Reply on RC2

Abstract. The short- and long-term variability of the surface spectral solar ultraviolet (UV) irradiance is investigated across Italy using high-quality ground-based measurements from three locations: Aosta (45.7∘ N, 7.4∘ E, 570 m a.s.l.), Rome (41.9∘ N, 12.5∘ E, 15 75 m a.s.l.), and Lampedusa (35.5∘ N, 12.6∘ E, 50 m a.s.l.). The three sites are characterized by different environmental conditions and represent almost the full latitudinal extent of the Italian territory. Data of two periods were analysed: 2006–2020 (all sites) and 1996–2020 (Rome only). The main objective of this study is to quantify the effect of the geopotential height (GPH) at 250 hPa on total ozone, and spectral irradiance at 307.5 and 324 nm. We first show that monthly anomalies in GPH, total ozone, and spectral irradiances are correlated amongst the three sites, suggesting that Italy is often affected by the same synoptical weather systems. We further find statistically significant anticorrelations between GPH and monthly anomalies in total ozone for all stations and months. Conversely, we identify positive correlations between GPH and monthly anomalies in spectral irradiance at 307.5 nm for most months. The influence of GPH on short-term variability also hold for long-term trends. For example, long-term changes in total ozone over the period 2006–2020 were associated with changes in GPH for all stations. This suggests that observed negative trends in total ozone were mainly driven by changes in lower-stratospheric ozone as upper-stratospheric ozone was increasing over this period. For several months of the year, positive trends in UV irradiance were observed, and we found that these trends were predominantly caused by changes in clouds and/or aerosols instead of total ozone. For the longer period of 1996–2020, a statistically significant annualized decrease in total ozone of ∼ 0.1 % per year was identified for Rome and could subsequently be attributed to decreasing lower-stratospheric ozone. While positive trends in spectral irradiance at 307.5 nm were observed for several months of this extended period, the negative trend in total ozone did not lead to a positive trend in the spectral irradiance at 307.5 nm in the deseasonalized data. Our study provides evidence that dynamical processes taking place in the troposphere lead to significant variability in total ozone and surface solar UV irradiance.

Abstract. We present the first spatial analysis of "fingerprints" of the El Niño/Southern Oscillation (ENSO) and atmospheric aerosol load after major volcanic eruptions (El Chichón and Mt. Pinatubo) in extreme low and high (termed ELOs and EHOs, respectively) and mean values of total ozone for the northern and southern mid-latitudes (defined as the region between 30° and 60° north and south, respectively). Significant influence on ozone extremes was found for the warm ENSO phase in both hemispheres during spring, especially towards low latitudes, indicating the enhanced ozone transport from the tropics to the extra-tropics. Further, the results confirm findings of recent work on the connection between the ENSO phase and the strength and extent of the southern ozone "collar". For the volcanic eruptions the analysis confirms findings of earlier studies for the northern mid-latitudes and gives new insights for the Southern Hemisphere. The results provide evidence that the negative effect of the eruption of El Chichón might be partly compensated by a strong warm ENSO phase in 1982–1983 at southern mid-latitudes. The strong west-east gradient in the coefficient estimates for the Mt. Pinatubo eruption and the analysis of the relationship between the AAO and ENSO phase, the extent and the position of the southern ozone "collar" and the polar vortex structure provide clear evidence for a dynamical "masking" of the volcanic signal at southern mid-latitudes. The paper also analyses the contribution of atmospheric dynamics and chemistry to long-term total ozone changes. Here, quite heterogeneous results have been found on spatial scales. In general the results show that EESC and the 11-yr solar cycle can be identified as major contributors to long-term ozone changes. However, a strong contribution of dynamical features (El Niño/Southern Oscillation (ENSO), North Atlantic Oscillation (NAO), Antarctic Oscillation (AAO), Quasi-Biennial Oscillation (QBO)) to ozone variability and trends is found at a regional level. For the QBO (at 30 and 50 hPa), strong influence on total ozone variability and trends is found over large parts of the northern and southern mid-latitudes, especially towards equatorial latitudes. Strong influence of ENSO is found over the Northern and Southern Pacific, Central Europe and central southern mid-latitudes. For the NAO, strong influence on column ozone is found over Labrador/Greenland, the Eastern United States, the Euro-Atlantic Sector, and Central Europe. For the NAO's southern counterpart, the AAO, strong influence on ozone variability and long-term changes is found at lower southern mid-latitudes, including the southern parts of South America and the Antarctic Peninsula, and central southern mid-latitudes.

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).

This study analyses the variability and trends of ultraviolet-B (UV-B, wavelength 280–320 nm) radiation that can cause DNA damage, which are caused by climate change due to enhanced greenhouse gas (GHG) concentrations. The analysis is based on DNA active irradiance, total ozone, total cloud cover, and surface albedo calculations with the EMAC Chemistry-Climate Model (CCM) free running simulations following the RCP-6.0 climate scenario for the period 1960–2100. The model output is evaluated with DNA active irradiance ground-based measurements, satellite SBUV (v8.7) total ozone measurements and satellite MODIS/Terra cloud cover data. The results show that the model reproduces the observed variability and change of total ozone, DNA active irradiance, and cloud cover for the period 2000–2018 quite well. Between 50° N–50° S, the DNA-damaging UV radiation is expected to decrease until 2050 and to increase thereafter, as it was shown previously by Eleftheratos et al. (2020). This change is associated with decreases in the model total cloud cover and insignificant trends in total ozone after about 2050. The new study confirms the previous work by adding more stations over low and mid-latitudes (13 instead of 5 stations). In addition, we include estimates from high latitude stations with long-term measurements of UV irradiance (2 stations in the northern high latitudes and 4 stations in the southern high latitudes greater than 55°). In contrast to the predictions for 50° N–50° S, it is shown that DNA active irradiance will continue to decrease after the year 2050 over high latitudes because of upward ozone trends. At latitudes poleward of 55° N, we estimate that DNA active irradiance will decrease by 10.6 ± 3.7 % from 2050 to 2100. Similarly, at latitudes poleward of 55° S, DNA active irradiance will decrease by 4.8 ± 2.9 % after 2050. The results for the high latitudes refer to the summer period and not to the seasons when ozone depletion occurs, i.e., in late winter and spring. The contributions of ozone, cloud and albedo trends on the DNA active irradiance trends are estimated and discussed.

Analysis of the monthly mean interannual variations of the total ozone (Total Ozone Mapping Spectrometer (TOMS, TOMS‐Meteor 3)), surface air temperature (National Centres for Environmental Prediction and Atmospheric Research (NCEP/NCAR) reanalysis) and their relations is conducted by means of the singular value decomposition (SVD) for each month December–March 1979–1994. It is shown that the total ozone increase (decrease) occurs almost over those geographical regions where cooling (warming) is observed both on the interannual and longer time‐scales Strong non‐local correlations were indicated between the total ozone changes over England in January and surface temperature variations in western Siberia, southern European Russia and the Azores in February. Similar relations occur for the total ozone (February) and surface temperature (March) changes. These results can be used for the empirical extended‐range forecasts of the anomalously cold winters in Europe and Russia using the total ozone data. Possible mechanisms for this non‐local linkage and its relation to the Arctic Oscillation are discussed.

The National Science Foundation scanning spectroradiometer at Palmer Station, Antarctica (64°46′S, 64°04′W), provides hourly ground‐based measurements of solar ultraviolet (UV) irradiance. In addition to defining the UV radiation environment of the region, these measurements allow the derivation of the column density of atmospheric ozone above the station nearly every daylight hour. This hourly time resolution, not generally available from other methods of monitoring Antarctic ozone abundances, enables the detection of large and rapid changes in total column ozone and UV surface irradiance associated with the dynamics of the polar vortex. Column ozone abundance is derived from a ratio of measured irradiances at 300 and 313.5 nanometers (nm) by means of theoretical calculation of this ratio as a function of total ozone amount. Noontime ozone abundances over Palmer Station obtained from this method agree with those obtained by the Total Ozone Mapping Spectrometer (TOMS) instrument aboard Nimbus 7 to within about 10% throughout the austral spring of 1988. Ozone recovery at Palmer Station, associated with the breakup of the polar vortex as indicated by TOMS satellite ozone observations, occurred rapidly within a 24‐hour period beginning in midafternoon on November 15. Over the Antarctic Peninsula, the 1989 ozone depletion was slightly greater than in 1988, the minimum noontime ozone abundances over Palmer Station as measured by the spectroradiometer being 194 and 166 Dobson units for October 14, 1988, and October 14, 1989, respectively. The 1989 ozone depletion however ended by November 5 over the Antarctic Peninsula, 10 days earlier than the 1988 event.

Variability of sunspot cycle QBO and total ozone over high altitude western Himalayan regions

Abstract. Continuous measurements of total ozone (by Dobson spectrophotometers) across the contiguous United States began in the early 1960s. Here, we analyze temporal and spatial variability and trends in total ozone from the five US sites with long-term records. While similar long-term ozone changes are detected at all five sites, we find differences in the patterns of ozone variability on shorter timescales. In addition to standard evaluation techniques, STL-decomposition methods (Seasonal Trend decomposition of time series based on LOESS (LOcally wEighted Scatterplot Smoothing)) are used to address temporal variability and "fingerprints" of dynamical features in the Dobson data. Methods from statistical extreme value theory (EVT) are used to characterize days with high and low total ozone (termed EHOs and ELOs, respectively) at each station and to analyze temporal changes in the frequency of ozone extremes and their relationship to dynamical features such as the North Atlantic Oscillation (NAO) and El Niño–Southern Oscillation. A comparison of the fingerprints detected in the frequency distribution of the extremes with those for standard metrics (i.e., the mean) shows that more fingerprints are found for the extremes, particularly for the positive phase of the NAO, at all five US monitoring sites. Results from the STL decomposition support the findings of the EVT analysis. Finally, we analyze the relative influence of low- and high-ozone events on seasonal mean column ozone at each station. The results show that the influence of ELOs and EHOs on seasonal mean column ozone can be as much as ±5 %, about as large as the overall long-term decadal ozone trends.

The morphology of temperature trends in the south polar stratosphere from 1979‐1985 is detailed. A significant cooling has occurred in the stratosphere over this period. South polar October mean temperatures have declined 18°K at 24 km over the seven year period. A large fraction of this decline occurs as a result of including 1979 temperatures in the trend. The years 1979 and 1982 differ from other years in that they include large mid‐October wave events which produce significantly warmer polar temperatures. Nevertheless, a significant temperature decline has still occurred in the stratosphere even when the dynamically active years are not included. Comparing two relatively dynamically quiet years, 1980 and 1985, the largest temperature decrease in the 30 mb temperature field occurs not at the pole but in the sub‐polar temperature maximum centered near 60°S. The changes in temperature of both the maximum and the polar low regions correlate fairly well with total ozone changes observed by TOMS.

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.

This assessment provides a comprehensive update of the effects of changes in stratospheric ozone and other factors (aerosols, surface reflectivity, solar activity, and climate) on the intensity of ultraviolet (UV) radiation at the Earth’s surface. The assessment is performed in the context of the Montreal Protocol on Substances that Deplete the Ozone Layer and its Amendments and Adjustments. Changes in UV radiation at low- and mid-latitudes (0–60°) during the last 25 years have generally been small (e.g., typically less than 4% per decade, increasing at some sites and decreasing at others) and were mostly driven by changes in cloud cover and atmospheric aerosol content, caused partly by climate change and partly by measures to control tropospheric pollution. Without the Montreal Protocol, erythemal (sunburning) UV irradiance at northern and southern latitudes of less than 50° would have increased by 10–20% between 1996 and 2020. For southern latitudes exceeding 50°, the UV Index (UVI) would have surged by between 25% (year-round at the southern tip of South America) and more than 100% (South Pole in spring). Variability of erythemal irradiance in Antarctica was very large during the last four years. In spring 2019, erythemal UV radiation was at the minimum of the historical (1991–2018) range at the South Pole, while near record-high values were observed in spring 2020, which were up to 80% above the historical mean. In the Arctic, some of the highest erythemal irradiances on record were measured in March and April 2020. For example in March 2020, the monthly average UVI over a site in the Canadian Arctic was up to 70% higher than the historical (2005–2019) average, often exceeding this mean by three standard deviations. Under the presumption that all countries will adhere to the Montreal Protocol in the future and that atmospheric aerosol concentrations remain constant, erythemal irradiance at mid-latitudes (30–60°) is projected to decrease between 2015 and 2090 by 2–5% in the north and by 4–6% in the south due to recovering ozone. Changes projected for the tropics are ≤ 3%. However, in industrial regions that are currently affected by air pollution, UV radiation will increase as measures to reduce air pollutants will gradually restore UV radiation intensities to those of a cleaner atmosphere. Since most substances controlled by the Montreal Protocol are also greenhouse gases, the phase-out of these substances may have avoided warming by 0.5–1.0 °C over mid-latitude regions of the continents, and by more than 1.0 °C in the Arctic; however, the uncertainty of these calculations is large. We also assess the effects of changes in stratospheric ozone on climate, focusing on the poleward shift of climate zones, and discuss the role of the small Antarctic ozone hole in 2019 on the devastating “Black Summer” fires in Australia. Additional topics include the assessment of advances in measuring and modeling of UV radiation; methods for determining personal UV exposure; the effect of solar radiation management (stratospheric aerosol injections) on UV radiation relevant for plants; and possible revisions to the vitamin D action spectrum, which describes the wavelength dependence of the synthesis of previtamin D3 in human skin upon exposure to UV radiation.Graphical abstract

Multiple regression methods are applied to estimate the solar cycle variation of (1) zonal mean ozone as a function of altitude and latitude using a combination of Nimbus 7 solar backscattered ultraviolet (SBUV) and National Oceanic and Atmospheric Administration (NOAA) 11 SBUV/2 ozone profile data for a 15‐year period; (2) total ozone as a function of latitude, longitude, and season using Nimbus 7 total ozone mapping spectrometer (TOMS) data for a 13.3‐year period; (3) lower stratospheric temperature as a function of latitude, longitude, and season using microwave sounding unit (MSU) Channel 4 data for a 16‐year period; and (4) lower stratospheric geopotential height as a function of latitude, longitude, and season in the northern hemisphere using Berlin height data for a 30‐year period. According to the SBUV‐SBUV/2 data, most (about 85%) of the 1.5–2% solar cycle variation of global mean total column ozone occurs in the lower stratosphere (altitudes < 28 km). Evidence is obtained for a related solar cycle variation of lower stratospheric temperature (50–150 mbar) and geopotential height (30, 50, and 100 mbar) with geographic dependences similar to that of the solar cycle variation of total ozone. Specifically, total ozone, lower stratospheric temperature, and lower stratospheric geopotential height have annual mean solar regression coefficients in the northern hemisphere that reach a maximum near 30°N latitude within a longitude sector extending from approximately 160°E to 250°E. Maximum variations from solar minimum to maximum in this sector are approximately 11 Dobson units, 0.8 K near 100 mbar, and 60 m at 50 mbar, respectively. Seasonal solar regression coefficients tend to be statistically significant over larger areas in summer but have larger amplitudes within limited regions in winter. These geographic similarities between total ozone, lower stratospheric temperature, and geopotential height solar coefficients suggest that changes in lower stratospheric dynamics between solar minimum and maximum may play an important role in driving the observed total ozone solar cycle variation. To test this hypothesis, a simplified perturbation ozone transport model is applied to calculate the expected total ozone variation owing to dynamical forcing for the calculated geopotential height solar coefficients, climatological ozone mixing ratios, and zonal winds. For the summer season during which the solar regression coefficients are significant over the largest area, both the amplitude and latitude dependence of the observed solar cycle ozone variation are approximately consistent with the model estimates.

Total ozone data for various regions of the world have been examined on a seasonal and annual basis. The increase in total ozone noted during the 1960's appears to have ended, and at United States and British stations there was a significant 2% reduction in ozone between 1970 and 1974, although the annual race of decrease diminished from 0.7% between 1971 and 1972 to 0.2% between 1973 and 1974. In the United States the 2% reduction returned the total ozone value nearly to its 1963 level, but in Great Britain (and at Huancayo, Peru) the decrease amounted to only one-third (one-fifth) of the increase during the 1960's. The existence of a quasi-biennial oscillation in total ozone, variable in phase, amplitude, and period, makes extremely difficult the detection of any effect on the ozone shield due to the large Russian nuclear explosions in 1961 and 1962. We fail to find an unambiguous effect, and suggest that, on a seasonal or annual basis, these nuclear tests could not have reduced the hemispheric total-ozone amount by more than 1–2%. There is no evidence of a decrease in total ozone over Japan during the passage of radioactive clouds following individual Chinese tests in 1968, 1969, and 1970. The coincidence of the breakdown of the quasi-biennial oscillation in total ozone at Aspendale and Brisbane in the exact season (spring, 1963) of the eruption of Mt. Agung (8°S) is noted. For whatever reason, it appears that the Southern Hemisphere ozone values were anomalously high (or indicated to be anomalously high) following the eruption, but the lengthening of the period of quasi-biennial oscillation during this time may also be influencing, or even dominating, the results. The sunspot-total ozone relation is reexamined using the available data. There is limited evidence of a nearly in-phase relationship; accordingly, the decrease in total ozone between 1970 and 1974 may be associated with the decrease in sunspot number following the sunspot maximum in 1969. There appears to have been some decrease in total ozone in north latitudes following intense solar flares (solar proton events) in 1960, 1966, and 1972, but the relationship is not conclusive.

The total column ozone variation over India has been studied using monthly mean ozone for the period of 18 years from 1981 to 1998, which were collected with Dobson Spectrophotometer at four Indian stations, New Delhi (28° N), Varanasi (24° N), Pune (18° N), and Kodaikanal (10° N). The effect of atmospheric phenomena like Quasi Biennial Oscillation (QBO), solar cycle and volcanic eruptions (El Chichon of 1982, Mt. Pinatubo of 1991) on ozone variations has also been explored in this paper. The results of the analysis has shown that the ozone variations in the tropical region seem to have mostly caused by the equatorial QBO in zonal wind at lower stratosphere and solar cycle. During the period 1981-90, there is no much year to year variability in total ozone, whereas during the period 1991-98, much variability is observed between year to year. The total ozone is decreased from 1991 to 1993 and later started recovering. The ozone anomalies calculated for the period 1981-98 showed positive anomaly for the period 1981-90 and negative anomaly for the period 1991-98. Though the contribution of QBO and solar cycle to total ozone for the two different periods of 1981-90 and 1991-98 is almost same, the total ozone amounts for the later period are lower. These lower ozone amounts may be attributed to the Mt. Pinatubo volcanic eruption.

The paper constitutes part 2 of a study performing a first systematic inter-model comparison of the atmospheric responses to stratospheric sulfate aerosol injections (SAI) at various latitudes as simulated by three state-of-the-art Earth System Models – CESM2(WACCM6), UKESM1.0, and GISS-E2.1-G. We use a set of five sensitivity experiments with constant annual injections of SO2 in the lower stratosphere at either 30° S, 15° S, 0°, 15° N or 30° N. We identify the similarities and differences in the simulated responses amongst the models as well as demonstrate the role of biases in the climatological circulation and specific aspects of the model microphysics and chemistry in driving the inter-model differences. Building on part 1 (Visioni et al., 2022), we explain the simulated differences in the aerosol spatial distribution between the models: CESM2 shows significantly faster shallow branches of the Brewer Dobson circulation facilitating transport of the relatively larger-sized aerosol to higher latitudes; UKESM shows a relatively isolated tropical pipe and older tropical age-of-air confining the relatively smaller-sized aerosols to the tropics; and the two GISS versions with either bulk or modal aerosol microphysics show elevated sulfate levels at higher latitudes as the result of smaller aerosol sizes and relatively stronger horizontal mixing (thus very young stratospheric age-of-air). We then elucidate the role of these factors in driving the stratospheric responses to SAI. We find a large spread in the magnitudes of the tropical lower stratospheric warming amongst the models, which can be partially attributed to the differences in aerosol distribution and sizes. Regarding the stratospheric ozone responses, we find a good agreement in the tropics between the models with modal 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 to the relative agreement at low latitudes, 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. Finally, we also find large differences in stratospheric water vapour responses amongst the models, with CESM2 and GISS with modal microphysics both showing significant increases in stratospheric water vapour under SAI consistent with the increase in cold point temperatures that were largely not reproduced in UKESM. For the GISS runs with bulk microphysics, the SAI simulations show contrastingly different stratospheric responses to the models using the modal aerosol treatment, including the absence of lower stratospheric warming as well as significant reductions in stratospheric water vapour and ozone. The results point towards the importance of detailed treatment of aerosol processes, although some problems in halogen chemistry in GISS are identified that require further attention. Overall, our results contribute to an increased understanding of the underlying physical mechanisms as well as the sources of uncertainty in model projections of climate impacts from SAI.