Tropical eruptions strong enough to inject into the stratosphere perturb the physical and the chemical states of the climate system for several years and longer . Although a range of gases are injected e.g., the global climate impacts stem from the aerosol produced from the injected sulfur dioxide (SO2). In the stratosphere volcanically injected SO2 is converted into sulfuric acid (H2SO4+H2O) aerosols that (i) reflect in the visible part of the solar spectrum, (ii) absorb terrestrial and solar infrared radiation, and (iii) provide surface for a large number of chemical reactions that alter the chemical composition of the stratosphere . The enhanced stratospheric aerosol increases the optical depth of the atmosphere, leading to a decrease in SW radiation in the troposphere and at the surface. Increased absorption of long wave and solar near-infrared radiation increases heating rates in the volcanic plume causing a pronounced warming of the tropical stratosphere. The perturbed vertical and meridional temperature gradients alter the stratospheric circulation and via interaction between the stratosphere and the troposphere affect surface climate. A prominent example for this mechanism is the winter warming pattern in the NH observed after several large tropical volcanic eruptions . Such surface temperature anomalies over Eurasia are related to a positive phase of the Arctic Oscillation, which is forced by interactions between the stratospheric polar vortex and tropospheric circulation patterns .

The overall impact of a tropical eruption on stratospheric ozone chemistry can be separated into the effect from (i) altered reaction rates due to change in temperature, (ii) enhanced heterogeneous chemistry from elevated sulfuric acid aerosol surface area density (SAD), (iii) the temperature and aerosol changes in modifying the occurrence and types of polar stratospheric clouds (PSCs), (iv) the composition changes induced by the dynamical perturbations in the stratosphere, and (v) changes in photolysis rates from the enhanced aerosol. The temperature change and the reactions on the heterogeneous aerosol surfaces mainly take place in the aerosol cloud. The heterogeneous conversion of nitrogen oxides (N2O5) into nitric acid (HNO3) slows down the NOx cycle of catalytic ozone destruction, with the effect of increasing ozone concentrations in the middle stratosphere . In the lower stratosphere, the Clx and HOx cycles are more important with the net chemical effect being ozone loss in the present-day atmosphere . The chemical ozone loss from an eruption in the present-day atmosphere is intensified at high latitudes by a strengthening of the polar vortex, which reduces temperatures, increasing PSC occurrence. Additionally, the presence of H2SO4 in the polar stratosphere in combination with colder temperatures facilitates the formation of liquid sulfuric acid ternary solution particles, which further increase SAD and therefore ozone loss .

The net effect of the chemical response further depends on the background composition of the atmosphere. The slow-down of the NOx cycle and heterogeneous reactions intensify the chlorine cycle of ozone destruction, but chlorine levels have undergone serious changes in the last decades . In the present-day atmosphere, the elevated halogen loading in the stratosphere means that the net chemical effect of the eruption on the global ozone abundance is a reduction . For low halogen loadings, however, the chemical effect of the eruption is to increase ozone globally . Furthermore, increase in the stratospheric water vapour concentrations associated with the warming of the tropical tropopause can accelerate the HOx cycle and reduce ozone even in the case of low halogen concentrations .

These effects on stratospheric ozone can broadly be classified into two distinct groups. The first involves composition-dynamical interactions associated with the radiative absorption of the volcanic aerosol, which we refer to as RAD-DYN. The second comprises the net change due to a large number of heterogeneous chemical reactions on the aerosol surface, which we refer to as HET-AER. Please note, that the RAD-DYN effect includes also chemical effects, for instance though changes in the reactions rates or PSC formation due to the temperature changes. Moreover, HET-AER processes affect also the radiation transfer through the atmosphere by changes in the chemical composition.

From the observations it is not possible to understand which processes are responsible for the ozone changes and how these changes affect the dynamics. Using model simulations, the attribution of the ozone changes after the Mt. Pinatubo eruption has been assessed in several studies . In general, these studies conclude that both mechanisms are important, with regional and seasonal differences.

In the tropics, total column ozone was observed to reduced after the eruption of Mt. Pinatubo which was a combined signal of a reduction in the lower stratosphere and an increase of ozone concentrations above . The reduction in the lower stratosphere has been attributed to a mix of the dynamic effect and heterogeneous chemical reactions . The former reduces ozone by enhanced up-welling of ozone poor air and the latter via chemical depletion by active chlorine. Above 18 hPa the NOx deactivation dominates, increasing ozone with further enhancement in the tropics due to modified photolysis . A negative feedback between the tropical ozone changes and aerosol heating has been suggested by , with an reduced heating of up to 25 % in the tropical middle stratosphere.

Ozone changes in the NH are thought to be primarily caused by the heterogeneous chemical reaction effects , in particular at high latitudes . By contrast, the increasing ozone observed in the Southern Hemisphere (SH) has been attributed to dynamical processes induced by the aerosol heating in combination with the phase of the Brewer–Dobson circulation or the Quasi-Biennial Oscillation (QBO) .

The stratospheric dynamic perturbation after volcanic eruptions originates mainly from the aerosol heating in the tropical lower stratosphere. Nevertheless, changes in the ozone concentrations also affect heating rates and therefore modulate the dynamic response to the eruption . Using observed ozone anomalies for the Mt. Pinatubo eruption, found a strengthening of the Arctic Oscillation (AO) in late winter and early spring after the eruption, which is explained by the cooling effect of the polar stratospheric ozone depletion. However, by forcing the model with observed ozone anomalies, a separation of the dynamical and chemical causes of the ozone changes is not possible. Similar results were found by who compared the Mt. Pinatubo eruption in simulations with and without ozone changes. For a different climate state (Mt. Tambora, 1815) without anthropogenic chlorine in the stratosphere, they found a very small influence of the ozone changes on the dynamical perturbation.

The purpose of this study is to deepen our understanding of the processes which drive ozone changes after a strong tropical volcanic eruption and how these changes modulate atmospheric dynamics in the stratosphere. Moreover, we assess the influence of the eruption strength on these changes and the role of different climate setting in moderating the dynamical responses, where the term climate setting describes a specific atmospheric composition of greenhouse gases and their effects on the climate system. To the best of our knowledge these questions have not been addressed before. Here, we use a set of ensemble sensitivity simulations performed by the atmosphere–ocean–chemistry–circulation model (AOCCM) SOCOL-MPIOM. To evaluate the dynamic response, this study focuses mainly on the NH and on the winter season.

The paper is structured as follows: Sect.  introduces the coupled AOCCM, the forcing data, and the setup of the experiments. The results are presented in Sect. , first for the simulated ozone changes associated with the different processes (Sect. ), followed by the dynamical changes (Sect. ). Section  focuses on interactions between the ozone chemistry and the dynamic response associated with the RAD-DYN effects. Finally, we discuss the results and present conclusive remarks in Sect. .

This study addresses the role of ozone changes for the dynamical perturbations of the stratosphere after strong tropical volcanic eruptions. Thereby, the underlying mechanisms and the influence of the climate setting and the eruption strengths are considered. The results are based on a number of ensemble sensitivity simulations with the AOCCM SOCOL-MPIOM, which allows us to separate the effect of heterogeneous chemical reactions from the warming effect of the aerosols for preindustrial and present-day conditions as well as three different eruption intensities.

In agreement with earlier studies we find that both processes, heterogeneous chemical reactions on the aerosol surface and the warming effect of the aerosols are important for the ozone changes after the eruption in an atmosphere with enhanced concentrations of ODS . A comparable case-study for the 1991 Mt. Pinatubo was performed by , with a similar separation between radiative-dynamical and heterogeneous chemical effects on the aerosols. They identified a combination of HET-AER and RAD-DYN processes to be responsible for the ozone anomalies in the SH. In particular they found similar anomalies in the residual mean circulation being responsible for reduced ozone in the tropics and enhanced ozone concentrations at mid-latitudes (compare Fig. b and their Fig. 7). This response, however, is limited to the early phase of the eruptions and in combination with the phase of the Brewer–Dobson circulation the anomaly-pattern is found only in the SH. In boreal winter, when tropical stratospheric temperature anomalies are at maximum in our experiments, our findings suggest a similar response of the residual mean circulation in the NH with positive column ozone anomalies at mid-latitudes. did not simulate positive column ozone anomalies in the NH related to radiative-dynamical changes (compare their Fig. 6) and concluded that NH ozone anomalies are mainly affected by HET-AER effects. This difference in the response is not yet understood, but may be related to differences in the aerosol forcings. Understanding the response to the RAD-DYN mechanism is of particular importance for volcanic eruptions under preindustrial conditions with low load of ozone depleting halogens, where chemical effects become weak and the response is dominated by radiative-dynamical effects.

The dynamical perturbation of the stratosphere is dominated by the RAD-DYN effect of the aerosols. During winter time, the warming in the tropical stratosphere increases the meridional temperature gradient towards the poles and strengthens the westerly circulation in the polar stratosphere. Ozone changes, related to heterogeneous chemical reactions on aerosols under present-day conditions weaken the warming in the tropical stratosphere and the NH polar vortex. By contrast, ozone changes related to the RAD-DYN mechanism amplify the warming in the subtropical and mid-latitude lower stratosphere, but weaken the temperature response in the middle stratosphere. The latter causes a slight weakening of the polar vortex. In agreement with and we found an intensification of the NH polar vortex in late winter due to ozone changes after strong volcanic eruptions in a present-day atmosphere. Our results show that both, HET-AER and RAD-DYN effects contribute to this intensification.

The atmospheric composition influences the response of the ozone chemistry and the dynamical response to the volcanic aerosols in a significant way. As already shown by heterogeneous chemical reactions on the aerosol surface cause pronounced global ozone depletion when stratospheric loads of ozone depleting halogens are high, but has only a weak positive effect in a preindustrial atmosphere. Furthermore, ozone depletion scales with the amplitude of eruptions under present-day conditions, but for a preindustrial atmosphere the response is independent from the eruption size . With the projected reduction of ODS the effect of heterogeneous chemical reactions can be expected to become less important for future eruptions. However, differences in the background state affect the results of this study as well. In recent decades, the stratosphere has undergone a pronounced cooling trend e.g., related to the ozone changes and the increasing levels of GHG . This cooling also affects the meridional temperature profiles in the stratosphere and the dynamics. In this study, significant differences in the vortex behaviour exist between the control simulations for preindustrial and present day, and these differences can be larger than the differences between the simulations perturbed by the volcanic eruption.

A direct comparison of our results to observations is difficult given the highly idealised character of our experiments. Nevertheless, one experiment, the 15 Tg experiment for the full aerosol effects under present-day conditions, can be considered similar to the perturbation from Pinatubo in 1991. Globally, the combined aerosol effect for the 15 Tg aerosol forcing results in an ozone loss of 10.4±3.7 DU 21 months after the start of the eruption. This agrees reasonably well with observations for Pinatubo , where a reduction of -13.9 DU was observed peaking in May 1993 (22 months after the eruption). Furthermore, the pattern of column ozone anomalies in is similar to the pattern found in the PD15 ensemble mean (Fig. ). After the eruption, negative anomalies are found in the tropics and positive anomalies in the sub-tropics and mid latitudes. The magnitude of both is stronger in SOCOL-MPIOM than in observations. In particular, the positive column ozone anomaly in the NH mid-latitudes is weaker in the data set. This may be related to the QBO effect or the Brewer–Dobson circulation , which modulated the ozone anomalies after Pinatubo. The modulating effect of the QBO, however, has been removed by the comparison to an ensemble of control simulations nudged by the same QBO reconstruction. Overestimated ozone anomalies in the PD15 ensemble may further be related to the too strong warming simulated in the tropical stratosphere by SOCOL-MPIOM. The remaining anomalies, i.e. the polar ozone depletion and the global reduction of columns ozone about 1 year after the eruptions agrees reasonable well with the simulated anomalies. For the vertical pattern of ozone changes e.g. the agreement between model results and observations is also reasonable, with the exception of stronger anomalies in the model, in particular in the tropics.

A caveat of this study is the overestimated warming in the tropical stratosphere, which may lead to an overestimation of the dynamical response and the resulting ozone changes. This needs to be considered when interpreting the results of the RAD-DYN and full forcing ensemble results. An overestimation of the lower stratospheric warming after volcanic eruptions is a feature common to many models e.g.. The maximum warming after the Pinatubo eruption in a multi-model ensemble average of 13 CMIP5 models model selection as in is 3.8±2.9 K for the temperature anomaly at 50 hPa, while reanalysis products and observations suggest temperature anomalies between 2.5 and 3 K . However, the phase of the QBO and other dynamical processes are suggested to reduce the stratospheric warming after Pinatubo . SOCOL-MPIOM simulates a warming of 8.7±1.2 K in the PD15 ensemble. Our results show that the overestimated warming is to some extent related to the ozone changes, which amplify the warming by about 0.9 K (15 Tg forcing at 50 hPa). The aerosol forcing applied in this study also contributes to the overestimated warming. We use a 2-D global aerosol forcing produced by an aerosol micro-physical model, which allows us to generate physical consistent forcing for different eruption intensities. This model-generated forcing differs from satellite based observations for Pinatubo in several aspects as described by. For comparison, we also perform eight simulations forced by realistic satellite based aerosol concentrations for the Mt. Pinatubo eruption SAGE_4λ in. In these simulations (not shown) the stratospheric warming is still overestimated (maximum temperature anomalies: 6.0±1.0 K), but the agreement to CMIP5 is better, in particular when the temperature effect of the ozone changes is considered.

In summary, we show that ozone is affected globally by a volcanic eruption for several years. Both effects, the radiative dynamical perturbation by the volcanic aerosols as well as the heterogeneous chemical reaction on the aerosols are important for the response of the ozone chemistry. The climate setting, in particular the atmospheric concentrations of ODS, has the strongest effects on the heterogeneous chemical effect on aerosol surfaces with pronounced global ozone depletion for present-day ODS concentrations (peak reductions of -13, -18, and -21 DU for the 15, 30, and 60 Tg eruptions, respectively) and slight ozone increase for preindustrial ODS concentrations (between 4 and 5 DU for all eruptions). Radiative dynamical ozone changes are positive for preindustrial and present-day conditions, but for present day the response is weakened by amplified polar ozone depletion (+5, +8, +12 DU peak column ozone anomalies for present day and +6, +14, +18 DU for preindustrial). The full effect of the volcanic aerosol, therefore, clearly differs between preindustrial and present day, with long lasting ozone depletion in a present-day atmosphere and positive ozone anomalies for preindustrial conditions. The response of stratospheric temperature and dynamics is dominated by the radiative heating effect of the aerosols. A small influence of the climate setting on the heating of the lower tropical stratosphere was found, with larger temperature anomalies for the present-day experiments. Dynamical radiative ozone changes further amplify the stratospheric temperature anomalies in the lower tropical stratosphere (and cause a cooling in higher levels). Ozone changes due to heterogeneous chemical reactions on the aerosols are responsible for a slight cooling of the tropical stratosphere. In winter and early spring after the eruption, the NH polar vortex is intensified, due to the radiative warming in the tropical stratosphere. Ozone changes, either due to radiative-dynamical effects or heterogeneous reaction on the aerosol surface, induce a slight weakening of the vortex in mid-winter. In late winter they cause a slight strengthening of the westerly circulation in the NH polar stratosphere.