Comment on acp-2022-372

Abstract. The paper constitutes Part 2 of a study performing a first systematic inter-model comparison of the atmospheric responses to stratospheric aerosol injection (SAI) at various single latitudes in the tropics, as simulated by three state-of-the-art Earth system models – CESM2-WACCM6, UKESM1.0, and GISS-E2.1-G. Building on Part 1 (Visioni et al., 2023) we demonstrate the role of biases in the climatological circulation and specific aspects of the model microphysics in driving the inter-model differences in the simulated sulfate distributions. We then characterize the simulated changes in stratospheric and free-tropospheric temperatures, ozone, water vapor, and large-scale circulation, elucidating the role of the above aspects in the surface SAI responses discussed in Part 1. We show that the differences in the aerosol spatial distribution can be explained by the significantly faster shallow branches of the Brewer–Dobson circulation in CESM2, a relatively isolated tropical pipe and older tropical age of air in UKESM, and smaller aerosol sizes and relatively stronger horizontal mixing (thus very young stratospheric age of air) in the two GISS versions used. We also find a large spread in the magnitudes of the tropical lower-stratospheric warming amongst the models, driven by microphysical, chemical, and dynamical differences. These lead to large differences in stratospheric water vapor responses, with significant increases in stratospheric water vapor under SAI in CESM2 and GISS that were largely not reproduced in UKESM. For ozone, good agreement was found in the tropical stratosphere amongst the models with more complex microphysics, with lower stratospheric ozone changes consistent with the SAI-induced modulation of the large-scale circulation and the resulting changes in transport. In contrast, we find a large inter-model spread in the Antarctic ozone responses that can largely be explained by the differences in the simulated latitudinal distributions of aerosols as well as the degree of implementation of heterogeneous halogen chemistry on sulfate in the models. The use of GISS runs with bulk microphysics demonstrates the importance of more detailed treatment of aerosol processes, with contrastingly different stratospheric SAI responses to the models using the two-moment aerosol treatment; however, some problems in halogen chemistry in GISS are also identified that require further attention. Overall, our results contribute to an increased understanding of the underlying physical mechanisms as well as identifying and narrowing the uncertainty in model projections of climate impacts from SAI.

Dynamical contributions to past long-term changes in the lower stratospheric ozone over the northern mid-latitudes are evaluated using a chemical transport model (CTM) forced by the horizontal wind of the Japanese 25-year Reanalysis (JRA-25). Two simulations (i.e., one is a simulation that prescribes the time-dependent vertical profile of halogens and the other is a simulation which uses the fixed vertical profile of halogens at 1979.) were conducted to estimate chemical and dynamical contributions to the long-term changes in stratospheric ozone during the last three decades. Different from previous similar studies using meteorological data of ECMWF (European Centre for Medium Weather Forecast) 40-year re-analysis (ERA-40), our current simulation does not show a large positive anomaly of simulated total ozone over northern mid-latitudes in the late 1980s, which is consistent with the observation. Because the trend of the fixed halogen simulation amounts to about two-third of that of the time-dependent halogen simulation during 1980–1993 in the northern mid-latitudes, it is evaluated that about two-thirds of the negative trend in total ozone comes from dynamics in the northern mid-latitudes. Since the increasing ozone from 1994 to 1998 is also represented in the fixed halogen simulation, it is considered that the increase of ozone was mainly due to dynamics as pointed out in previous studies. However the dynamical contribution to the trend after 1994 could not be evaluated in our simulation because of simulated ozone gap in 1998. In the same manner, it is evaluated that about two-thirds of the negative ozone trend in the lower stratosphere comes from dynamics in the northern mid-latitudes from 1980 to the mid-1990s. The simulation results indicate that the effect of transport (dynamical influence) is predominant for the negative ozone trend in the lower stratosphere from 1980 to mid-1990s, while the upper stratospheric ozone trend is strongly influenced by long-term changes in halogens (chemical influence).

In this study the short- and long-term variability of the surface spectral solar ultraviolet (UV) irradiance are investigated over Italy using high quality ground based measurements from three sites located at quite different environmental conditions, and covering the full latitudinal extent of the Italian territory: Aosta (45.7° N, 7.4° E, 570 m a.s.l.), Rome (41.9° N, 12.5° E, 75 m a.s.l.), and Lampedusa (35.5° N, 12.6° E, 50 m a.s.l.). The variability of the irradiances at 307.5 nm, 324 nm, and of the ratio between the 307.5 nm and the 324 nm irradiances were investigated with respect to the corresponding variability in total ozone and the geopotential height at 250 hPa (GPH). The study was performed for two periods: 2006–2020 for all stations, and 1996–2020 only for Rome. A statistically significant correlation between the GPH and total ozone monthly anomalies was found for all stations and all seasons of the year. A corresponding statistically significant correlation was also found in most cases between the GPH and the 307.5 nm irradiance monthly anomalies. The correlation between GPH anomalies at different sites was statistically significant, possibly explaining the strong and significant correlation between the total ozone monthly anomalies at the three sites. A statistically significant decrease of total ozone, of ~0.1 %/year was found for Rome for the period 1996–2020, which however did not induce increasing trends in irradiance at 307.5 nm (neither increasing trends in the ratio between the 307.5 nm and the 324 nm irradiances) at SZA = 67°. Further analyses revealed positive trends in the ratio and the 307.5 nm irradiance at smaller solar zenith angles (SZA), which can be attributed to the fact that total ozone decrease is driven by a decrease in the lower stratosphere while upper stratospheric ozone increases, and the effect of changes of upper stratospheric ozone becoming disproportionately larger for increasing SZA. It was also showed that long-term changes in total ozone follow changes in GPH, which is an additional indication that negative trends in total ozone are mainly driven by changes in lower stratospheric ozone. An anti-correlation between the GPH long-term changes and total ozone was also evident for all stations in 2006–2020. Positive trends in UV irradiance for this latter period which were possibly driven by changes in clouds and/or aerosols were found for Rome and Aosta. This study clearly points out the significance of dynamical processes which take place in the troposphere for the variability of total ozone and surface solar UV irradiance.

This study examined the vertical profile and ozone variation in the lower stratosphere in Indonesia using AQUA/AIRS and AURA/MLS satellite data. Ozone in the stratosphere as an important component in atmospheric chemistry as well as radiation and climate becomes important to learn. Seasonal vertical profiles, time-varying vertical profiles, ozone seasonal variations at pressures of 100 hPa and 70 hPa, ozone time series at pressures of 100 hPa and 70 hPa from 2002 to 2016, average zonal variation (influence latitude) ozone concentrations in Indonesia, and the comparison of the vertical profiles of ozone, HCl, and N2O in the lower stratosphere layer in Indonesia from MLS/AURA satellite data have been done to understand the variation of ozone in the lower stratosphere in Indonesia. The results show that the vertical ozone profile increased from the tropopause then the lower stratosphere to the middle stratosphere with the highest concentration of up to 9 ppmv. The monthly variation of ozone climatology at 70 hPa shows a pattern with a maximum peak in August while at a pressure of 100 hPa it indicates the presence of two maximum peaks in June and September. Time series of ozone at 100 hPa and 70 hPa from 2002 to 2016 show that ozone concentration tended to be constant with very small concentration increase occurring at 70 hPa and 100 hPa. Zonal variation shows the lowest ozone concentration at zero degrees latitude compared to other latitudes, and the vertical profile of ozone has the same pattern as HCl while it is different from N2O. Vertical-time series cross section of ozone and Potential Vorticity (PV) in upper troposphere show influenced of stratospheric air into upper tropospheric air which showed by higher ozone and PV value and can reach until pressure 150 hPa occurred in the same season.

Ozone in the stratosphere protects life on Earth by absorbing much of the ultraviolet solar radiation, as well as being a major source of ozone to the troposphere. A number of factors can influence stratospheric ozone levels and explain past, present and future changes. While there is a large literature exploring changes in stratospheric ozone due to, for example, ozone depleting substances (ODSs) and climate change, the study of its interactions with the rest of the Earth system is relatively recent. The work presented in this thesis investigates changes in stratospheric ozone and its links with other elements of the Earth system (tropospheric chemical composition and climate), using observations, a global chemistry-climate model (CESM1-WACCM), and existing multi-model output. This work evaluates past changes and explores future evolution of stratospheric ozone and associated climate impacts using multi-model output (ACCMIP simulations) from the pre-industrial period to the end of the 21st century. ACCMIP multi-model mean total column ozone trends compare favourably against observations. These models show a strong link between the Antarctic ozone hole and surface climate, which demonstrates that stratospheric ozone changes are coupled with the troposphere. A series of sensitivity simulations is conducted to investigate multi-decadal variability. A striking finding is that low frequency variability of ozone in the tropical middle stratosphere resembles multi-decadal climate variability in Pacific Ocean sea surface temperatures. This analysis also shows that internally generated climate variability is the leading factor explaining recent negative trends of mid-stratospheric tropical ozone. Finally, an additional set of sensitivity simulations is performed to quantify future radiative forcing between 2000s and 2100s that results from changes in ozone due to climate change, ODSs and methane concentrations. These results highlight the importance of stratosphere-troposphere exchange, as well as the key role of the stratosphere controlling the tropospheric ozone forcing.

Abstract. Sulfate geoengineering (SG) methods based on lower stratospheric tropical injection of sulfur dioxide (SO2) have been widely discussed in recent years, focusing on the direct and indirect effects they would have on the climate system. Here a potential alternative method is discussed, where sulfur emissions are located at the surface or in the troposphere in the form of carbonyl sulfide (COS) gas. There are two time-dependent chemistry–climate model experiments designed from the years 2021 to 2055, assuming a 40 Tg-Syr-1 artificial global flux of COS, which is geographically distributed following the present-day anthropogenic COS surface emissions (SG-COS-SRF) or a 6 Tg-Syr-1 injection of COS in the tropical upper troposphere (SG-COS-TTL). The budget of COS and sulfur species is discussed, as are the effects of both SG-COS strategies on the stratospheric sulfate aerosol optical depth (∼Δτ=0.080 in the years 2046–2055), aerosol effective radius (0.46 µm), surface SOx deposition (+8.9 % for SG-COS-SRF; +3.3 % for SG-COS-TTL), and tropopause radiative forcing (RF; ∼-1.5 W m−2 in all-sky conditions in both SG-COS experiments). Indirect effects on ozone, methane and stratospheric water vapour are also considered, along with the COS direct contribution. According to our model results, the resulting net RF is −1.3 W m−2, for SG-COS-SRF, and −1.5 W m−2, for SG-COS-TTL, and it is comparable to the corresponding RF of −1.7 W m−2 obtained with a sustained injection of 4 Tg-Syr-1 in the tropical lower stratosphere in the form of SO2 (SG-SO2, which is able to produce a comparable increase of the sulfate aerosol optical depth). Significant changes in the stratospheric ozone response are found in both SG-COS experiments with respect to SG-SO2 (∼5 DU versus +1.4 DU globally). According to the model results, the resulting ultraviolet B (UVB) perturbation at the surface accounts for −4.3 % as a global and annual average (versus −2.4 % in the SG-SO2 case), with a springtime Antarctic decrease of −2.7 % (versus a +5.8 % increase in the SG-SO2 experiment). Overall, we find that an increase in COS emissions may be feasible and produce a more latitudinally uniform forcing without the need for the deployment of stratospheric aircraft. However, our assumption that the rate of COS uptake by soils and plants does not vary with increasing COS concentrations will need to be investigated in future work, and more studies are needed on the prolonged exposure effects to higher COS values in humans and ecosystems.

Abstract. The vertical profile of global-mean stratospheric temperature changes has traditionally represented an important diagnostic for the attribution of the cooling effects of stratospheric ozone depletion and CO2 increases. However, CO2-induced cooling alters ozone abundance by perturbing ozone chemistry, thereby coupling the stratospheric ozone and temperature responses to changes in CO2 and ozone-depleting substances (ODSs). Here we untangle the ozone-temperature coupling and show that the attribution of global-mean stratospheric temperature changes to CO2 and ODS changes (which are the true anthropogenic forcing agents) can be quite different from the traditional attribution to CO2 and ozone changes. The significance of these effects is quantified empirically using simulations from a three-dimensional chemistry-climate model. The results confirm the essential validity of the traditional approach in attributing changes during the past period of rapid ODS increases, although we find that about 10% of the upper stratospheric ozone decrease from ODS increases over the period 1975–1995 was offset by the increase in CO2, and the CO2-induced cooling in the upper stratosphere has been somewhat overestimated. When considering ozone recovery, however, the ozone-temperature coupling is a first-order effect; fully 2/5 of the upper stratospheric ozone increase projected to occur from 2010–2040 is attributable to CO2 increases. Thus, it has now become necessary to base attribution of global-mean stratospheric temperature changes on CO2 and ODS changes rather than on CO2 and ozone changes.

<strong class="journal-contentHeaderColor">Abstract.</strong> Lagrangian trajectories have previously been used to reconstruct water vapor variability in the lower stratosphere, where the sensitivity of surface radiation to changes in the water vapor concentration is strongest, by obtaining temperature histories of air parcels that ascend from the troposphere to the stratosphere through the tropical tropopause layer (TTL). Models and theory predict an acceleration of the Brewer-Dobson Circulation (BDC) and deceleration of the Walker Circulation (WC) with surface warming, and both of these will drive future changes to transport across the TTL. Here, we examine the response of TTL transport during boreal winter to idealized changes in the BDC and WC by comparing the temperature histories of trajectories computed with ERA5 data to those calculated using the same data but with altered vertical and zonal wind velocities. We find that lower stratospheric water vapor mixing ratios calculated from trajectories' cold point temperatures can increase by about 1.6 ppmv (about 50 %) when only zonal winds are slowed, while changes to vertical winds have a negligible impact on water vapor concentrations. This change follows from a decrease in zonal sampling of the temperature field by trajectories, which weakens the "cold trap" mechanism of dehydration as TTL transport evolves. As the zonal winds of the TTL decrease, the fraction of air that passes through the cold trap while ascending to the stratosphere will decrease and the coldest average temperature experienced by parcels will increase. Some of the resultant moistening may be negated by a decreased rate of temperature change following the cold point, which will allow more ice to gravitationally settle before sublimating outside of the cold trap. This result presents a mechanism for a stratospheric water vapor feedback that can exist without changes to TTL temperatures.

Stratosphere-to-troposphere transport (STT) is an important source of ozone for the troposphere, particularly over western North America. STT in this region is predominantly controlled by a combination of the variability and location of the Pacific jet stream and the amount of ozone in the lower stratosphere, two factors which are likely to change if greenhouse gas concentrations continue to increase. Here we use Whole Atmosphere Community Climate Model experiments with a tracer of stratospheric ozone (O3S) to study how end-of-the-century Representative Concentration Pathway (RCP) 8.5 sea surface temperatures (SSTs) and greenhouses gases (GHGs), in isolation and in combination, influence STT of ozone over western North America relative to a preindustrial control background state.&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160; We find that O3S increases up to 39% at 700 hPa over western North America in response to RCP8.5 forcing with the largest increases occurring during late winter and tapering off during spring and summer. The GHGs are primarily responsible for these tropospheric O3S changes. Both the future SSTs and the future GHGs accelerate the Brewer Dobson circulation, which increases extratropical lower stratospheric ozone mixing ratios. While the GHGs promote a more zonally symmetric lower stratospheric ozone change due to enhanced ozone production and some transport, the SSTs increase lower stratospheric ozone predominantly over the North Pacific via transport associated with a stationary planetary-scale wave. Ozone accumulates in the trough of this anomalous wave and is reduced over the wave&amp;#8217;s ridges, illustrating that the composition of the lower stratospheric ozone reservoir in the future is dependent on the stationary planetary-scale wave response to future SSTs. In addition, the future SSTs are found to prompt most changes to the large-scale circulation in the troposphere and stratosphere compared to the effect of the GHGs. These changes include modifying the position and speed of the future North Pacific jet, lifting the tropopause, accelerating both the Brewer-Dobson Circulation&amp;#8217;s shallow and deep branches, and enhancing two-way isentropic mixing in the stratosphere.

Abstract. Accurate quantification of long-term trends in stratospheric ozone can be challenging due to their sensitivity to natural variability, the quality of the observational datasets, and non-linear changes in forcing processes as well as the statistical methodologies. Multivariate linear regression (MLR) is the most commonly used tool for ozone trend analysis; however, the complex coupling in many atmospheric processes can make it prone to the issue of over-fitting when using the conventional ordinary-least-squares (OLS) approach. To overcome this issue, here we adopt a regularized (ridge) regression method to estimate ozone trends and quantify the influence of individual processes. We use the Stratospheric Water and OzOne Satellite Homogenized (SWOOSH) merged dataset (v2.7) to derive stratospheric ozone profile trends for the period 1984–2020. Besides SWOOSH, we also analyse a machine-learning-based satellite-corrected gap-free global stratospheric ozone profile dataset from a chemical transport model (ML-TOMCAT) and output from a chemical transport model (TOMCAT) simulation forced with European Centre for Medium-Range Weather Forecasts (ECMWF) ERA5 reanalysis. For 1984–1997, we observe smaller negative trends in the SWOOSH stratospheric ozone profile using ridge regression compared to OLS. Except for the tropical lower stratosphere, the largest differences arise in the mid-latitude lowermost stratosphere (&gt;4 % per decade difference at 100 hPa). From 1998 and the onset of ozone recovery in the upper stratosphere, the positive trends estimated using the ridge regression model (∼1 % per decade near 2 hPa) are smaller than those using OLS (∼2 % per decade). In the lower stratosphere, post-1998 negative trends with large uncertainties are observed and ridge-based trend estimates are somewhat smaller and less variable in magnitude compared to the OLS regression. Aside from the tropical lower stratosphere, the largest difference is around 2 % per decade at 100 hPa (with ∼3 % per decade uncertainties for individual trends) in northern mid-latitudes. For both time periods the SWOOSH data produce large negative trends in the tropical lower stratosphere with a correspondingly large difference between the two trend methods. In both cases the ridge method produces a smaller trend. The regression coefficients from both OLS and ridge models, which represent ozone variations associated with natural processes (e.g. the quasi-biennial oscillation, solar variability, El Niño–Southern Oscillation, Arctic Oscillation, Antarctic Oscillation, and Eliassen–Palm flux), highlight the dominance of dynamical processes in controlling lower-stratospheric ozone concentrations. Ridge regression generally yields smaller regression coefficients due to correlated explanatory variables, and care must be exercised when comparing fit coefficients and their statistical significance across different regression methods. Comparing the ML-TOMCAT-based trend estimates with the ERA5-forced model simulation, we find ML-TOMCAT shows significant improvements with much better consistency with the SWOOSH dataset, despite the ML-TOMCAT training period overlapping with SWOOSH only for the Microwave Limb Sounder (MLS) measurement period. The largest inconsistencies with respect to SWOOSH-based trends post-1998 appear in the lower stratosphere where the ERA5-forced model simulation shows positive trends for both the tropics and the mid-latitudes. The large differences between satellite-based data and the ERA5-forced model simulation confirm significant uncertainties in ozone trend estimates, especially in the lower stratosphere, underscoring the need for caution when interpreting results obtained with different regression methods and datasets.

Reanalysis data sets are widely used to understand atmospheric processes and past variability, and are often used to stand in as "observations" for comparisons with climate model output. Because of the central role of water vapor (WV) and ozone (O3) in climate change, it is important to understand how accurately and consistently these species are represented in existing global reanalyses. In this paper, we present the results of WV and O3 intercomparisons that have been performed as part of the SPARC (Stratosphere-troposphere Processes and their Role in Climate) Reanalysis Intercomparison Project (S-RIP). The comparisons cover a range of timescales and evaluate both inter-reanalysis and observation-reanalysis differences. We also provide a systematic documentation of the treatment of WV and O3 in current reanalyses to aid future research and guide the interpretation of differences amongst reanalysis fields. The assimilation of total column ozone (TCO) observations in newer reanalyses results in realistic representations of TCO in reanalyses except when data coverage is lacking, such as during polar night. The vertical distribution of ozone is also relatively well represented in the stratosphere in reanalyses, particularly given the relatively weak constraints on ozone vertical structure provided by most assimilated observations and the simplistic representations of ozone photochemical processes in most of the reanalysis forecast models. However, significant biases in the vertical distribution of ozone are found in the upper troposphere and lower stratosphere in all reanalyses. In contrast to O3, reanalysis estimates of stratospheric WV are not directly constrained by assimilated data. Observations of atmospheric humidity are typically used only in the troposphere, below a specified vertical level at or near the tropopause. The fidelity of reanalysis stratospheric WV products is therefore mainly dependent on the reanalyses' representation of the physical drivers that influence stratospheric WV, such as temperatures in the tropical tropopause layer, methane oxidation, and the stratospheric overturning circulation. The lack of assimilated observations and known deficiencies in the representation of stratospheric transport in reanalyses result in much poorer agreement amongst observational and reanalysis estimates of stratospheric WV. Hence, stratospheric WV products from the current generation of reanalyses should generally not be used in scientific studies.

Sulfate geoengineering (SG) methods based on lower stratospheric tropical injection of sulfur dioxide (SO2) have been widely discussed in recent years, focusing on the direct and indirect effects they would have on the climate system. Here a potential alternative method is discussed, where sulfur emissions are located at the surface in the form of carbonyl sulfide (COS) gas. A time-dependent chemistry-climate model experiment is designed from year 2021 to 2055, assuming a 40 Tg-S/yr artificial global flux of COS, geographically distributed following the present day anthropogenic COS surface emissions. The budget of COS and sulfur species is discussed, as well as the effects of SG-COS on the stratospheric sulfate aerosol optical depth (Δ τ = 0.080 in years 2046–2055), aerosol effective radius (0.46 μm), surface SOx deposition (+8.7 %) and tropopause radiative forcing (RF) (−2.0 W/m2 for clear sky conditions and −1.5 W/m2 including the cloud adjustment). Indirect effects on ozone, methane and stratospheric water vapor are also considered, along with the COS direct contribution (with an overall gas phase global radiative forcing of +0.23 W/m2). According to our model results, the resulting net RF of this SG-COS experiment is −1.3 W/m2 for the year 2050, and it is comparable to the corresponding RF of −1.7 W/m2 obtained with a sustained injection of 4 Tg-S/yr in the tropical lower stratosphere in the form of SO2 (SG-SO2, able to produce a comparable increase of the sulfate aerosol optical depth). Significant changes of the stratospheric ozone response are found in SG-COS with respect to SG-SO2 (+4.9 DU versus +1.5 DU, globally). According to the model results, the resulting UVB perturbation at the surface accounts to −4.3 % as a global-annual average (versus −2.4 % in the SG-SO2 case), with a springtime Antarctic decrease of −2.7 % (versus a +5.8 % increase in the SG-SO2 experiment). Overall, we find that an increase in COS surface emission may be feasible, and produce a more latitudinally-uniform forcing without the need for the deployment of stratospheric aircrafts.

Cosmic rays (CRs) play an important role in affecting planetary and interstellar climate and environment. Here, we apply the CR-driven electron-induced reaction (CRE) theory of ozone depletion to obtain a quantitative understanding of spatiotemporal variations in Earth's lower-stratospheric ozone (LSO) and temperature, which provide fingerprints for the mechanisms of ozone depletion and examine the impact of nonhalogen greenhouse gases on the ozone layer. We first show from observations that both LSO and temperature display pronounced 11-y cyclic variations over Antarctica and mid-latitudes, while weak (no apparent) cyclic variations over the tropics. These observations are consistent with the prediction by the CRE theory. Second, our no-parameter CRE theoretical calculations give the vertical profile of ozone loss in perfect agreement with observations at the Antarctic Syowa station and reproduce well the time-series variations of both LSO and temperature in the polar, mid-latitude, and tropical regions, including the previously reported large ozone depletion in the lower stratosphere over the tropics. The results also demonstrate that both LSO and temperature are controlled by CRs and ozone-depleting substances (ODSs) only. Moreover, CRE calculations exhibit complex phenomena in future trends of LSO and temperature, which are strongly affected by the future trend of CR fluxes. The latter might even lead to almost no recovery of the ozone hole over Antarctica and no returning to the 1980 level over the tropics by 2100. This study greatly improves quantitative understanding of ozone depletion and climate in the global lower stratosphere and offers predictions on future trends.

A series of studies were performed concerning the response of low‐latitude ozone and temperature in the stratosphere and mesosphere to short‐term solar ultraviolet variability associated with the rotation of the sun. The studies are based on Nimbus 7 Limb Infrared Monitor of the Stratosphere (LIMS) stratospheric ozone and temperature data, Nimbus 7 solar backscattered ultraviolet (SBUV) stratospheric ozone and 205‐nm solar ultraviolet data, Solar Mesosphere Explorer (SME) 1.27‐μm mesospheric O3 data, SME 121.6‐nm solar ultraviolet data, and Nimbus 7 Stratosphere and Mesosphere Sounder (SAMS) stratospheric and mesospheric temperature data. Using a longer temperature time series than has been used in the past for such studies, response times of temperature to solar UV variability in the stratosphere are found to be unexpectedly long (6 days at 2 mbar) and to become much shorter in the mesosphere (1 day at 0.01 mbar). Maximum sensitivity of temperature to solar variability (0.3 K/percent 205‐nm radiation) is found to occur near 70 km. The coupling of the temperature and ozone response to solar UV variability has been isolated by studying ozone responses with and without temperature effects. Temperature effects tend to increase rather than decrease the amplitude and to shift the response time of stratospheric ozone to solar variability to an earlier time. Using a longer ozone time series than has been used in the past for such studies, the stratospheric ozone response with no correction for temperature effects is found to be approximately a 0.4% increase at 3 mbar for a 1.0% increase in 205‐nm solar radiation. In the mesosphere a major systematic ozone decrease has been detected near 0.05 mbar (∼70 km), with increased solar Lyman α (121.6 nm) radiation (−0.14% ozone decrease for a 1% increase in solar Lyman α). This may be caused by solar Lyman α photodissociating H2O vapor producing HOx, with subsequent destruction of Ox. Higher in the mesosphere, where H2O mixing ratios should be much lower, ozone is found to increase with increasing solar UV. Observed responses of HNO3 and NO2 to solar UV variability are also briefly discussed. The theoretical response of middle atmosphere species and temperature to solar UV variability is discussed in detail in a companion paper (Brasseur et al., this issue).

Hydrofluorocarbons (HFCs) have been increasingly replacing chlorofluorocarbons and hydrochlorofluorocarbons. Although their direct chemical ozone‐depleting potential is negligible, as potent greenhouse gases they modify atmospheric temperature and circulation patterns, thereby indirectly influencing stratospheric ozone recovery. Measurements and model projections must continue to evaluate HFC limitation measures and assess the long‐term impact of HFCs on the atmospheric radiation budget and stratospheric ozone. In this study, we present multi‐member ensemble simulations designed to estimate the impact of HFCs on stratospheric temperature, ozone and circulation changes at the end of the century. We compared simulations with and without HFCs for two three‐dimensional chemistry‐climate models that use the same chemistry module but different physical schemes. At low and mid‐latitudes, temperature and ozone responses were comparable for both models and in general agreement with previous studies. HFCs induced a marked temperature increase up to about 10–20 hPa and vertically alternating positive and negative ozone anomalies. We explained this pattern by competing effects of vertical motion (low and middle stratosphere) and temperature (upper stratosphere) anomalies. At northern high latitudes, there were strong discrepancies with previous studies and between the models themselves, attributed to differences in ozone anomalies caused by wave activity during winter. Quantitatively, we found a net positive, but small, HFC impact on total ozone amounts. Largest anomalies were less than 1% in the winter polar stratosphere. Our results indicate that increasing HFC amounts will likely have a limited impact on stratospheric ozone recovery within this century, with large uncertainty in the polar regions.