Tropospheric ozone variations governed by changes in stratospheric circulation

Transcription

1 SUPPLEMENTARY INFORMATION DOI: /NGEO2138 Tropospheric ozone variations governed by changes in stratospheric circulation Jessica L. Neu* (1), Thomas Flury (1,2), Gloria L. Manney (3,4), Michelle L. Santee (1), Nathaniel J. Livesey (1), John Worden (1) (1) JPL/Caltech (2) Now at Swiss Federal Office of Public Health (3) NorthWest Research Associates (4) Also at New Mexico Institute of Mining and Technology TES and MLS measurements The TES 1,2 and MLS 3 instruments were launched on the NASA Earth Observing System Aura satellite in July The Aura satellite is in a sun-synchronous polar orbit and provides near-global coverage from 82S-82N with a 16-day repeat cycle. TES is a Fourier Transform Spectrometer that measures in the thermal infrared ( cm -1 ). While TES has several observational modes and measures multiple species, here we use only Global Survey nadir ozone measurements, which have a footprint of 5.3x8.5 km and an along-track separation of ~182 km. There are ~3400 TES measurements per Global Survey when measurements are taken over the entire latitude range of the orbit. In cloud-free conditions, TES nadir ozone profiles have approximately 4 degrees of freedom for signal (DOFS), with ~2 in the troposphere and ~2 in the stratosphere (below ~5 hpa). This is equivalent to a vertical resolution of ~6-7 km. TES retrievals and error estimation are based on the optimal estimation approach 4,5. In the tropical and Northern Hemisphere (NH) middle to upper troposphere, the TES version 4 ozone measurements used here are positively biased with respect to ozonesondes by 10-20%, and there are relatively small seasonal differences in the bias in northern midlatitudes 6,7. The TES observation error of ~20% includes effects from measurement noise and radiative interferences NATURE GEOSCIENCE 1

2 that are effectively random. We aggregate approximately 5000 observations in the 30-50N monthly mean, reducing this error to approximately 0.3%, which is small enough to be ignored. We use TES data from June 2005-Dec Prior to June 2005, TES alternated nadirviewing scans with limb-viewing scans, resulting in ~1/3 the number of nadir ozone profiles per Global Survey compared to measurements taken after that date. Due to instrument aging, the latitudinal coverage was reduced in June 2008 to 60S-82N and again in July 2008 to 50S-70N. From January to March 2010, the instrument was offline as a result of problems with the scanning mechanism. When operations resumed in April 2010, the latitude coverage was further reduced to 30S-50N. TES measurements are available beyond December 2010, but while the data quality remains unchanged, multiple long data gaps make the more recent data poorly suited for this study. TES currently operates only in Special Operations mode over selected regions. For more information, see MLS measures thermal emission from Earth s limb in five broad spectral regions from 118 GHz to 2.5 THz. MLS measurements have a horizontal (along-track) resolution of km and a spacing of 165 km, with ~3500 profiles per day. The vertical resolution of the measurements is ~3 4 km. As for TES, MLS retrievals are performed via optimal estimation 4,8. In the lower stratosphere, the version 2.2 MLS ozone measurements used here are biased high by 10-20% relative to measurements from other satellite instruments, aircraft, and balloons 9. The precision on the ~5000-profile 40-50N monthly averages shown here is better than 0.2%. A more recent version of MLS ozone (v3.3) is available, but was not used here due to unphysical vertical oscillations in tropical ozone 10 that complicated interpretation of the mean El Niño and La Niña ozone distributions (Figure 3). Replacing the 40-50N v2.2 ozone time series with v3.3 has no impact on our results. Version 3.3 water vapor measurements, which we use to infer the 2

3 magnitude of the tropical upwelling as described below, are measured with a single-profile accuracy of ~8% and a precision of ~10% in the lower stratosphere 11. More information can be found at Mechanisms of QBO and ENSO Impacts on the Stratospheric Circulation and Ozone The QBO is a quasi-periodic oscillation in zonal winds in the tropical stratosphere characterized by descending easterly and westerly wind regimes, with a period of months 12. Since the QBO zonal wind signal descends from the upper stratosphere to the tropopause, at any given time the sign of the tropical wind changes with height: easterlies overlie westerlies during the easterly shear phase, while westerlies overlie easterlies during the westerly shear phase. The QBO modulates tropical upwelling directly through thermal wind balance: the vertical wind shear induces a two-celled meridional circulation centered on the tropics, with anomalous upwelling in the tropics during the easterly shear phase and downwelling during the westerly shear phase 12,13. During the equinoxes, the QBO circulation cells are approximately symmetric about the equator and confined to the tropics, with downward (upward) flow in the subtropics to balance the equatorial upwelling (downwelling) 12, and thus have little impact on the net tropical upwelling or on the large-scale downwelling at midlatitudes. During the solstices, however, the summer hemisphere cell is virtually nonexistent and the winter hemisphere cell extends much deeper into midlatitudes, modulating both the mean tropical upwelling and mean midlatitude downwelling 14,15. The QBO hpa Easterly and Westerly shear phases all occur during NH winter; thus the direct QBO circulation impacts the entire NH tropical-midlatitude large-scale circulation. 3

4 In addition to the direct modulation of the circulation by the QBO through thermal wind balance, changes in subtropical zonal wind structure associated with the QBO alter the propagation and dissipation of planetary-scale waves in the winter hemisphere extratropics 16,17,18. These waves play a predominant role in driving the circulation 19,20,21. They propagate on the westerly winds of the winter hemisphere and break near the transition between extratropical westerlies and tropical easterlies, resulting in rapid mixing of air in the stratospheric midlatitude surf zone 22. Conservation of angular momentum requires a poleward flow to balance the wave momentum deposition, and this flow is balanced by upwelling in the tropics and downwelling at middle and high latitudes 19,20, with most mass transport in the winter hemisphere. The QBO modulates the latitude of the planetary-scale wave breaking region and thus indirectly impacts the strength of the overturning circulation 15,23. This mechanism explains the existence of QBO variability in ozone at high latitudes, beyond the extent of the QBO direct circulation cell 15,23. Studies indicate that ENSO also alters the stratospheric circulation by modulating the propagation and dissipation of the waves that drive it, which include not only planetary-scale waves, but also synoptic-scale waves and gravity waves 24,25,26. There is, however, disagreement as to the exact mechanisms involved. Some models suggest that ENSO primarily impacts the circulation via increased (decreased) upward propagation of planetary-scale waves during El Niño (La Niña) 24, while others attribute the response primarily to a shift in the location of gravity wave breaking resulting from changes in the strength of the subtropical jet 25, 26 or to anomalous synoptic-scale wave activity in the SH 25. Unlike the QBO, ENSO affects tropospheric ozone through mechanisms other than the STE ozone flux. In the tropics, as seen in Figure 3 and discussed in the manuscript, ENSO-driven shifts in the location of convective activity have a measureable effect on tropospheric ozone; in 4

5 fact, an ozone-based ENSO index has been derived from tropical tropospheric column ozone measurements 27. ENSO also alters biomass burning activity, and in some cases increased tropical tropospheric ozone during El Niño has been linked to increased precursor emissions from biomass burning 28,29,30. We examined the relationship between biomass burning and tropospheric ozone, both averaged over 30-50N, using time-varying biomass burning emissions from the Global Fire Emission Database version 3 (GFED3) 31. We found no correlation between changes in tropospheric ozone and changes in biomass burning in northern midlatitudes during the time period. ENSO has also been shown to have an impact on midlatitude storm tracks 32, and in one study a shift in the storm track over Asia, which altered the lofting of ozone precursor emissions into the free troposphere 33, led to increased midlatitude tropospheric ozone over Europe during El Niño 34. However, the Asian contribution to the total ozone column over Europe was ¼ the contribution of the enhanced STE ozone flux following El Niño Derivation of Tropical Upwelling Rates Water vapor abundances near the tropical tropopause show a pronounced seasonal cycle, with low values during NH winter and high values during summer. Because the tropics are relatively isolated from midlatitudes throughout most of the stratosphere 35, the seasonal cycle signal is carried upward by the stratospheric circulation from ~100 hpa to ~10 hpa with little attenuation except just above the tropopause, where there is rapid transport of air to midlatitudes. The propagation speed of the resulting water vapor tape recorder signal 36 is thus a good approximation to the vertical component of the stratospheric residual (mass transport) circulation 37, which cannot be directly measured. 5

6 Our tropical upwelling rates 38 are calculated from daily zonal mean water vapor values averaged between 8S-8N. We calculate the optimal time lag for the correlation between a segment of the water vapor profile spanning three pressure levels (for example: 56, 46, and 38 hpa) and the segment of the profile starting one pressure level higher (for example: 46, 38, and 32 hpa) on subsequent days 39. Using geopotential height from MLS to estimate the altitude of the pressure levels, the vertical velocity for the midpoint of each layer is calculated from the distance between the pressure levels divided by the lag. We then average the velocity over the hpa region and compute monthly mean values. While the MLS water vapor measurements represent the upwelling in the deep tropics, most models and analyses indicate that the annual mean net upwelling region extends to the subtropics (~35S-35N) in the middle stratosphere 40,41. We have thus compared variability in the hpa residual vertical velocity over the past 30 years averaged over 10S-10N to that averaged over 35S-35N for the Whole Atmosphere Chemistry Climate Model 42 (WACCM) nudged to the Modern Era Retrospective Analysis (MERRA) wind fields 43, which have realistic ENSO and QBO variability, to examine whether the variations in deep tropical upwelling are representative of the broader upwelling region. The 35S-35N residual vertical velocity variability is highly correlated with the variability over 10S-10N (R=0.63) but is ~35% smaller in magnitude. It is likely that at least some of this difference arises from fact that the QBO-induced direct circulation cell is strongest near the equator Stratosphere-Troposphere Ozone Correlations The use of correlations between stratospheric and tropospheric ozone to attribute changes in tropospheric ozone to the STE ozone flux has been criticized because it is really changes in mass 6

7 flux, rather than changes in stratospheric ozone, that drive variability in the STE flux 44. However, in reality, stratospheric ozone variability is tightly coupled to variability in the mass flux 44,45. It is clear in our study that the changes in northern midlatitude lower stratospheric ozone we observe are, in fact, driven by changes in the mass flux since 1) the ozone variability is correlated with the variability in tropical upwelling and 2) while decreases in ozone could theoretically be driven by chemical loss or increased transport through the lower branch of the circulation, the increases we see during El Niño/easterly shear QBO can only arise from increased transport poleward and downward from the ozone maximum. To test the validity of the correlation between stratospheric and tropospheric ozone as a proxy for the STE ozone flux, we replaced the STE flux from the University of California, Irvine chemistry-transport model (UCI CTM) in the analysis described in Section 5 below with the 40-50N lower stratospheric ozone from the model, and find almost no difference in the relationship with tropospheric ozone Comparison to Chemistry-Transport Model Output A recent study using the UCI-CTM with linearized stratospheric ozone chemistry and a simple parameterized boundary layer ozone sink found a compact linear relationship between the annual mean hemispherically averaged STE ozone flux (diagnosed from the modeled ozone tendency at each timestep 46 and mean tropospheric ozone, with slope of 0.03 ppb/tg 47. This translates to a 25% increase in STE ozone flux resulting in a ~7% increase in mean tropospheric ozone. We have re-run the same simulations with full tropospheric chemistry and analyzed the relationship between the monthly mean STE ozone flux averaged over 40-50N and 500 hpa ozone averaged over 30-50N in order to compare directly with the observations shown here. We 7

8 find that a 25% change in the STE flux in the model results in a 2% change in 500 hpa ozone, in very good agreement with our observational results Southern Hemisphere Ozone Response to and ENSO/QBO In Figure 3 of the manuscript, we show latitude-height cross-sections of the mean ozone anomalies for the time period of the strongest La Niña/westerly shear QBO (3a, November August 2008) and strongest El Niño/easterly shear QBO (3b, June 2009-August 2010) in the record. The ozone anomalies are not symmetric between the hemispheres, particularly in the UTLS region and the troposphere. The primary reason for the asymmetry is the synchronicity between ENSO/QBO and the seasonal cycle. The upwelling anomalies associated with both the La Niña/westerly shear QBO and El Niño/easterly shear QBO occur during NH fall/winter, when most of the stratospheric circulation mass flux flows into the NH. The poleward and downward flow through the upper branch of the circulation in the summer hemisphere is very weak, and most transport that does occur is through the lower branch of the circulation, which is driven primarily by synoptic-scale rather than planetary-scale waves 48. The fact that the SH UTLS ozone anomalies are the same sign as those in the tropics likely reflects the dominance of this lower branch transport during these time periods. Tropospheric ozone is largely decoupled from stratospheric ozone during the summer, when the STE ozone flux is at a minimum Long-Term Changes in Tropical Upwelling Stratosphere-resolving chemistry-climate models predict increases in tropical upwelling for 100 hpa <p< 1 hpa over the next century in response to increasing greenhouse gases 40,41. For the 8

9 hpa vertical domain of our water vapor observations, the increase in upwelling seen in models is ~15-35% 40,41, which is comparable in magnitude to the interannual variability in upwelling we derive from MLS water vapor. However, the modeled long-term changes in the circulation are calculated as the average over the entire upwelling region, which extends into the subtropics, while our observations represent upwelling in the deep tropics. We use WACCM 42 along with the Canadian Middle Atmosphere Model (CMAM) 49 to examine whether the magnitude of the long-term change in the deep tropics (10S-10N) is similar to that over the entire upwelling region (30S-30N). The mean increase in upwelling is 14% for WACCM and 28% for CMAM when averaged over 10S-10N versus 13% for WACCM and 27% for CMAM when averaged over 30S-30N, indicating that the long-term changes in the circulation in the deep tropics and over the entire upwelling region are indeed similar CAM-Chem Simulation The CAM-Chem results described in the manuscript are from a version simulation based on the Chemistry Climate Model Validation activity Ref-B2 protocol 41, with prescribed GHG concentrations from the Intergovernmental Panel on Climate Change A1b scenario 50 and ozone depleting substances from the World Meteorological Organization A1 scenario 51 adjusted to include a 2030 phaseout of HCFCs 41. Sea surface temperatures and sea ice concentrations were prescribed from a Community Climate System Model version 3 simulation 52. The critical difference between the CAM-Chem simulation discussed here and the Ref-B2 protocol is that ozone and aerosol precursors were held constant in the CAM-Chem simulation, so that long-term changes in tropospheric ozone are primarily driven by changes in temperature, humidity, production of NO x by lightning, and the STE ozone flux 53,54. The critical 9

10 difference between the CAM-Chem simulation discussed here and the Ref-B2 protocol is that ozone and aerosol precursors were held constant in the CAM-Chem simulation, so that long-term changes in tropospheric ozone are primarily driven by changes in temperature, humidity, production of NO x by lightning, and the STE ozone flux 53,54. Increasing humidity results in increased ozone loss through the reaction of O( 1 D) with water vapor, but this reaction is of primary importance in the tropics and boundary layer (particularly over oceans), which is also where the largest increases in humidity are expected to occur 53. Some models predict small changes in lightning NO x over the next century, but the impact on ozone is expected to be largest in the tropical upper troposphere 53,54. Thus, we expect the 5% increase in northern midlatitude tropospheric ozone in the CAM-Chem simulation to be primarily associated with increases in temperature and the STE ozone flux. Furthermore, because chemical ozone depletion is small in the NH, the changes in stratospheric ozone and the STE ozone flux from are dominated by the increase in the stratospheric circulation rather than ozone recovery over this period hpa temperatures in NH midlatitudes are expected to increase by ~2 o C by ; given a value for the ozone dependence on temperature of 2 ppb/ o C 55, the associated increase in mid-tropospheric ozone would be ~6%. This is a maximum estimate because the ozonetemperature dependence was derived from surface observations 55, where NO x concentration and chemical ozone production are much larger than in the mid-troposphere. If the modeled 24% increase in midlatitude lower stratospheric is associated with a 2% increase in tropospheric ozone as suggested by the interannual variability from the model and the observations, then the increases in mid-tropospheric associated with temperature and with the STE ozone flux are within a factor of 3 of one another

11 221 References Beer, R., T. Glavich, A. & Rider, D. M. Tropospheric Emission Spectrometer for the Earth Observing System s Aura satellite. Appl. Opt. 40, (2001) Beer, R., TES on the Aura mission: Scientific objectives, measurements, and analysis overview. IEEE Trans. Geosci. Remote Sens. 44, (2006) Waters, J. W., et al. The Earth Observing System Microwave Limb Sounder (EOS MLS) on the Aura satellite. IEEE Trans. Geosci. Remote Sens. 44, (2006) Rodgers, C. Inverse Methods for Atmospheric Sounding: Theory and Practice (World Scientific, London, 2000) Bowman, K. W., et al. Tropospheric Emission Spectrometer: Retrieval method and error analysis. IEEE Trans. Geosci. Remote Sens. 44, (2006) Nassar, R., et al. Validation of Tropospheric Emission Spectrometer (TES) nadir ozone profiles using ozonesonde measurements. J. Geophys. Res. 113, D15S17 (2008) Boxe, C. S., et al. Validation of northern latitude Tropospheric Emission Spectrometer stare ozone profiles with ARC-IONS sondes during ARCTAS: sensitivity, bias and error analysis. Atmos. Chem. Phys. 10, (2010). 11

12 Livesey, N.J., Snyder, W.V., Read, W.G. & Wagner, P.A. Retrieval algorithms for the EOS Microwave Limb Sounder (MLS) instrument. IEEE Trans. Geosci. Remote Sens. 4, (2006) Froidevaux, L., et al. Validation of Aura Microwave Limb Sounder stratospheric and mesospheric ozone measurements. J. Geophys. Res. 113, D15S20 (2008) Livesey, N. J., et al. Interrelated variations of O 3, CO and deep convection in the tropical/subtropical upper troposphere observed by the Aura Microwave Limb Sounder (MLS) during Atmos. Chem. Phys. 13, (2013) Read, W. G., et al. Aura Microwave Limb Sounder upper tropospheric and lower stratospheric H2O and relative humidity with respect to ice validation. J. Geophys. Res. 112, D24S35 (2007) Baldwin, M. P., et al. The quasi-biennial oscillation, Rev. Geophys. 39(2), (2001) Plumb, R. A. & Bell, R. C. A model of the quasi-biennial oscillation on an equatorial beta- plane. Q.J.R. Meteorol. Soc. 108, (1982) Kinnersley, J. S. Seasonal Asymmetry of the Low- and Middle-Latitude QBO Circulation Anomaly. J. Atmos. Sci. 56, (1999). 12

13 Kinnersley, J. S. & Tung, K. K. Mechanisms for the Extratropical QBO in Circulation and Ozone. J. Atmos. Sci. 56, (1999) Holton, J. R. & Tan, H.C. The Influence of the Equatorial Quasi-Biennial Oscillation on the Global Circulation at 50 mb. J. Atmos. Sci. 37, (1980) Holton, J. R. & Tan, H. C. The quasi-biennial oscillation in the Northern Hemisphere lower stratosphere. J. Meteor. Soc. Japan 60, (1982) Dunkerton, T.J. & Baldwin, M. P. Quasi-biennial modulation of planetary wave fluxes in the northern hemisphere winter. J. Atmos. Sci. 48, (1991) Haynes, P. H., McIntyre, M. E., Shepherd, T. G., Marks, C. J., & Shine, K. P.: On the Downward Control of Extratropical Diabatic Circulations by Eddy-Induced Mean Zonal Forces. J. Atmos. Sci. 48, (1991) Holton, J. R., Haynes, P. H., McIntyre, M. E., Douglass, A. R., Rood, R. B. & Pfister, L. Stratosphere-troposphere exchange, Rev. Geophys. 33, (1995) Zhou, T., Geller, M. A. & Lin, W. An observational study on the latitudes where wave forcing drives Brewer-Dobson upwelling. J. Atmos. Sci. 69, (2012)

14 McIntyre, M. E. & Palmer, T. N. The surf zone in the stratosphere. J. Atmos. Terr. Phys. 46, (1984) Tung, K. K. & Yang, H. Global QBO in Circulation and Ozone. Part II: A Simple Mechanistic Model. J. Atmos. Sci. 51, (1994) Garcia-Herrera, R., Calvo, N., Garcia, R.R. & Giorgetta, M.A. Propagation of ENSO temperature signals into the middle atmosphere: A comparison of two general circulation models and ERA-40 reanalysis data. J. Geophys. Res. 111, D06101 (2006) Simpson I. R., Shepherd, T. G. & Sigmond, M. Dynamics of the lower stratospheric circulation response to ENSO. J. Atmos. Sci. 68, (2011) Calvo, N., Garcia, R. R., Randel, W. J. & Marsh, D. R. Dynamical mechanism for the increase in tropical upwelling in the lowermost tropical stratosphere during warm ENSO events. J. Atmos. Sci. 67, (2010) Ziemke, J. R., Chandra, S., Oman, L. D. & Bhartia, P. K. A new ENSO index derived from satellite measurements of column ozone. Atmos. Chem. Phys. 10, (2010) Logan, J. A., et al. Effects of the 2006 El Niño on tropospheric composition as revealed by data from the Tropospheric Emission Spectrometer (TES). Geophys. Res. Lett. 35, L03816 (2008). 14

15 Nassar, R., et al. Analysis of tropical tropospheric ozone, carbon monoxide, and water vapor during the 2006 El Niño using TES observations and the GEOS-Chem model. J. Geophys. Res. 114, D17304 (2009) Livesey, N. J., et al. Interrelated variations of O 3, CO and deep convection in the tropical/subtropical upper troposphere observed by the Aura Microwave Limb Sounder (MLS) during Atmos. Chem. Phys. 13, (2013) van der Werf, G. R., et al. Global fire emissions and the contribution of deforestation, savanna, forest, agricultural, and peat fires ( ). Atmos. Chem. Phys. 10, (2010) Eichler, T. & Higgins, W. Climatology and ENSO-Related Variability of North American Extratropical Cyclone Activity. J. Climate 19, (2006) Liu, H., et al. Transport pathways for Asian pollution outflow over the Pacific: Interannual and seasonal variations. J. Geophys. Res. 108, D (2003) Koumoutsaris, S., Bey, I., Generoso, S. & Thouret, V. Influence of El Niño Southern Oscillation on the interannual variability of tropospheric ozone in the northern midlatitudes. J. Geophys. Res. 113, D19301 (2008)

16 Plumb, R. A. A tropical pipe model of stratospheric transport. J. Geophys. Res. 101, (1996) Mote, P. W., et al. An atmospheric tape recorder: The imprint of tropical tropopause temperatures on stratospheric water vapor. J. Geophys. Res. 101, (1996) Schoeberl, M. R., et al. Comparison of lower stratospheric tropical mean vertical velocities. J. Geophys. Res. 113, (2008) Flury, T., Wu, D. L. & Read, W. G. Variability in the speed of the Brewer Dobson circulation as observed by Aura/MLS. Atmos. Chem. Phys. 13, (2013) Niwano, M., Yamazaki, K. & Shiotani, M. Seasonal and QBO variations of ascent rate in the tropical lower stratosphere as inferred from UARS HALOE trace gas data. J. Geophys. Res. 108, 4794 (2003) Butchart, N., et al. Simulations of anthropogenic change in the strength of the Brewer- Dobson circulation. Clim. Dynam. 27, (2006) Eyring, V., Shepherd, T. G., & Waugh, D. W. (eds). SPARC Report on the Evaluation of Chemistry Climate Models, SPARC Report No. 5, WCRP-132, WMO/TD-No.1526 (2010)

17 Kinnison, D. E., et al. Sensitivity of chemical tracers to meteorological parameters in the MOZART-3 chemical transport model, J. Geophys. Res., 112, D20302 (2007) Rienecker, M. M., et al. MERRA: NASA s Modern-Era Retrospective Analysis for Research and Applications. J. Climate, 24, (2011) Voulgarakis, A., Hadjinicolaou, P. & Pyle, J. A. Increases in global tropospheric ozone following an El Niño event: Examining stratospheric ozone variability as a potential driver. Atmos. Sci. Lett. 12, (2011) Salby, M. L. & Callaghan, P. F. Influence of the Brewer-Dobson circulation on stratosphere-troposphere exchange, J. Geophys. Res. 111, D21106 (2006) Hsu, J., Prather, M. J. & Wild, O. Diagnosing the stratosphere-to-troposphere flux of ozone in a chemistry transport model. J. Geophys. Res. 110, D19305 (2005) Hsu, J. & Prather, M. J. Stratospheric variability and tropospheric ozone. J. Geophys. Res. 114, D06102 (2009) Plumb, R. A. Stratospheric transport. J. Meteor. Soc. Japan 80, (2002) Hegglin, M. I. & Shepherd, T. G. Large climate-induced changes in ultraviolet index and stratosphere-to-troposphere ozone flux, Nature Geosci., 2, (2009). 17

18 Solomon, S., et al. (eds). Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, 2007 (Cambridge Univ. Press, Cambridge, 2007) WMO (World Meteorological Organization) Scientific Assessment of Ozone Depletion: 2006, Global Ozone Research and Monitoring Project - Report No. 50 (Geneva, Switzerland 2007) Collins, W. D., et al. The Community Climate System Model Version 3 (CCSM3). J. Climate 19, (2006) Stevenson, D., et al. Impacts of climate change and variability on tropospheric ozone and its precursors, Faraday Discuss. 130, (2005) Stevenson, D., et al. Tropospheric ozone changes, radiative forcing and attribution to emissions in the Atmospheric Chemistry and Climate Model Intercomparison Project (ACCMIP). Atmos. Chem. Phys. 13, (2013) Bloomer, B. J., Stehr, J. W., Piety, C. A., Salawitch, R. J. & Dickerson, R. R. Observed relationships of ozone air pollution with temperature and emissions, Geophys. Res. Lett. 36, L09803 (2009) 18