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The impact of ozone-depleting substances on global lower-stratospheric temperature trends is widely recognized. In the tropics, however, understanding lower-stratospheric temperature trends has proven more challenging. While the tropical lower-stratospheric cooling observed from 1979 to 1997 has been linked to tropical ozone decreases, those ozone trends cannot be of chemical origin, as active chlorine is not abundant in the tropical lower stratosphere. The 1979–97 tropical ozone trends are believed to originate from enhanced upwelling, which, it is often stated, would be driven by increasing concentrations of well-mixed greenhouse gases. This study, using simple arguments based on observational evidence after 1997, combined with model integrations with incrementally added single forcings, argues that trends in ozone-depleting substances, not well-mixed greenhouse gases, have been the primary driver of temperature and ozone trends in the tropical lower stratosphere until 1997, and this has occurred because ozone-depleting substances are key drivers of tropical upwelling and, more generally, of the entire Brewer–Dobson circulation.

The recent hiatus in global-mean surface temperature warming was characterized by a Eurasian winter cooling trend, and the cause(s) for this cooling is unclear. Here we show that the observed hiatus in Eurasian warming was associated with a recent trend toward weakened stratospheric polar vortices. Specifically, by calculating the change in Eurasian surface air temperature associated with a given vortex weakening, we demonstrate that the recent trend toward weakened polar vortices reduced the anticipated Eurasian warming due to increasing greenhouse gas concentrations. Those model integrations whose stratospheric vortex evolution most closely matches that in reanalysis data also simulate a hiatus. While it is unclear whether the recent weakening of the midwinter stratospheric polar vortex was forced, a properly configured model can simulate substantial deviations of the polar vortex on decadal timescales and hence such hiatus events, implying that similar hiatus events may recur even as greenhouse gas concentrations rise.

A series of simulations using the NASA Goddard Earth Observing System Chemistry Climate Model are analyzed in order to assess changes in the Brewer–Dobson Circulation (BDC) over the past 55 years. When trends are computed over the past 55 years, the BDC accelerates throughout the stratosphere, consistent with previous modeling results. However, over the second half of the simulations (i.e., since the late 1980s), the model simulates structural changes in the BDC as the temporal evolution of the BDC varies between regions in the stratosphere. In the mid-stratosphere in the midlatitude Northern Hemisphere, the BDC does not accelerate in the ensemble mean of our simulations despite increases in greenhouse gas concentrations and warming sea surface temperatures, and it even decelerates in one ensemble member. This deceleration is reminiscent of changes inferred from satellite instruments and in situ measurements. In contrast, the BDC in the lower stratosphere continues to accelerate. The main forcing agents for the recent slowdown in the mid-stratosphere appear to be declining ozone-depleting substance (ODS) concentrations and the timing of volcanic eruptions. Changes in both mean age of air and the tropical upwelling of the residual circulation indicate a lack of recent acceleration. We therefore clarify that the statement that is often made that climate models simulate a decreasing age throughout the stratosphere only applies over long time periods and is not necessarily the case for the past 25 years, when most tracer measurements were taken.

Satellite instruments show a cooling of global stratospheric temperatures over the whole data record (1979–2014). This cooling is not linear and includes two descending steps in the early 1980s and mid-1990s. The 1979–1995 period is characterized by increasing concentrations of ozone-depleting substances (ODSs) and by the two major volcanic eruptions of El Chichon (1982) and Mount Pinatubo (1991). The 1995–present period is characterized by decreasing ODS concentrations and by the absence of major volcanic eruptions. Greenhouse gas (GHG) concentrations increase over the whole time period. In order to isolate the roles of different forcing agents in the global stratospheric temperature changes, we performed a set of simulations using the NASA Goddard Earth Observing System Chemistry-Climate Model with prescribed sea surface temperatures. We find that in our model simulations the cooling of the stratosphere from 1979 to present is mostly driven by changes in GHG concentrations in the middle and upper stratosphere and by GHG and ODS changes in the lower stratosphere. While the cooling trend caused by increasing GHGs is roughly constant over the satellite era, changing ODS concentrations cause a significant stratospheric cooling only up to the mid-1990s, when they start to decrease because of the implementation of the Montreal Protocol. Sporadic volcanic events and the solar cycle have a distinct signature in the time series of stratospheric temperature anomalies but do not play a statistically significant role in the long-term trends from 1979 to 2014. Several factors combine to produce the step-like behavior in the stratospheric temperatures: in the lower stratosphere, the flattening starting in the mid-1990s is due to the decrease in ozone-depleting substances; Mount Pinatubo and the solar cycle cause the abrupt steps through the aerosol-associated warming and the volcanically induced ozone depletion. In the middle and upper stratosphere, changes in solar irradiance are largely responsible for the step-like behavior of global temperature anomalies, together with volcanically induced ozone depletion and water vapor increases in the post-Pinatubo years.

Using a global climate model (Hadley Centre Global Environment Model version 2-Carbon Cycle Stratosphere ) with a well-resolved stratosphere, we test the sensitivity of volcanic aerosol plume dispersion to meteorological conditions by simulating 1 day Mount Pinatubo-like eruptions on 10 consecutive days. The dispersion of the volcanic aerosol is found to be highly sensitive to the ambient meteorology for low-altitude eruptions (16–18 km), with this variability related to anomalous anticyclonic activity along the subtropical jet, which affects the permeability of the tropical pipe and controls the amount of aerosol that is retained by the tropical reservoir. Conversely, a high-altitude eruption scenario (19–29 km) exhibits low meteorological variability. Overcoming day-to-day meteorological variability by spreading the emission over 10 days is shown to produce insufficient radiative heating to loft the aerosol into the stratospheric tropical aerosol reservoir for the low eruption scenario. This results in limited penetration of aerosol into the southern hemisphere (SH) in contrast to the SH transport observed after the Pinatubo eruption. Our results have direct implications for the accurate simulation of past/future volcanic eruptions and volcanically forced climate changes, such as Intertropical Convergence Zone displacement.

The authors present a new method to diagnose the middle-atmosphere climate sensitivity by extending the climate feedback–response analysis method (CFRAM) for the coupled atmosphere–surface system to the middle atmosphere. The middle-atmosphere CFRAM (MCFRAM) is built on the atmospheric energy equation per unit mass with radiative heating and cooling rates as its major thermal energy sources. MCFRAM preserves CFRAM’s unique feature of additivity, such that partial temperature changes due to variations in external forcing and feedback processes can be added to give a total temperature change for direct comparison with the observed temperature change. In addition, MCFRAM establishes a physical relationship of radiative damping between the energy perturbations associated with various feedback processes and temperature perturbations associated with thermal responses. In this study, MCFRAM is applied to both observations and model output fields to diagnose the middle-atmosphere climate sensitivity. The authors found that the largest component of the middle-atmosphere temperature response to the 11-yr solar cycle (solar maximum vs solar minimum) is the partial temperature change due to the variation of the solar flux. Increasing CO2 cools the middle atmosphere, whereas the partial temperature change due to changes in O3 can be either positive or negative. The application of MCFRAM to model dynamical fields reconfirms the advantage of introducing the residual circulation to characterize middle-atmosphere dynamics in terms of the partial temperature changes. The radiatively driven globally averaged partial temperature change is approximately equal to the observed temperature change, ranging from −0.5 K near 25 km to −1.0 K near 70 km between solar maximum and solar minimum.

Sulfate geoengineering has been proposed as an affordable and climate-effective means to temporarily offset the warming produced by the increase of well-mixed greenhouse gases (WMGHGs). This technique would likely have to be applied while and after global intergovernmental measures on emissions of WMGHGs are implemented in order to achieve surface temperature stabilization. The direct radiative effects of sulfur injection in the tropical lower stratosphere can be summarized as increasing shortwave scattering with consequent tropospheric cooling and increasing longwave absorption with stratospheric warming. Indirect radiative effects are related to induced changes in the ozone distribution; stratospheric water vapor abundance,;formation and size of upper-tropospheric cirrus ice particles; and lifetime of long-lived species, namely CH4 in connection with OH changes through several photochemical mechanisms. Direct and indirect effects of sulfate geoengineering both concur to determine the atmospheric response. A review of previous studies on these effects is presented here, with an outline of the important factors that control the amount of sulfur dioxide to be injected in an eventual realization of the experiment. However, we need to take into account that atmospheric models used for these studies have shown a wide range of climate sensitivity and differences in the response to stratospheric volcanic aerosols. In addition, large uncertainties exist in the estimate of some of these aerosol effects.

This paper examines the impact of geoengineering via stratospheric sulfate aerosol on the quasi-biennial oscillation (QBO) using the NASA Goddard Earth Observing System (GEOS-5) Chemistry Climate Model. We performed four 30-year simulations with a continuous injection of sulfur dioxide on the equator at 0 degree longitude. The four simulations differ by the amount of sulfur dioxide injected (5Tg per year and 2.5 Tg per year) and the altitude of the injection (16km-25km and 22km-25km). We find that such an injection dramatically alters the quasi-biennial oscillation, prolonging the phase of easterly shear with respect to the control simulation. In the case of maximum perturbation, i.e. highest stratospheric aerosol burden, the lower tropical stratosphere is locked into a permanent westerly QBO phase. This locked QBO westerly phase is caused by the increased aerosol heating and associated warming in the tropical lower stratosphere.

Geoengineering with stratospheric sulfate aerosols has been proposed as a means of temporarily cooling the planet, alleviating some of the side effects of anthropogenic CO2 emissions. However, one of the known side effects of stratospheric injections of sulfate aerosols under present-day conditions is a general decrease in ozone concentrations. Here we present the results from two general circulation models and two coupled chemistry-climate models within the experiments G3 and G4 of the Geoengineering Model Intercomparison Project. On average, the models simulate in G4 an increase in sulfate aerosol surface area density similar to conditions a year after the Mount Pinatubo eruption and a decrease in globally averaged ozone by 1.1−2.1 DU (Dobson unit, 1 DU = 0.001 atm cm) during the central decade of the experiment (2040–2049). Enhanced heterogeneous chemistry on sulfate aerosols leads to an ozone increase in low and middle latitudes, whereas enhanced heterogeneous reactions in polar regions and increased tropical upwelling lead to a reduction of stratospheric ozone. The increase in UV-B radiation at the surface due to ozone depletion is offset by the screening due to the aerosols in the tropics and midlatitudes, while in polar regions the UV-B radiation is increased by 5% on average, with 12% peak increases during springtime. The contribution of ozone changes to the tropopause radiative forcing during 2040–2049 is found to be less than −0.1 W m−2. After 2050, because of decreasing ClOx concentrations, the suppression of the NOx cycle becomes more important than destruction of ozone by ClOx, causing an increase in total stratospheric ozone.

Observations have shown that the mass of nitrogen dioxide decreased at both southern and northern midlatitudes in the year following the eruption of Mt. Pinatubo, indicating that the volcanic aerosol had enhanced nitrogen dioxide depletion via heterogeneous chemistry. In contrast, the observed ozone response showed a northern midlatitude decrease and a small southern midlatitude increase. Previous simulations that included an enhancement of heterogeneous chemistry by the volcanic aerosol but no other effect of this aerosol produce ozone decreases in both hemispheres, contrary to observations. The authors’ simulations show that the heating due to the volcanic aerosol enhanced both the tropical upwelling and Southern Hemisphere extratropical downwelling. This enhanced extratropical downwelling, combined with the time of the eruption relative to the phase of the Brewer–Dobson circulation, increased Southern Hemisphere ozone via advection, counteracting the ozone depletion due to heterogeneous chemistry on the Pinatubo aerosol.

Black carbon (BC) and mineral dust are among the most abundant insoluble aerosol components in the atmosphere. When released, most BC and dust particles are externally mixed with other aerosol species. Through coagulation with particles containing soluble material and condensation of gases, the externally mixed particles may obtain a liquid coating and be transferred into an internal mixture. The mixing state of BC and dust aerosol particles influences their radiative and hygroscopic properties, as well as their ability of forming ice crystals. We introduce the new aerosol microphysics submodel MADE-in, implemented within the ECHAM/MESSy Atmospheric Chemistry global model (EMAC). MADE-in is able to track mass and number concentrations of BC and dust particles in their different mixing states, as well as particles free of BC and dust. MADE-in describes these three classes of particles through a superposition of seven log-normally distributed modes, and predicts the evolution of their size distribution and chemical composition. Six out of the seven modes are mutually interacting, allowing for the transfer of mass and number among them. Separate modes for the different mixing states of BC and dust particles in EMAC/MADE-in allow for explicit simulations of the relevant aging processes, i.e. condensation, coagulation and cloud processing. EMAC/MADE-in has been evaluated with surface and airborne measurements and mostly performs well both in the planetary boundary layer and in the upper troposphere and lowermost stratosphere.

[1] We use the GEOS-5 general circulation model to simulate the transport of the volcanic cloud from an eruption similar to the 1991 eruption of Mount Pinatubo. The simulated aerosol optical thickness and transport of the volcanic cloud are in good agreement with observations of the actual Pinatubo eruption from the Stratospheric Aerosol and Gas Experiment II (SAGE II) and the Advanced Very High Resolution Radiometer (AVHRR) and with vertical profiles of sulfur dioxide observed by the Microwave Limb Sounder (MLS). We tested the importance of initial conditions corresponding to the specific meteorological situation at the time of the eruption by comparing results when GEOS-5 is initialized using Modern Era Retrospective Analyses for Research and Applications (MERRA) reanalysis fields with results when it is initialized from an existing model run. We found no significant difference in the transport of the cloud. We show how the inclusion of the interaction between volcanic sulfate aerosol and radiation is essential for a reliable simulation of the transport of the volcanic cloud. The absorption of longwave radiation by the volcanic sulfate largely induces the rising of the volcanic cloud up to the middle stratosphere and the divergent motion from the latitude of the eruption to the tropics. Our simulations indicate that the cloud is transported to the Northern Hemisphere through a lower stratospheric pathway and to middle and high latitudes of the Southern Hemisphere through a middle stratospheric pathway, centered at about 30 hPa. The direction of the middle stratospheric pathway depends on the season of the eruption.

Global aerosol models are used to study the distribution and properties of atmospheric aerosol particles as well as their effects on clouds, atmospheric chemistry, radiation, and climate. The present article provides an overview of the basic concepts of global atmospheric aerosol modeling and shows some examples from a global aerosol simulation. Particular emphasis is placed on the simulation of aerosol particles and their effects within global climate models.

Black carbon (BC) and mineral dust are among the dominant atmospheric ice nuclei, i.e. aerosol particles that can initiate heterogeneous nucleation of ice crystals. When released, most BC and dust particles are externally mixed with other aerosol compounds. Through coagulation with particles containing soluble material and condensation of gases, externally mixed particles may obtain a coating and be transferred into an internal mixture. The mixing state of BC and dust aerosol particles influences their radiative and hygroscopic properties, as well as their ability of building ice crystals. We introduce the new aerosol microphysics submodel MADE-IN, implemented within the ECHAM/MESSy Atmospheric Chemistry global model (EMAC). MADE-IN is able to track separately mass and number concentrations of BC and dust particles in their different mixing states, as well as particles free of BC and dust. MADE-IN describes these three classes of particles through a superposition of seven log-normally distributed modes, and predicts the evolution of their size distribution and chemical composition. Six out of the seven modes are mutually interacting, allowing for the transfer of mass and number among them. Separate modes for the different mixing states of BC and dust particles in EMAC/MADE-IN allow for explicit simulations of the relevant aging processes, i.e. condensation, coagulation and cloud processing. EMAC/MADE-IN has been evaluated with surface and airborne measurements and performs well both in the planetary boundary layer and in the upper troposphere and lowermost stratosphere. Such a model represents a highly appropriate tool for the study of the concentration and composition of potential atmospheric ice nuclei.