemble visualization software Met.3D, allowing for interactive specification of clustering parameters and for interactive visual analysis, including 3-D elements. A case study using ensemble prediction data of sudden stratospheric warmings (SSWs) is presented, demonstrating how visualizing similarity between clusterings with different parameters can aid the interpretation of the data.

In this study, we perform dedicated model experiments with the ECHAM/MESSy (European Center Hamburg Model / Modular Earth Submodel System) Atmospheric Chemistry (EMAC, Jöckel et al. 2005, 2010, 2016) model version 2.54.0.3. to quantify the modification of the circulation response to CO2 forcing by stratospheric ozone. Specifically, we compare simulations with pre-industrial (piCtrl) and with quadrupled CO2 climate conditions, in which stratospheric ozone is either held fixed (4xCO2piO3) or is adapted to the new climate state (4xCO2Ctrl). The diagnosticof the residual circulation and mean age of air shows that ozone changes dampen the CO2-induced BDC acceleration by up to 20%. This damping of the BDC acceleration is caused by an enhancement of the meridional temperature gradient in the lower stratosphere in summer, thereby leading to stronger stratospheric easterlies that suppress wave propagation. Additionally, we find a systematic weakening of the polar vortices in winter and spring. In the SH, ozone reduces the CO2-induced delay of the final warming date by 50 %. The output data that are used for this study are located in the following files: The zonal meanozone fields (for the pre-industrial and 4xCO2 condition) are located in these files: abr4xCO2_O3_ml47_zm_1960-1989_clim.nc; piCtrl_O3_ml47_zm_1960-1989_clim.nc We use v* and w* from the TEM monthly mean climatology data (files with the ending ...TEM_ymm.nc)for calculating the mass streamfunction as well as for the tropical upwelling.In addition, the TEM climatology contains also the EP-flux and its divergence (EPFD). The AoA climatology data (files with the ending ...AoA_1980-2029_ymm.nc) is also used for investigation of the mass transport represented by the BDC. We apply themonthly mean climatology of the temperature, specific humidity, zonal wind and gravity wave drag fields from the files with the ending...plev-ymm.nc. The monthly mean climatology of the shortwave heating rates are in the files with the ending ...._heatsw_pl_ymm.nc. The interaction between the stratosphere and troposphere and the abruptness of the final warming areanalyzed with daily climatological datafrom the files which end with ...._geopot_yd.nc. The standard deviation of these climatologies that are located in the files with the ending ...ymstd.nc (for monthly data) or ...ydstd.nc (for daily data) is used for the significance tests. The tropopause data (for the pre-industrial and 4xCO2 condition) can be found in these two files:C2-4xCO2ctrl_1980-2029_tropop-p-ymm.nc; C2-4xCO2ctrl_1980-2029_tropop-p-ymm.nc For the detection of the final warming date following by the method of Charlton and Polvani (2007), the daily mean climatology of the zonal mean zonal wind(file with the ending..10hPa_60degN/S.nc)is applied.

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.

A severe cold spell with surface temperatures reaching 10 K below its climatology hit Eurasia during late February/early March 2018. This cold spell was associated with a Scandinavian blocking pattern followed by an extreme negative North Atlantic Oscillation (NAO) phase. Here we explore the predictability of this cold spell/NAO event using ensemble forecasts from the Subseasonalto-Seasonal (S2S) archive of the European Centre for Medium-Range Weather Forecasts. We find that this event was predicted with the observed strength roughly 10 days in advance. However, the probability of the cold spell occurring doubled up to 25 days in advance, when a sudden stratospheric warming (SSW) occurred. Our results indicate that the amplitude of the cold spell was increased by a regime shift to the negative NAO phase at the end of February, which was likely favoured by the SSW. We quantify the contribution of the SSW to the enhanced extended-range forecast skill for this particular event by running forecast ensembles in which the evolution of the stratosphere is nudged to (a) the observed evolution, and (b) a time-invariant state. In the experiment with nudging to the observed stratospheric evolution, the probability of a strong cold spell occurring is enhanced to 45%, while it is at its climatological value of 5% when the stratosphere is nudged to a time-invariant state. These results showing enhanced predictability of surface extremes following SSWs extend previous observational evidence, which is mostly based on composite analyses, to a single event. Our results suggest that it is the subsequent evolution throughout the lower stratosphere following the SSW, rather than the occurrence of the SSW itself, that is crucial in coupling to large-scale tropospheric flow patterns. However, we caution that probabilistic gain in predictability alone is insufficient to conclude a causal link between the SSW and the cold spell event.

Eine Erhöhung der atmosphärischen CO2-Konzentration verändert die Temperaturen und Dynamik der Stratosphäre und somit auch die Ozonkonzentrationen. Da Ozon sensibel auf langwellige, als auch auf kurzwellige Strahlung reagiert, wirkt die veränderte Ozonverteilung wiederum auf die Temperatur und modifiziert somit auch die CO2-induzierte Veränderung der stratosphärischen Dynamik. Um diese genau untersuchen zu können, haben wir Modellsimulationen mit dem Klima-Chemie-Modell EMAC durchgeführt, bei denen die CO2-Konzentration im Vergleich zur Zeit der Vorindustrialisierung vervierfacht wurde. Bei den 4xCO2-Simulationen wurde dazu einmal eine nicht veränderte (vorindustrielle) und einmal eine veränderte Ozonverteilung aus einer vorhergegangenen 4xCO2-Simulation vorgeschrieben. Es zeigt sich hierbei, dass die Änderung im Ozon zu einer Verstärkung der stratosphärischen Ostwinde im Sommer, sowie eine Abschwächung des Polarwirbels in beiden Hemisphären führt. Während die hohe Variabilität in der Nordhemisphäre zwar zu keinen eindeutigen Ergebnissen führt, sind diese in der Südhemisphäre statistisch signifikant. Zudem ist die Saison des Polarwirbels durch die CO2-bedingte Veränderung des Ozonfeldes kürzer. Des Weiteren wird die Beschleunigung der Brewer-Dobson-Zirkulation, welche durch eine CO2-Erhöhung hervorgerufen wird, auf der Sommerhemisphäre durch den Ozoneinfluss gedämpft. Dies beeinflusst wiederum den Transport von den Tropen zu den Extra-Tropen. Die Ozon-bedingten Änderungen in der stratosphärischen Zirkulation beeinflussen auch die troposphärische Zirkulation und haben einen Effekt auf den troposphärischen Polarfrontjet. Dieser erfährt durch eine veränderte Ozonverteilung auf der Südhemisphäre eine systematisch schwächere Verschiebung in Richtung Pol als mit einer gleichbleibenden Ozonverteilung. Wir diskutieren die hier aufgeführten Ergebnisse und stellen diese im Kontext zum Einfluss von stratosphärischem Ozon auf die Dynamik im Klimawandel dar.

The Brewer–Dobson circulation (BDC) is a key feature of the stratosphere that models need to accurately represent in order to simulate surface climate variability and change adequately. 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 the sign of BDC trends between observations and models in the middle and upper stratosphere. Nevertheless, the large uncertainty in the observational trend estimates opens the door to compatibility. In particular, when accounting for the limited sampling of the observations, model and observational trend error bars overlap in 40 % of the simulations with available output. The increasing CO2 simulations feature an acceleration of the BDC but reveal a large spread in the middle-to-upper stratospheric trends, possibly related to the parameterized gravity wave forcing. The very close connection between the shallow branch of the residual circulation 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.

Analysis of stratospheric transport from an observational point of view is frequently realized by evaluation of the 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. It is so far primarily based on model work. This paper introduces a modified version of an inverse method to infer age spectra from mixing ratios of short-lived trace gases and investigates its basic principle in an idealized model simulation. For a full description of transport seasonality the method includes an imposed seasonal cycle to gain multimodal spectra. An ECHAM/MESSy Atmospheric Chemistry (EMAC) model simulation is utilized for a general proof of concept of the method and 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. It is assessed whether the modified inverse method in combination with the seasonal cycle can provide matching age spectra when chemistry is well-known. 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 timescales. Results indicate that the modified inverse age spectra match the annual and seasonal pulse age spectra well on global scale beyond 1.5 years of 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 of 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. Tests with a focus on future application to observational data imply that subsets of trace gases with 5 to 10 species are sufficient for deriving well-matching age spectra. These subsets can also compensate for an average uncertainty of up to ±20 % in the knowledge of chemical lifetime if a deviation of circa ±10 % in modal age and amplitude of the resulting spectra is tolerated.

Major disruptions to the stratospheric circulation have far-reaching effects, including increased likelihood of snowstorms, growth of the ozone hole, and interference in global telecommunications.

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.

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.

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.

Comprehensive global climate simulations are still conducted in fairly low resolution. Current general circulation models therefore rely on gravity wave parameterisations to simulate atmospheric dynamics correctly. Among other parameters, the surface wind determines gravity wave launching in orographic gravity wave parameterisations. However, the mountainous terrain in regions where orographic gravity waves occur suggests larger surface wind variability on unresolved topography than the model grid box wind can provide. To account for this variability, we here present a stochastic modification of the low-level wind direction when it is used in the orographic gravity wave scheme of the EMAC (ECHAM MESSy Atmospheric Chemistry) model. For our first application, we implemented a random normal function to evoke a modest deviation of the wind direction at each time step when it is used in the subgrid scale scheme.An EMAC simulation shows that this gravity wave modification locally leads to significant changes of orographic gravity wave drag, but this does not result in significant annual or seasonal differences in temperatures or winds. However, the Arctic polar vortex is stretched and its center shifts in February. Moreover, we find a shift in the Antarctic polar vortex breakdown date, resulting in a significant zonal mean temperature change in October and possibly in an alleviation of the EMAC low bias in Antarctic polar vortex strength. In this presentation, we discuss our results, the method and possible further developments like allowing gusts in the modified scheme.

The stratospheric transport circulation, or Brewer-Dobson Circulation (BDC), is often conceptually seperated into advection along the residual circulation and two-way mixing. In particular the latter part has recently been found to exert a strong influence on inter-model differences of mean age of Air (AoA), a common measure of the BDC. However, the precise reason for model differences in two-way mixing remains unknown, as many modelcomponents in multi-model projects differ. One component that likely plays an important role is model resolution, both vertically and horizontally. To analyse this aspect, we carried out a set of simulations with identical and constant year 2000 climate forcing varying the spectral horizontalresolution (T31,T42,T63,T85) and the number of vertical levels (L31,L47,L90). We find that increasing the vertical resolution leads to an increase in mean AoA. Most of this change can be attributed to aging by mixing. The mixing efficiency, defined as the ratio of isentropic mixing strength and the diabatic circulation, shows the same dependency on vertical resolution. While horizontal resolution changes do not systematically change mean AoA, we dofind a systematic decrease in the mixing efficiency with increasing horizontal resolution. Non-systematic changes in the residual circulation partly compensate the mixing efficiency changes, leading to the non-systematic mean AoA changes. The mixing efficiency changes with vertical and horizontal resolution are consistent with expectations on the effects of numerical dispersion on mean AoA. To further investigate the most relevant regions of mixing differences, we analyse height-resolved mixing efficiency differences. Overall, this work will help to shed light on the underlying reasons for the large biases of climate models in simulating stratospheric transport.

Sudden stratospheric warmings (SSWs) are impressive fluid dynamical events in which large and rapid temperature increases in the winter polar stratosphere (⁓10‐50km) are associated with a complete reversal of the climatological wintertime westerly winds. SSWs are caused by the breaking of planetary‐scale waves that propagate upwards from the troposphere. During an SSW, the polar vortex breaks down, accompanied by rapid descent and warming of air in polar latitudes, mirrored by ascent and cooling above the warming. The rapid warming and descent of the polar air column affects tropospheric weather, shifting jet streams, storm tracks, and the Northern Annular Mode, making cold air outbreaks over North America and Eurasia more likely. SSWs affect the atmosphere above the stratosphere, producing widespread effects on atmospheric chemistry, temperatures, winds, neutral (non‐ionized) particles and electron densities, and electric fields. These effects span both hemispheres. Given their crucial role in the whole atmosphere, SSWs are also seen as a key process to analyze in climate change studies and subseasonal to seasonal prediction. This work reviews the current knowledge on the most important aspects of SSWs, from the historical background to dynamical processes, modelling, chemistry, and impact on other atmospheric layers. ISSN:8755-1209 ISSN:0096-1043 ISSN:1944-9208

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.

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.

Mean age of air (AoA) is a common diagnostic for the stratospheric overturning circulation in both climate models and observations. Observations of AoA mostly base on measurements of SF6, which is an almost ideal AoA tracer because its emssions across the recent decades increased nearly linearly and it is fairly stable in the troposphere and stratosphere. Over the last ten years, however, researchers were puzzled as to why AoA climatologies and trends of model simulations and observational data do not coincide. AoA in climate models is generally much lower than in observations and models show a clear decrease of AoA over time while measurements show a non-significant increase.What is commonly not considered in the models is that SF6 has chemical sinks in the mesosphere, and these lead to apparently older air in the stratosphere. In our experiment, we explicitely calculate SF6 sinks based on physical processes in simulations with the global chemistry-climate model EMAC (ECHAM MESSy Atmospheric Chemistry). We show that considering the SF6 removal reactions strongly increases stratospheric AoA and leads to much better agreement between the climatologies of EMAC and MIPAS (Michelson Interferometer for Passive Atmospheric Sounding) satellite observations. Moreover, the stratospheric AoA trend over the recent decades reverses sign when we derive it from SF6 with sinks. This means that the trend can such be reconciled with the trend that has been derived from long-term balloon-borne measurements. Our specifically designed sensitivity studies moreover reveal that this positive trend results neither from circulation changes, nor from variations of the reactive species involved in mesospheric SF6 depletion. Instead, it is generated through the temporally growing influence of the SF6 sinks themselves, an effect that overcompensates the negative trend resulting from the accelerating stratospheric overturning circulation.

A severe cold spell with surface temperatures reaching 10 K below climatology hit Eurasia during late February/early March 2018. This cold spell was associated with a Scandinavian blocking pattern followed by an extreme negative North Atlantic Oscillation (NAO) phase. Here we explore the predictability of this cold spell/NAO event using ensemble forecasts from the Subseasonal-to-Seasonal (S2S) archive. We find that this event was predicted with the observed strength roughly 10 days in advance. However, the probability of occurrence of the cold spell was doubled up to 25 days in advance, when a sudden stratospheric warming (SSW) occurred. Our results indicate that the amplitude of the cold spell was increased by the regime shift to the negative NAO phase at the end of February, which was likely favored by the SSW. We quantify the contribution of the SSW to the enhanced extended-range forecast skill for this particular event by running forecast ensembles in which the evolution of the stratosphere is nudged to a) the observed evolution, and b) a time invariant state. In the experiment with the observed stratospheric evolution nudged, the probability of occurrence of a strong cold spell is enhanced to 45%, while it is at its climatological value of 5% when the stratosphere is nudged to a time invariant state. These results showing enhanced predictability of surface extremes following SSWs extend previous observational evidence, which is mostly based on composite analyses, to a single event. Our results support that it is the subsequent evolution throughout the lower stratosphere following the SSW, rather than the occurrence of the SSW itself, that is crucial in coupling to large-scale tropospheric flow patterns. However, we caution that probabilistic gain in predictability alone is insufficient to conclude about a causal link between the SSW and the cold spell event.

A dry dynamical-core model is used to investigate the regime behavior of the polar vortex under the influence of tropical upper-tropospheric warming. Up to 5 K temperature increase in this region, the polar vortex strength and variability hardly changes. Only for temperature increases above 8 K the polar night jet speeds up by approximately 20 m s−1 and the probability of sudden stratospheric warmings is strongly reduced.A comparison of climatological-mean differences of the zonal-mean zonal winds between the two regimes and the first empirical orthogonal function of the zonal-mean zonal wind closest to the regime transition at around 7.5 K temperature increase reveals that the system oscillates between both regimes at the regime transition. Every regime is present for a long time accounting for the peaked autocorrelation time scale being distinctive of a regime transition. From a dynamical point of view the strong polar vortex regime is characterized by less negative Eliassen-Palm (EP) flux divergence in the stratosphere and an equatorward refraction of EP flux in the midlatitudes compared to the weak polar vortex regime.In order to quantify the influence of the polar vortex on the tropospheric circulation during tropospheric warming, another set of tropical upper-tropospheric heating simulations without a polar vortex is performed. This reveals that the latitudes of the tropospheric jets in both sets of simulations coincide for tropical upper-tropospheric warmings up to 5 K, or equivalently, when the polar vortex is in its weak regime. However, when the polar vortex starts to transition to the strong regime, i.e. for tropospheric warmings above 5 K, the poleward contraction of the tropospheric jet is strongly enhanced compared to the set of simulations without polar vortex.

Midwinter sudden stratospheric warmings (SSWs), characterised by the reversal of the temperature gradient poleward of 60°N and the 10 hPa climatological zonal mean wind from westerly to easterly at 60°N, are known to have pronounced impacts on tropospheric circulation which lead to regional changes in temperature, precipitation and other meteorological variables. Such abrupt events are furthermore known to be associated with large-scale changes in the distribution of stratospheric chemistry constituents, such as ozone (O3) and water vapour (H2O), although the implications for stratosphere-troposphere exchange (STE) have not been previously investigated. The evolution of O3 and H2O anomalies during an SSW life cycle are first examined from the surface up to 1 hPa using specified-dynamics simulations from the European Centre for Medium-Range Weather Forecasts – Hamburg (ECHAM)/Modular Earth Submodel System (MESSy) Atmospheric Chemistry (EMAC) model over the period 1979-2013. We show that significant positive anomalies in O3 occur around the onset of an SSW in the middle to lower stratosphere, with persistence timescales of around 50 days in the upper troposphere-lower stratosphere (UTLS). Similarly, we find significant H2O anomalies in the lowermost stratosphere (± 25 %) for up to 75 days. The extent and magnitude of the anomalies are largely confirmed in both Copernicus Atmospheric Monitoring Service (CAMS) reanalysis and ozonesonde measurements at five different Arctic stations. These chemical perturbations result in local temperature changes of up to 2 K, which may impact numerical weather prediction (NWP) of the tropospheric response to SSWs. Evaluation of the vertical residual velocity (w*) support the notion of transport changes being the driver of the temporal evolution of the anomalies. Using a stratospheric-tagged O3 tracer, a signal for enhanced STE of ozone is subsequently inferred (~ 5-10 %), which is maximised around 50 days after the SSW onset date. We furthermore attempt to elucidate STE transport pathways using a tropopause fold identification algorithm applied to ERA-Interim during this period, and assess such changes in folding frequency and distribution during such events. Our results highlight that SSWs can induce significantly disturbed O3 and H2O distributions in the UTLS, leading to enhanced STE of O3, with potentially significant implications for radiative fluxes, atmospheric heating rates and air quality.

The stratospheric Brewer--Dobson circulation (BDC) is an important element of climate system as it determines the concentration of radiatively active trace gases like water vapor, ozone and aerosol above the tropopause. Climate models predict that increasing greenhouse gas levels speed up the stratospheric circulation. BDC changes is substantially modulated by different modes of climate variability (QBO, ENSO, solar cycle), including the volcanic aerosols. However, such variability is often not reliably included or represented in current climate model simulations, challenging the evaluation of models’ behavior against observations and constituting a major uncertainty in current climate simulations. Here, we investigate the main differences between the reanalysis and the CCMI/CMIP6 climate models’ response to stratospheric volcanic forcings regarding the depth/strength of the stratospheric BDC, with a focus on potential changes in the deep and shallow circulation branches. We also discuss the key reasons of the discrepancies (incl. uncertainties associated with volcanological forcing datasets and missing direct aerosol heating in the reanalysis) in the BDC response between reanalysis-driven and climate model simulations in the lower, mid and upper stratosphere. Finally, we assess the dynamical mechanisms involved in the volcanically-induced BDC changes to understand the opposite regime between lower, middle and upper stratosphere after the Mt Pinatubo eruption.

Energy and momentum deposition from planetary-scale Rossby waves as well as from small-scale gravity waves (GWs) largely control stratospheric dynamics. Interactions between these different wave types, however, complicate the quantification of their individual contribution to the overall dynamical state of the middle atmosphere. In state-of-the-art general circulation models (GCMs), the majority of the GW spectrum cannot be resolved and therefore has to be parameterised. This is commonly implemented in two discrete schemes, one for GWs that originate from flow over orographic obstacles and one for all other kinds of GWs (non-orographic GWs). In this study, we attempt to gain a deeper understanding of the interactions of resolved with parameterised wave driving and of their influence on the stratospheric zonal winds and on the Brewer–Dobson circulation (BDC). For this, we set up a GCM time slice experiment with two sensitivity simulations: one without orographic GWs and one without non-orographic GWs. Our findings include an acceleration of the polar vortices, which has historically been one of the main reasons for including explicit GW parameterisations in GCMs. Further, we find inter-hemispheric differences in BDC changes when omitting GWs that can be explained by wave compensation and amplification effects. These are partly evoked through local changes in the refractive properties of the atmosphere caused by the omitted GW drag and a thereby increased planetary wave propagation. However, non-local effects on the flow can act to suppress vertical wave fluxes into the stratosphere for a very strong polar vortex. Moreover, we study mean age of stratospheric air to investigate the impact of missing GWs on tracer transport. On the basis of this analysis, we suggest that the larger ratio of planetary waves to GWs leads to enhanced horizontal mixing, which can have a large impact on stratospheric tracer distributions.

While models of the Earth system gain more and more complexity, a need emerges to establish model hierarchies and to utilize simplified models to improve process understanding. The Modular Earth Submodel System (MESSy) was developed with the aim to provide an environment to allow for model configurations and set-ups of varying complexity, and as of now the hierarchy reaches from a chemical box model to the full coupled ECHAM/MESSy Atmospheric Chemistry (EMAC) Chemistry-Climate model. In the current study, we present and document the development of a new simplified set-up within the ECHAM/MESSy model, namely the dry dynamical core model set-up ECHAM/MESSy IdeaLized (EMIL). The set-up is achieved by the implementation of a new submodel for relaxation of temperature and horizontal winds to given background values (the RELAX submodel), which replaces all other physics submodels in the EMIL set-up. The RELAX submodel incorporates options to set the needed parameters (e.g. equilibrium temperature, relaxation time and damping coefficient) to functions used frequently in the past (given by Held and Suarez, 1994; Polvani and Kushner, 2002). Test simulations with the EMIL model set-up show that results from earlier studies with other dry dynamical core models are reproduced under same set-ups. Furthermore, modifications to the previously used set-ups are tested, with the following main findings: 1) lowering the equilibrium temperature in the lower stratosphere at winter polar high latitudes to more realistic values (i.e. below observed temperatures) results in high latitudes temperature profiles in the model closer to observations, and 2) when replacing the idealized topography to generate planetary waves by mid-tropospheric wave-like heating (as suggested in a previous study), the response of the tropospheric jet to changes in the equilibrium temperature is strongly damped, indicating that the wave-like heating has to be used with care. As application examples, we present simulations with simplified chemistry to study the impact of dynamical variability and idealized changes on tracer transport, and simulations of idealized monsoon circulations forced by localized heating. The ability to simulate dynamical systems and to incorporate passive and chemical active tracers in the EMIL set-up demonstrates the potential for future studies of tracer transport in the idealized dynamical model.

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.

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 timescales using seven reanalyses and observational data. Our comparison of the different reanalyses focuses especially 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 (70–130° E × 15–30° N) on the south-eastern border of the SAH. A composite analysis of precipitation and outgoing long-wave radiation 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 weaker (stronger) precipitation over the western Pacific.

Climate model simulations show an acceleration of the Brewer–Dobson circulation (BDC) in response to climate change. While the general mechanisms for the BDC strengthening are widely understood, there are still open questions concerning the influence of the details of the wave driving. Mean age of stratospheric air (AoA) is a useful transport diagnostic for assessing changes in the BDC. Analyzing 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 20 and 25 geopotential kilometers (gpkm) and 20–50∘ N and S). We show that the occurrence of AoA trend minima in those regions is directly related to the climatological AoA distribution, which is sensitive to an upward shift of the circulation in response to climate change. 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 focusing on 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. 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. Czech Science Foundation (GACˇ R) | Ref. 16- 01562J Czech Science Foundation (GACˇ R) | Ref. 18-01625S Ministerio de Ciencia e Innovación | Ref. CGL2015-71575-P Xunta de Galicia | Ref. ED481B 2018/103 Ministerio de Economía y Competitividad | Ref. RYC-2013-14560

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 %).

This study investigates the abrupt and severe water vapour decline in the stratosphere beginning in the year 2000 (the "millennium water vapour drop") and other similarly strong stratospheric water vapour reductions by means of various simulations with the state-of-the-art Chemistry-Climate Model (CCM) EMAC (ECHAM/MESSy Atmospheric Chemistry Model). The model simulations differ with respect to the prescribed sea surface temperatures (SSTs) and whether nudging is applied or not. The CCM EMAC is able to most closely reproduce the signature and pattern of the water vapour drop in agreement with those derived from satellite observations if the model is nudged. Model results confirm that this extraordinary water vapour decline is particularly obvious in the tropical lower stratosphere and is related to a large decrease in cold point temperature. The drop signal propagates under dilution to the higher stratosphere and to the poles via the Brewer–Dobson circulation (BDC). We found that the driving forces for this significant decline in water vapour mixing ratios are tropical sea surface temperature (SST) changes due to a coincidence with a preceding strong El Niño–Southern Oscillation event (1997/1998) followed by a strong La Niña event (1999/2000) and supported by the change of the westerly to the easterly phase of the equatorial stratospheric quasi-biennial oscillation (QBO) in 2000. Correct (observed) SSTs are important for triggering the strong decline in water vapour. There are indications that, at least partly, SSTs contribute to the long period of low water vapour values from 2001 to 2006. For this period, the specific dynamical state of the atmosphere (overall atmospheric large-scale wind and temperature distribution) is important as well, as it causes the observed persistent low cold point temperatures. These are induced by a period of increased upwelling, which, however, has no corresponding pronounced signature in SSTs anomalies in the tropics. Our free-running simulations do not capture the drop as observed, because a) the cold point temperature has a low bias and thus the water vapour variability is reduced and b) because they do not simulate the appropriate dynamical state. Large negative water vapour declines are also found in other years and seem to be a feature which can be found after strong combined El Niño/La Niña events if the QBO west phase during La Niña changes to the east phase.

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.

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.

Three types of reference simulations, as recommended by the Chemistry–Climate Model Initiative (CCMI), have been performed with version 2.51 of the European Centre for Medium-Range Weather Forecasts – Hamburg (ECHAM)/Modular Earth Submodel System (MESSy) Atmospheric Chemistry (EMAC) model: hindcast simulations (1950–2011), hindcast simulations with specified dynamics (1979–2013), i.e. nudged towards ERA-Interim reanalysis data, and combined hindcast and projection simulations (1950–2100). The manuscript summarizes the updates of the model system and details the different model set-ups used, including the on-line calculated diagnostics. Simulations have been performed with two different nudging set-ups, with and without interactive tropospheric aerosol, and with and without a coupled ocean model. Two different vertical resolutions have been applied. The on-line calculated sources and sinks of reactive species are quantified and a first evaluation of the simulation results from a global perspective is provided as a quality check of the data. The focus is on the intercomparison of the different model set-ups. The simulation data will become publicly available via CCMI and the Climate and Environmental Retrieval and Archive (CERA) database of the German Climate Computing Centre (DKRZ). This manuscript is intended to serve as an extensive reference for further analyses of the Earth System Chemistry integrated Modelling (ESCiMo) simulations.

The 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 by the residual circulation and two-way mixing; we quantify the effects of these processes using data from the CCM inter-comparison projects CCMVal-2 (Chemistry–Climate Model Validation Activity 2) and CCMI-1 (Chemistry–Climate Model Initiative, phase 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 in AoA by mixing. The mixing efficiency varies strongly between the models from 0.24 to 1.02. 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 parameterized wave forcing do not appear to be related to differences in mixing efficiency or AoA., Atmospheric Chemistry and Physics, 18 (9), ISSN:1680-7375, ISSN:1680-7367

Because tropospheric ozone is both a greenhouse gas and harmful air pollutant, it is important to understand how anthropogenic activities may influence its abundance and distribution through the 21st century. Here, we present model simulations performed with the chemistry–climate model SOCOL, in which spatially disaggregated chemistry and transport tracers have been implemented in order to better understand the distribution and projected changes in tropospheric ozone. We examine the influences of ozone precursor emissions (nitrogen oxides (NOx), carbon monoxide (CO) and volatile organic compounds (VOCs)), climate change (including methane effects) and stratospheric ozone recovery on the tropospheric ozone budget, in a simulation following the climate scenario Representative Concentration Pathway (RCP) 6.0 (a medium-high, and reasonably realistic climate scenario). Changes in ozone precursor emissions have the largest effect, leading to a global-mean increase in tropospheric ozone which maximizes in the early 21st century at 23% compared to 1960. The increase is most pronounced at northern midlatitudes, due to regional emission patterns: between 1990 and 2060, northern midlatitude tropospheric ozone remains at constantly large abundances: 31% larger than in 1960. Over this 70-year period, attempts to reduce emissions in Europe and North America do not have an effect on zonally averaged northern midlatitude ozone because of increasing emissions from Asia, together with the long lifetime of ozone in the troposphere. A simulation with fixed anthropogenic ozone precursor emissions of NOx, CO and non-methane VOCs at 1960 conditions shows a 6% increase in global-mean tropospheric ozone by the end of the 21st century, with an 11 % increase at northern midlatitudes. This increase maximizes in the 2080s and is mostly caused by methane, which maximizes in the 2080s following RCP 6.0, and plays an important role in controlling ozone directly, and indirectly through its influence on other VOCs and CO. Enhanced flux of ozone from the stratosphere to the troposphere as well as climate change-induced enhancements in lightning NOx emissions also increase the tropospheric ozone burden, although their impacts are relatively small. Overall, the results show that under this climate scenario, ozone in the future is governed largely by changes in methane and NOx; methane induces an increase in tropospheric ozone that is approximately one-third of that caused by NOx. Climate impacts on ozone through changes in tropospheric temperature, humidity and lightning NOx remain secondary compared with emission strategies relating to anthropogenic emissions of NOx, such as fossil fuel burning. Therefore, emission policies globally have a critical role to play in determining tropospheric ozone evolution through the 21st century.

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 3 hPa (~ 40.5 km) 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.