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Variability in the stratosphere, especially extreme events such as Sudden Stratospheric Warmings (SSWs), can impact surface weather. Understanding stratospheric prediction uncertainty is therefore crucial for skillful surface weather forecasts on weekly to monthly timescales. Using ECMWF subseasonal hindcasts, this study finds that stratospheric uncertainty is most strongly linked to tropospheric uncertainty over the North Pacific and Northern Europe, regions that can modulate but also respond to stratospheric variability, suggesting a two-way propagation of uncertainty. A case study of the 2018 SSW event shows an initial poleward and upward propagation of uncertainty from tropical convection, followed by a downward propagation where ensemble members that accurately predict the SSW also better at predicting its downward impacts. These findings highlight the locations in the troposphere that are linked to stratospheric uncertainty and suggest that improved model representation of tropospheric mechanisms linked to polar vortex variability could enhance both stratospheric and extratropical surface prediction. [ABSTRACT FROM AUTHOR]

Solar radiation modification by a sustained deliberate source of SO2 into the stratosphere (strat-SRM) has been proposed as an option for climate intervention. Global interactive aerosol–chemistry–climate models are often used to investigate the potential cooling efficiencies and associated side effects of hypothesized strat-SRM scenarios. A recent model intercomparison study for composition–climate models with interactive stratospheric aerosol suggests that the modeled climate response to a particular assumed injection strategy depends on the type of aerosol microphysical scheme used (e.g., modal or sectional representation) alongside host model resolution and transport. Compared to short-duration volcanic SO2 emissions, the continuous SO2 injections in strat-SRM scenarios may pose a greater challenge to the numerical implementation of microphysical processes such as nucleation, condensation, and coagulation. This study explores how changing the time steps and sequencing of microphysical processes in the sectional aerosol–chemistry–climate model SOCOL-AERv2 (40 mass bins) affects model-predicted climate and ozone layer impacts considering strat-SRM by SO2 injections of 5 and 25 Tg(S) yr −1 at 20 km altitude between 30° S and 30° N. The model experiments consider the year 2040 to be the boundary conditions for ozone-depleting substances and greenhouse gases (GHGs). We focus on the length of the microphysical time step and the call sequence of nucleation and condensation, the two competing sink processes for gaseous H2SO4. Under stratospheric background conditions, we find no effect of the microphysical setup on the simulated aerosol properties. However, at the high sulfur loadings reached in the scenarios injecting 25 Tg(S) yr −1 of SO2 with a default microphysical time step of 6 min, changing the call sequence from the default "condensation first" to "nucleation first" leads to a massive increase in the number densities of particles in the nucleation mode (R<0.01 µm) and a small decrease in coarse-mode particles (R>1 µm). As expected, the influence of the call sequence becomes negligible when the microphysical time step is reduced to a few seconds, with the model solutions converging to a size distribution with a pronounced nucleation mode. While the main features and spatial patterns of climate forcing by SO 2 injections are not strongly affected by the microphysical configuration, the absolute numbers vary considerably. For the extreme injection with 25 Tg(S) yr −1 , the simulated net global radiative forcing ranges from - 2.3 to - 5.3 Wm-2 , depending on the microphysical configuration. Nucleation first shifts the size distribution towards radii better suited for solar scattering (0.3 µm

Stratospheric ozone is expected to recover by the mid-century due to the success of the Montreal Protocol in regulating the emission of ozone-depleting substances (ODSs). In the Arctic, ozone abundances are projected to surpass historical levels due to the combined effect of decreasing ODSs and elevated greenhouse gases (GHGs). While long-term changes in stratospheric ozone have been shown to be a major driver of future surface climate in the Southern Hemisphere during summertime, the dynamical and climatic impacts of elevated ozone levels in the Arctic have not been investigated. In this study, we use two chemistry climate models (the SOlar Climate Ozone Links – Max Planck Ocean Model (SOCOL-MPIOM) and the Community Earth System Model – Whole Atmosphere Community Climate Model (CESM-WACCM)) to assess the climatic impacts of future changes in Arctic ozone on stratospheric dynamics and surface climate in the Northern Hemisphere (NH) during the 21st century. Under the high-emission scenario (RCP8.5) examined in this work, Arctic ozone returns to pre-industrial levels by the middle of the century. Thereby, the increase in Arctic ozone in this scenario warms the lower Arctic stratosphere; reduces the strength of the polar vortex, advancing its breakdown; and weakens the Brewer–Dobson circulation. The ozone-induced changes in springtime generally oppose the effects of GHGs on the polar vortex. In the troposphere, future changes in Arctic ozone induce a negative phase of the Arctic Oscillation, pushing the jet equatorward over the North Atlantic. These impacts of future ozone changes on NH surface climate are smaller than the effects of GHGs, but they are remarkably robust among the two models employed in this study, canceling out a portion of the GHG effects (up to 20 % over the Arctic). In the stratosphere, Arctic ozone changes cancel out a much larger fraction of the GHG-induced signal (up to 50 %–100 %), resulting in no overall change in the projected springtime stratospheric northern annular mode and a reduction in the GHG-induced delay of vortex breakdown of around 15 d. Taken together, our results indicate that future changes in Arctic ozone actively shape the projected changes in the stratospheric circulation and their coupling to the troposphere, thereby playing an important and previously unrecognized role as a driver of the large-scale atmospheric circulation response to climate change. [ABSTRACT FROM AUTHOR]

Solar radiation management as a sustained deliberate source of SO2 into the stratosphere (strat-SRM) has been proposed as an option for climate intervention. Global interactive aerosol-chemistry-climate models are often used to investigate the potential cooling efficiencies and side effects of hypothesised strat-SRM scenarios. A recent strat-SRM model intercomparison study for composition-climate models with interactive stratospheric aerosol suggests that the modelled climate response to a particular assumed injection strategy, depends on the type of aerosol microphysical scheme used (e.g., modal or sectional representation), alongside also host model resolution and transport. Compared to short-duration volcanic SO2 emission, the continuous SO2 injections in strat-SRM scenarios may pose a greater challenge to the numerical implementation of of microphysical processes such as nucleation, condensation, and coagulation. This study explores how changing the timesteps and sequencing of microphysical processes in the sectional aerosol-chemistry-climate model SOCOL-AERv2 (40 size bins) affect model predicted climate and ozone layer impacts considering strat-SRM SO2 injections of of 5 and 25 Tg(S) yr-1 at 20 km altitude between 30° S and 30° N. The model experiments consider year 2040 boundary conditions for ozone depleting substances and green house gases. We focus on the length of the microphysical timestep and the call sequence of nucleation and condensation, the two competing sink processes for gaseous H2SO4. Under stratospheric background conditions, we find no effect of the microphysical setup on the simulated aerosol properties. However, at the high sulfur loadings reached in the scenarios injecting 25 Mt/yr of sulfur with a default microphysical timesetp of 6 min, changing the call sequence from the default 'condensation first' to 'nucleation first' leads to a massive increase in the number densities of particles in the nucleation mode (R < 0.01 μm) and a small decrease in coarse mode particles (R > 1 μm). As expected, the influence of the call sequence becomes negligible when the microphysical timestep is reduced to a few seconds, with the model solutions converging to a size distribution with a pronounced nucleation mode. While the main features and spatial patterns of climate forcing by SO2 injections are not strongly affected by the microphysical configuration, the absolute numbers vary considerably. For the extreme injection with 25 Tg(S) yr-1, the simulated net global radiative forcing ranges from -2.3 W m-2 to -5.3 W m-2, depending on the microphysical configuration. "Nucleation first" shifts the size distribution towards radii better suited for solar scattering (0.3 μm < R < 0.4 μm), enhancing the intervention efficiency. The size-distribution shift however generates more ultra-fine aerosol particles, increasing the surface area density, resulting in 10 DU less ozone (about 3 % of total column) in the northern midlatitudes and 20 DU less ozone (6 %) over the polar caps, compared to the 'condensation first' approach. Our results suggest that a reasonably short microphysical time step of 2 minutes or less must be applied to accurately capture the magnitude of the H2SO2 supersaturation resulting from SO2 injection scenarios or volcanic eruptions. Taken together these results underscore how structural aspects of model representation of aerosol microphysical processes become important under conditions of elevated stratospheric sulfur in determining atmospheric chemistry and climate impacts. [ABSTRACT FROM AUTHOR]

Sulfate aerosol in the stratosphere is an important climate driver, causing solar dimming in the years after major volcanic eruptions. Hence, a growing number of general circulation models are adapting interactive sulfur and aerosol schemes to improve the representation of relevant chemical processes and associated feedbacks. However, uncertainties of these schemes are not well constrained. Stratospheric sulfate is modulated by natural emissions of sulfur-containing species, including volcanic eruptive, and anthropogenic emissions. Model intercomparisons have examined the effects of volcanic eruptions, whereas the background conditions of the sulfur cycle have not been addressed in a global model intercomparison project. Assessing background conditions in global models allows us to identify model discrepancies as they are masked by large perturbations such as volcanic eruptions, yet may still matter in the aftermath of such a disturbance. Here, we analyze the atmospheric burden, seasonal cycle, and vertical and meridional distribution of the main sulfur species among nine global atmospheric aerosol models that are widely used in the stratospheric aerosol research community. We use observational and reanalysis data to evaluate model results. Overall, models agree that the three dominant sulfur species in terms of burdens (sulfate aerosol, OCS, and SO2) make up about 98 % of stratospheric sulfur and 95 % of tropospheric sulfur. However, models vary considerably in the partitioning between these species. Models agree that anthropogenic emission of SO2 strongly affects the sulfate aerosol burden in the Northern Hemispheric troposphere, while its importance is very uncertain in other regions. The total deposition of sulfur varies among models, deviating by a factor of two, but models agree that sulfate aerosol is the main form in which sulfur is deposited. Additionally, the partitioning between wet and dry deposition fluxes is highly model dependent. We investigate the areas of greatest variability in the sulfur species burdens and find that inter-model variability is low in the tropics and increases towards the poles. Seasonality in the southern hemisphere is depicted very similar among models. Differences are largest in the dynamically active northern hemispheric extratropical region, hence some of the differences could be attributed to the differences in the representation of the stratospheric circulation among underlying general circulation models. This study highlights that the differences in the atmospheric sulfur budget among the models arise from the representation of both chemical and dynamical processes, whose interplay complicates the bias attribution. Several problematic points identified for individual models are related to the specifics of the chemistry schemes, model resolution, and representation of cross-tropopause transport in the extratropics. Further model intercomparison research is needed focusing on the clarification of the reasons for biases, given also the importance of this topic for the stratospheric aerosol injection studies. [ABSTRACT FROM AUTHOR]

Stratospheric ozone is expected to recover by mid-century due to the success of the Montreal Protocol in regulating the emission of ozone-depleting substances (ODSs). In the Arctic, ozone abundances are projected to surpass historical levels due to the combined effect of decreasing ODSs and elevated greenhouse gases (GHGs). While ozone recovery has been shown to be a major driver of future surface climate in the Southern Hemisphere during summertime, the dynamical and climatic impacts of elevated ozone levels in the Arctic have not been investigated. In this study, we use two chemistry climate models (SOCOL-MPIOM and CESM-WACCM) to assess the climatic impacts of Arctic ozone recovery on stratospheric dynamics and surface climate in the Northern Hemisphere (NH) during the 21st century. Under the high-emission scenario (RCP8.5) examined in this work, Arctic ozone returns to pre-industrial levels by the middle of the century. Thereby, it warms the lower Arctic stratosphere, reduces the strength of the polar vortex, advancing its breakdown, and weakening the Brewer-Dobson circulation. In the troposphere, Arctic ozone recovery induces a negative phase of the Arctic Oscillation, pushing the jet equatorward over the Atlantic. These impacts of ozone recovery in the NH are smaller than the effects of GHGs, but they are remarkably robust among the two models employed in this study, cancelling out some of the GHG effects. Taken together, our results indicate that Arctic ozone recovery actively shapes the projected changes in the stratospheric circulation and their coupling to the troposphere, thereby playing an important and previously unrecognized role as driver of the large-scale atmospheric circulation response to climate change [ABSTRACT FROM AUTHOR]

The Geoengineering Model Intercomparison Project (GeoMIP) is a coordinating framework, started in 2010, that includes a series of standardized climate model experiments aimed at understanding the physical processes and projected impacts of solar geoengineering. Numerous experiments have been conducted, and numerous more have been proposed as "test-bed" experiments, spanning a variety of geoengineering techniques aimed at modifying the planetary radiation budget: stratospheric aerosol injection, marine cloud brightening, surface albedo modification, cirrus cloud thinning, and sunshade mirrors. To date, more than 100 studies have been published that used results from GeoMIP simulations. Here we provide a critical assessment of GeoMIP and its experiments. We discuss its successes and missed opportunities, for instance in terms of which experiments elicited more interest from the scientific community and which did not, and the potential reasons why that happened. We also discuss the knowledge that GeoMIP has contributed to the field of geoengineering research and climate science as a whole: what have we learned in terms of intermodel differences, robustness of the projected outcomes for specific geoengineering methods, and future areas of model development that would be necessary in the future? We also offer multiple examples of cases where GeoMIP experiments were fundamental for international assessments of climate change. Finally, we provide a series of recommendations, regarding both future experiments and more general activities, with the goal of continuously deepening our understanding of the effects of potential geoengineering approaches and reducing uncertainties in climate outcomes, important for assessing wider impacts on societies and ecosystems. In doing so, we refine the purpose of GeoMIP and outline a series of criteria whereby GeoMIP can best serve its participants, stakeholders, and the broader science community. [ABSTRACT FROM AUTHOR]

The Montreal Protocol and its amendments (MPA) have been a huge success in preserving the stratospheric ozone layer from being destroyed by unabated chlorofluorocarbons (CFCs) emissions. The phase out of CFCs has not only prevented serious impacts on our health and climate, but also avoided strong alterations of atmospheric circulation patterns. With the Earth System Model SOCOLv4, we study the dynamical and climatic impacts of a scenario with unabated CFC emissions by 2100, disentangling radiative and chemical (ozone-mediated) effects of CFCs. In the stratosphere, chemical effects of CFCs (i.e. the resulting ozone loss) are the main drivers of circulation changes, weakening wintertime polar vortices and speeding up the Brewer-Dobson circulation. These dynamical impacts during wintertime are due to low-latitude ozone depletion and resulting reduction of the equator-to-pole temperature gradient. In Southern Hemisphere (SH) summer, the vortex strengthens, similar due to the effects of the Antarctic ozone hole over the second half of the 20th century. Furthermore, the winter and spring vortex variability increases in the SH, whereas it decreases in summer and fall. This seasonal variation of wind speed in the stratosphere has regional implications on the tropospheric circulation modes. We find coherent changes in the troposphere, such as negative Southern Annular mode (SAM) and North Atlantic Oscillation (NAO) during seasons with a weaker vortex (winter and spring); the opposite occurs during seasons with stronger westerlies in the stratosphere (summer). In the troposphere, radiative heating by CFCs prevails throughout the year, shifting the SAM into a positive phase and canceling out the ozone-induced effects on the NAO. Furthermore, global warming is amplified by 1.9 K with regionally up to 12 K increase over Eastern Canada and Western Arctic. Our study sheds light into the adverse effects of a non-adherence to the MPA on the global atmospheric circulation, uncovering the roles of the underlying physical mechanisms. In so doing, our study emphasizes the importance of the MPA for Earth's climate, to avoid regional amplifications of negative climate impacts. [ABSTRACT FROM AUTHOR]