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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.

Thermodynamical and dynamical aspects of the climate system response to anthropogenic forcing are often considered in two distinct frameworks: the former in the context of the forcing–feedback framework, and the latter in the context of eddy–mean flow feedbacks and large-scale thermodynamic constraints. Here we use experiments with the dynamical core of a general circulation model (GCM) to provide insights into the relationships between these two frameworks. We first demonstrate that the climate feedbacks and climate sensitivity in a dynamical core model are determined by its prescribed thermal relaxation time scales. We then perform two experiments: one that explores the relationships between the thermal relaxation time scale and the climatological circulation, and a second that explores the relationships between the thermal relaxation time scale and the circulation response to a global warming–like forcing perturbation. The results indicate that shorter relaxation time scales (i.e., lower climate sensitivities in the context of a dynamical core model) are associated with 1) a more vigorous large-scale circulation, including increased thermal diffusivity and stronger and more poleward storm tracks and eddy-driven jets, and 2) a weaker poleward displacement of the storm tracks and eddy-driven jets in response to the global warming–like forcing perturbation. Interestingly, the circulation response to the forcing perturbation effectively disappears when the thermal relaxation time scales are spatially uniform, suggesting that the circulation response to homogeneous forcing requires spatial inhomogeneities in the local feedback parameter. Implications for anticipating the circulation response to global warming and thermodynamic constraints on the circulation are discussed.

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.

The wintertime extratropical general circulation may be viewed as being primarily governed by interactions between Rossby waves and the background flow. These Rossby waves propagate vertically and meridionally away from their sources and amplify within the core of the tropopause-level jet, which acts as a waveguide. The strength of this waveguide is in part controlled by tropopause sharpness, which itself is a function of the strength of tropopause inversion layer (TIL), a layer of enhanced static stability just above the tropopause. Here, we report a strong relation between interannual-to-multidecadal variations in the strength of the midlatitude TIL and jet latitude in a reanalysis and climate models. Similar relationships hold for the variability across climate models. Experiments with a mechanistic model show that a sharper tropopause promotes an intensified general circulation and an equatorward shifted jet., The 28th IUGG General Assembly (IUGG2023) (Berlin 2023)

The Brewer-Dobson circulation is the wave driven meridional circulation of the stratosphere it plays an important role in determining the transport of trace gases and aerosols within the stratosphere, affecting the lifetimes of ozone depleting substances, and the global radiation budget. The overturning part of the circulation is usually split into a deep and a shallow branch. Here, we investigate the dynamical driving of these circulation branches, aiming for a dynamical separation of different circulation regimes.For that purpose, we use data from fifth generation atmospheric reanalysis ERA5 by the European Centre for Medium-range Weather Forecasts (ECMWF) and apply the Transformed Eulerian Mean approach to estimate the overturning mass flux and the related wave forcing in the stratosphere. We find that reducing the horizontal resolution of the data from 0.3 to 1 degree does not affect results significantly. However, reducing temporal resolution from 1 to 6 hours has a significant effect on the structure of daily mean upwelling, but this effect is much less pronounced for monthly means. Eliassen-Palm flux divergence is used as a diagnostic to estimate the wave propagation in the stratosphere. Using Fourier transformation for spectral decomposition we estimate the contribution from different waves to the driving of the deep and shallow branches. In particular, it is found that the deep branch has a strong seasonality with maximum in winter, while the shallow branch is less affected by the change of seasons. On the one hand, we find a strong correlation between variability in residual circulation velocity along the deep branch and large scale waves with wavenumber 3 or smaller. On the other hand, variability of medium and small scale waves with wavenumber 4 or greater correlates strongly with variability in the shallow branch circulation velocities. The change between these two dynamical regimes happens at a level close to 37 hPa. These results were further tested by applying a downward control calculation, showing that indeed the large-scale, planetary waves (wavenumber less than 4) account for the largest part of deep branch variability, while smaller waves (wavenumber larger than 3) account for the largest part of shallow branch variability. Based on these results, we propose a physical definition of the different Brewer-Dobson circulation branches, with the deep branch defined as driven by planetary waves (wave numbers 1-3) and located above 35 hPa, whereas the shallow branch being located below that level and driven by smaller-scale waves (wave numbers 4 and greater).

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.

Volcanic eruptions that inject sulfur dioxide into the stratosphere have the potential to alter large-scale circulation patterns, such as the quasi-biennial oscillation (QBO), which can affect weather and transport of chemical species. Here, we conduct simulations of tropical volcanic eruptions using the UM-UKCA aerosol-climate model with an explicit representation of the QBO. Eruptions emitting 60 Tg of SO2 (i.e. the magnitude of the 1815 Mt. Tambora eruption) and 15 Tg of SO2 (i.e. the magnitude of the 1991 Mt. Pinatubo eruption) were initiated at the Equator during two different QBO states. We show that tropical eruptions delay the progression of the QBO phases, with the magnitude of the delay dependent on the initial wind shear in the lower stratosphere and a much longer delay when the shear is easterly than when it is westerly. The QBO response in our model is driven by vertical advection of momentum by the stronger tropical upwelling caused by heating due to the increased volcanic sulfate aerosol loading. Direct aerosol-induced warming with subsequent thermal wind adjustment, as proposed by previous studies, is found to only play a secondary role. This interpretation of the response is supported by comparison with a simple dynamical model. The dependence of the magnitude of the response on the initial QBO state results from differences in the QBO secondary circulation. In the easterly shear zone of the QBO, the vertical component of the secondary circulation is upward and reinforces the anomalous upwelling driven by volcanic aerosol heating, whereas in the westerly shear zone the vertical component is downward and opposes the aerosol-induced upwelling. We also find a change in the latitudinal structure of the QBO, with the westerly phase of the QBO strengthening in the hemisphere with the lowest sulfate aerosol burden. Overall, our study suggests that tropical eruptions of Pinatubo magnitude or larger could force changes to the progression of the QBO, with particularly disruptive outcomes for the QBO if the eruption occurs during the easterly QBO shear.

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)

The poleward expansion of the Hadley cells is one of the most robust modeled responses to increasing greenhouse gas concentrations. There are many proposed mechanisms for expansion, and most are consistent with modeled changes in thermodynamics, dynamics, and clouds. The adjustment of the eddies and the mean flow to greenhouse gas forcings, and to one another, complicates any effort toward a deeper understanding. Here we modify the Gray Radiation and Moist Aquaplanet (GRANDMA) model to uncouple the eddy and mean flow responses to forcings. When eddy forcings are held constant, the purely axisymmetric response of the Hadley cell to a greenhouse gas–like forcing is an intensification and poleward tilting of the cell with height in response to an axisymmetric increase in angular momentum in the subtropics. The angular momentum increase drastically alters the circulation response compared to axisymmetric theories, which by nature neglect this adjustment. Model simulations and an eddy diffusivity framework demonstrate that the axisymmetric increase in subtropical angular momentum—the direct manifestation of the radiative–convective equilibrium temperature response—drives a poleward shift of the eddy stresses which leads to Hadley cell expansion. Prescribing the eddy response to the greenhouse gas–like forcing shows that eddies damp, rather than drive, changes in angular momentum, moist static energy transport, and momentum transport. Expansion is not driven by changes in baroclinic instability, as would otherwise be diagnosed from the fully coupled simulation. These modeling results caution any assessment of mechanisms for circulation change within the fully coupled wave–mean flow system.

Funding: OA acknowledges support by ISF Grant 1022/21. Regions along the edges of the tropics host vast populations and ecosystems which are sensitive to climate change. Here we examine the extent of tropical land areas in the ERA5 and MERRA‐2 reanalyses and in high‐emission scenarios of 45 models participating in phases 5 and 6 of the Coupled Model Intercomparison Project (CMIP5/6). Based on the definition of tropical climate land areas as regions where the diurnal temperature range exceeds the seasonal temperature range, we find a net reduction of tropical land area with global warming. This change is primarily due to an increased seasonal temperature range driven by enhanced summer warming, which in turn is largely driven by reduced evaporative cooling. The reduction of tropical climate area is consistent with a narrowing of the tropical rain belt and with an equatorward and poleward expansion of the subtropical dry zones. Understanding the links between these trends requires further study. Publisher PDF

Volcanic eruptions that inject sulfur dioxide into the stratosphere have the potential to alter large-scale circulation patterns, such as the quasi-biennial oscillation (QBO), which can affect weather and transport of chemical species. Here, we conduct simulations of tropical volcanic eruptions using the UM-UKCA aerosol-climate model with an explicit representation of the QBO. Eruptions emitting 60 Tg of SO2 (i.e. the magnitude of the 1815 Mt. Tambora eruption) and 15 Tg of SO2 (i.e. the magnitude of the 1991 Mt. Pinatubo eruption) were initiated at the Equator during two different QBO states. We show that tropical eruptions delay the progression of the QBO phases, with the magnitude of the delay dependent on the initial wind shear in the lower stratosphere and a much longer delay when the shear is easterly than when it is westerly. The QBO response in our model is driven by vertical advection of momentum by the stronger tropical upwelling caused by heating due to the increased volcanic sulfate aerosol loading. Direct aerosol-induced warming with subsequent thermal wind adjustment, as proposed by previous studies, is found to only play a secondary role. This interpretation of the response is supported by comparison with a simple dynamical model. The dependence of the magnitude of the response on the initial QBO state results from differences in the QBO secondary circulation. In the easterly shear zone of the QBO, the vertical component of the secondary circulation is upward and reinforces the anomalous upwelling driven by volcanic aerosol heating, whereas in the westerly shear zone the vertical component is downward and opposes the aerosol-induced upwelling. We also find a change in the latitudinal structure of the QBO, with the westerly phase of the QBO strengthening in the hemisphere with the lowest sulfate aerosol burden. Overall, our study suggests that tropical eruptions of Pinatubo magnitude or larger could force changes to the progression of the QBO, with particularly disruptive outcomes for the QBO if the eruption occurs during the easterly QBO shear.

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.

Idealised baroclinic-wave life-cycle experiments are a widely used tool to study fundamental characteristics of mid-latitude baroclinic instability. A typical life-cycle evolves from an initialised baroclinically unstable jet through an exponential growth phase of a particular unstable wave mode, followed by wave breaking during the mature phase, and wave-mean flow interaction driving a jet shift during the decay phase. Many authors distinguish between life-cycles with predominantly anticyclonic (LC1) and cyclonic (LC2) wave breaking and the transition between the two flavours is typically controlled via the strength of cyclonic meridional wind shear in the initial conditions. While baroclinic wave growth has traditionally been triggered via a specified initial perturbation with fixed zonal wave number, this study extends the concept of baroclinic-wave life-cycles by analysing the influence of random initial perturbations without any preferred zonal dependency on the life-cycle evolution. We find that the growth phase shows a robust LC1-LC2 distinction as a function of initialised meridional shear, while a preference for LC1-like characteristics is observed during the decay phase for all life-cycles with non-monochromatic initial perturbations. In particular, the persistent cut-off cyclones that typically form for LC2 initialisations are found to eventually become unstable – the earlier during the life-cycle the stronger the initial noise perturbations. All non-monochromatic life-cycles result in a poleward jet shift in their final state, regardless of the strength of the initial shear. Consistently, anticyclonic wave breaking tends to be predominant during the mature and decay phases, even for LC2 initialisations. Equatorward jet shifts associated with cyclonic wave breaking still exist, although purely as a transient interim state. We show that wave-wave interactions resulting from the initialised random wave spectrum play an important role during all phases of the life-cycle.

Both theory and climate model results suggest that the Brewer-Dobson circulation should strengthen in the stratosphere with increasing greenhouse gas concentrations. Directly measuring the circulation strength is not possible, so verification of this sensitivity has been limited to indirect inferences from observed tracer fields of long-lived species. These methods, however, are complex and accumulation of the data required for them is difficult. When limiting discussion to the tropical lower stratosphere, ozone concentrations have shown to be consistent with an accelerating circulation. These measurements are particularly useful because of the long timeseries available from multiple datasets, but they have only been used for indirect investigations of the circulation strength, up until now.In this work, we invert the ozone balance equation to solve for upwelling. By limiting the investigation to 70 hPa in the southern tropics and estimating upwelling anomalies from the long-term mean (and not the absolute value of upwelling) most chemical terms and both horizontal and vertical mixing can be neglected, and calculation of the remaining terms is straight-forward. To verify the validity of the method, a calculation of upwelling is performed using climate model data, from which a comparison of actual upwelling and upwelling from the inverse method can be made. The seasonal cycle of upwelling anomalies is compared to upwelling anomalies from reanalyses and model results, and trends and variability are discussed.

A ubiquitous cold signal near the tropopause, here called “tropopause layer cooling” (TLC), has been documented in deep convective regions such as tropical cyclones (TCs). Temperature retrievals from the Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) reveal cooling of order 0.1–1 K day−1on spatial scales of order 1000 km above TCs. Data from the Cloud Profiling Radar (onboardCloudSat) and from the Cloud–Aerosol Lidar with Orthogonal Polarization [onboard theCloud–Aerosol Lidar and Infrared Pathfinder Satellite Observations(CALIPSO)] are used to analyze cloud distributions associated with TCs. Evidence is found that convective clouds within TCs reach the upper part of the tropical tropopause layer (TTL) more frequently than do convective clouds outside TCs, raising the possibility that convective clouds within TCs and associated cirrus clouds modulate TLC. The contribution of clouds to radiative heating rates is then quantified using theCloudSatandCALIPSOdatasets: in the lower TTL (below the tropopause), clouds produce longwave cooling of order 0.1–1 K day−1inside the TC main convective region, and longwave warming of order 0.01–0.1 K day−1outside; in the upper TTL (near and above the tropopause), clouds produce longwave cooling of the same order as TLC inside the TC main convective region, and up to one order of magnitude smaller outside. Considering that clouds also produce shortwave warming, it is suggested that cloud radiative effects inside and outside TCs only explain modest amounts of TLC while other processes must provide the remaining cooling.

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 Arctic Oscillation (AO) describes a seesaw pattern of variations in atmospheric mass over the polar cap. It is by now well established that the AO pattern is in part determined by the state of the stratosphere. In particular, sudden stratospheric warmings (SSWs) are known to nudge the tropospheric circulation toward a more negative phase of the AO, which is associated with a more equatorward-shifted jet and enhanced likelihood for blocking and cold air outbreaks in mid-latitudes. SSWs are also thought to contribute to the occurrence of extreme AO events. However, statistically robust results about such extremes are difficult to obtain from observations or meteorological (re-)analyses due to the limited sample size of SSW events in the observational record (roughly six SSWs per decade). Here we exploit a large set of extended-range ensemble forecasts within the subseasonal-to-seasonal (S2S) framework to obtain an improved characterization of the modulation of AO extremes due to stratosphere–troposphere coupling. Specifically, we greatly boost the sample size of stratospheric events by using potential SSWs (p-SSWs), i.e., SSWs that are predicted to occur in individual forecast ensemble members regardless of whether they actually occurred in the real atmosphere. For example, the S2S ensemble of the European Centre for Medium-Range Weather Forecasts gives us a total of 6101 p-SSW events for the period 1997–2021. A standard lag-composite analysis around these p-SSWs validates our approach; i.e., the associated composite evolution of stratosphere–troposphere coupling matches the known evolution based on reanalysis data around real SSW events. Our statistical analyses further reveal that following p-SSWs, relative to climatology, (1) persistently negative AO states (>1 week duration) are 16 % more likely; (2) the likelihood for extremely negative AO states (<-3σ) is enhanced by about 40 %–80 %, while that for extremely positive AO states (>+3σ) is reduced to almost zero; (3) approximately 50 % of extremely negative AO states that follow SSWs may be attributable to the SSW, whereas about one-quarter of all extremely negative AO states during winter may be attributable to SSWs. A corresponding analysis relative to strong stratospheric vortex events reveals similar insights into the stratospheric modulation of positive AO extremes. However, conclusions in terms of causality remain difficult, in part due to unconsidered confounding factors.

The connection between the polar stratospheric vortex and the vertical component of the Eliassen-Palm flux in the lower stratosphere and upper troposphere is examined in model level data from the ERA-5 reanalysis. The particular focus of this work is on the conditions that lead to upward wave propagation between the tropopause and the bottom of the vortex near 100 hPa. The ability of four different versions of the index of refraction to capture this wave propagation are evaluated. The original Charney and Drazin index of refraction includes terms ignored by Matsuno that are shown to be critical for understanding upward wave propagation just above the tropopause in both the climatology and during extreme heat flux events. By adding these terms to the Matsuno index of refraction, it is possible to construct a useful tool that describes wave flux immediately above the tropopause and at the same time also describes the role of meridional variations within the stratosphere. It is shown that a stronger tropopause inversion layer tends to restrict upward wave propagation. It is also shown that while only 38% of extreme wave-1 Eliassen-Palm flux vertical component (Fz) at 100hPa events are preceded by extreme Fz at 300hPa, there are almost no extreme events at 100hPa in which the anomaly at 300hPa is of opposite sign or very weak. Overall, wave propagation near the tropopause is sensitive to vertical gradients in buoyancy frequency, and these vertical gradients may not be accurately captured in models or reanalysis products especially with lower vertical resolutions.

Variations in the width and strength of the Hadley cells are associated with many radiative, thermodynamic, and dynamical forcings. The physical mechanisms driving these responses remain unclear, in part because of the interactive nature of eddy‐mean flow adjustment. Here, a modeling framework is developed which separates the mean flow and time‐mean eddy flow in a gray radiation general circulation model with simple representations of ocean heat transport and ozone. In the absence of eddies, with moist convection and weak numerical damping, the Hadley cell is confined to the upper troposphere and has a vanishingly small poleward momentum flux. Eddies allow the cell to extend down to the surface, double its heat transport, and flux momentum poleward, the latter two being basic consequences of a deepening of the circulation. Because of convection and damping—which mimics, in part, the effect of eddy stresses—previous work may have underestimated the impact of eddies on earth's circulation. Quasigeostrophic eddy fluxes are sufficient to produce Hadley and Ferrel cells, but with a substantially greater Hadley cell strength than when all eddy impacts are considered, including eddy fluxes of moisture, mass, and momentum and eddy impacts on surface fluxes and clouds.

Previous studies have documented a poleward shift in the subsiding branches of Earth’s Hadley circulation since 1979 but have disagreed on the causes of these observed changes and the ability of global climate models to capture them. This synthesis paper reexamines a number of contradictory claims in the past literature and finds that the tropical expansion indicated by modern reanalyses is within the bounds of models’ historical simulations for the period 1979–2005. Earlier conclusions that models were underestimating the observed trends relied on defining the Hadley circulation using the mass streamfunction from older reanalyses. The recent observed tropical expansion has similar magnitudes in the annual mean in the Northern Hemisphere (NH) and Southern Hemisphere (SH), but models suggest that the factors driving the expansion differ between the hemispheres. In the SH, increasing greenhouse gases (GHGs) and stratospheric ozone depletion contributed to tropical expansion over the late twentieth century, and if GHGs continue increasing, the SH tropical edge is projected to shift further poleward over the twenty-first century, even as stratospheric ozone concentrations recover. In the NH, the contribution of GHGs to tropical expansion is much smaller and will remain difficult to detect in a background of large natural variability, even by the end of the twenty-first century. To explain similar recent tropical expansion rates in the two hemispheres, natural variability must be taken into account. Recent coupled atmosphere–ocean variability, including the Pacific decadal oscillation, has contributed to tropical expansion. However, in models forced with observed sea surface temperatures, tropical expansion rates still vary widely because of internal atmospheric variability.

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.

Planetary waves and gravity waves are the key drivers of middle atmospheric circulation and variability. While planetary waves are well resolved in climate models, inaccuracies in representation of gravity waves in climate models persist. Inaccuracies in representation of gravity waves limit our understanding of the planetary wave-gravity wave interactions that can be crucial during the Antarctic polar vortex breakdown. Moreover, "missing" gravity wave drag around 60oS in the upper stratosphere is considered to be responsible for the "cold-pole" bias in comprehensive climate models that employ parameterizations to appproximately represent the gravity wave drag.We illustrate the strength of the high-resolution ERA-5 reanalysis in resolving a broad spectrum of gravity waves in southern hemisphere midlatitudes and to estimate their contribution to the momentum budget around 60oS. We find that most of the resolved mountain waves excited over the Andes and Antarctic peninsula propagate away from their source and deposit momentum around 60oS over the Southern Ocean. Further, a composite analysis around 60oS during the vortex breakdown period using ERA-5 reveals considerably large fractional contribution of resolved + parameterized GWD towards the vortex deceleration. Upto 30 days prior to the breakdown, a balance between the Coriolis acceleration and the planetary wave deceleration provides a weak net deceleration of the mean winds, following which, they provide a net acceleration of the mean winds. The gravity waves, however, provide a steady deceleration of the mean winds throughout the breakdown period. The resolved drag in ERA-5 accounts for as much as one-fourth of the zonal wind deceleration at 60oS and 10 hPa, while the parameterized drag in ERA-5 accounts for more than one-half of the zonal wind deceleration. The findings establish the crucial role of gravity waves in wintertime stratospheric circulation and opens avenues for further stratospheric gravity wave analysis using ERA-5.

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.

A pronounced signature of stratosphere–troposphere coupling is a robust negative anomaly in the surface northern annular mode (NAM) following sudden stratospheric warming (SSW) events, consistent with an equatorward shift in the tropospheric jet. It has previously been pointed out that tropospheric synoptic-scale eddy feedbacks, mainly induced by anomalies in the lowermost extratropical stratosphere, play an important role in creating this surface NAM signal. Here, we use the basic set-up of idealised baroclinic life cycles to investigate the influence of stratospheric conditions on the behaviour of tropospheric synoptic-scale eddies. Particular attention is given to the enhancement of the tropospheric eddy response by surface friction and the sensitivity to wind anomalies in the lower stratosphere. We find systems that include a tropospheric jet only (modelling post-SSW conditions) to be characterised by an equatorward shift in the tropospheric jet in the final state of the life cycle, relative to systems that include a representation of the polar vortex (mimicking more undisturbed stratospheric wintertime conditions), consistent with the observed NAM response after SSWs. The corresponding relative surface NAM signal is increased if the system includes surface friction, presumably due to a direct coupling of the eddy field at tropopause level to the surface winds. We further show that the jet shift signal observed in our experiments is mainly caused by changes in the zonal wind structure of the lowermost stratosphere, while changes in the wind structure of the middle and upper stratosphere have almost no influence.

Atmospheric planetary waves play a fundamental role in driving stratospheric dynamics, including sudden stratospheric warming (SSW) events. It is well established that the bulk of the planetary wave activity originates near the surface. However, recent studies have pointed to a planetary wave source near the tropopause that may play an important role in the development of SSWs. Here we analyze the dynamical origin of this wave source and its impact on stratosphere–troposphere coupling, using an idealized model and a quasi-reanalysis. It is shown that the tropopause-level planetary wave source is associated with nonlinear wave–wave interactions, but it can also manifest as an apparent wave source due to transient wave decay. The resulting planetary waves may then propagate deep into the stratosphere, where they dissipate and may help to force SSWs. Our results indicate that SSWs preceded by both the tropopause and the surface wave-source events tend to be followed by a weakened tropospheric zonal flow several weeks later. However, while in the case of a preceding surface wave-source event this downward impact is found mainly poleward of 60∘ N, it appears to be the strongest between 40 and 60∘ N for SSWs preceded by tropopause wave-source events. This suggests that tropopause wave-source events could potentially serve as an additional predictor of not only SSWs but also their downward impact as well.

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.

The dry dynamical core represents one of the simplest possible numerical models for studying the response of the extratropical circulation to climate change. In the model, the circulation is forced by relaxing temperature to a notional “equilibrium” using linear damping. The linear damping coefficient plays an essential role in governing the structure of the circulation. But despite decades of research with the dry dynamical core, the role of the damping coefficient in governing the circulation has received relatively little scrutiny.In this work, we systematically vary the damping coefficient in a dry dynamical core in order to understand how the amplitude of the damping influences extratropical dynamics. Critically, we prove that the local climate feedback parameter is proportional to the damping coefficient – that is, the damping timescale is a measure of climate sensitivity for the dry atmosphere. The key finding is that the steady-state extratropical circulation responds to changes in this climate sensitivity.Longer damping timescales (i.e. higher climate sensitivities) lead to a less dynamically active extratropical circulation, stronger and more persistent annular modes, and equatorward shifts in the jet. When perturbed with climate change-like forcings, changing the damping timescale can also change the dynamical response to the forcing. We argue that understanding the response of the circulation to climate change is critically dependent on understanding its climate sensitivity, and consider how climate sensitivity might be inferred from its effect on the circulation in the dry model and more complex general circulation models.

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.

The importance of understanding the dynamical coupling of troposphere and stratosphere to make accurate weather and climate predictions is well-known. Over the past years and decades various signatures of such acoupling have been discovered. A very robust result, for example, seems to be an equatorward shift of the tropospheric eddy driven jet following sudden stratospheric warming events, where the westerly winds of the stratospheric polar vortex weaken or even reverse. However, many aspects of this fundamental coupling are still not fully understood and research on how the state of the stratosphere can influence the tropospheric circulation and what dynamical processes are involved is still ongoing.An important such process arises due to the interaction of a sharp, localised maximum in potential vorticity gradient near the tropopause with baroclinic eddies in the troposphere. Here, we analyse the sensitivity of baroclinic wave development and evolution to changes of various basic state characteristics, by performing a series of idealised baroclinic eddy life cycle experiments. Special attention is paid to sensitivities associated with the dynamical state of the stratosphere. We find that the final (steady) state of the life cycle simulations corresponds to an equatorward shift of the tropospheric jet in cases where the initial conditions do not include a stratospheric polar vortex (such as following sudden warming events) compared to those that do. These results further support the idea that the stratospheric state can strongly influence tropospheric dynamics and, in particular, highlight the robustness of the jet shift response following sudden warmings, that can be seen in a range of observations and numerical model experiments.

Recent work has highlighted that not all periods with anomalous heat flux at 100hPa were preceded by anomalous heat flux in the troposphere (Birner and Alberts 2017; White et al 2019; Camara et al 2019), and the goal of this work is to understand the factors that govern the efficiency of upward wave propagation near the tropopause. The index of refraction of Matsuno (1970) has been used to offer guidance on the direction of wave propagation within the stratosphere. Specifically, waves are preferentially refracted towards regions with a more positive index of refraction and ducted away from regions in which the index of refraction is more negative. However, the index of refraction was derived under the assumption that buoyancy frequency is constant at all height levels, which is clearly not true near the tropopause. This assumption allowed Matsuno to ignore certain height dependent buoyancy frequency terms, and here we explore the impact of these terms near the tropopause.Using the dataset of the European Center for Medium-Range Weather Forecasts Reanalysis version 5 (ERA5) we defined 'transmitting' composites consisting of more efficient upward propagation events between 300hPa and 100hPa. Similarly, periods of less efficient upward propagation events between 300hPa and 100hPa are composited as 'decaying' events. We computed the index of refraction profile using a median, percentage of negative days and the trimmed mean (Wilks 2011), and also consider the terms neglected by Matsuno. We find that the index of refraction can account for the difference between the decaying and transmitting composite.

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.