On the interpretation of seasonal variations of stratospheric ozone

Insight into the causes of the annual and semi-annual ozone oscillations may be gained from the analysis of photochemical model behavior. In this paper, the monthly variations of the ozone mixing ratio computed by the two-dimensional photochemical model of Garcia and Solomon (1983, J. geophys. Res. 88, 1379) are Fourier-analyzed and compared with SBUV observations of ozone mixing ratio. Remarkably good qualitative agreement between the model calculations and the observations is found. Analysis of computed transport and chemical production and destruction rates reveals the causes of the modelled seasonal ozone variations.

It will be shown that at high latitudes and low altitudes, modelled ozone abundances increase in the winter due to transport and decrease in the summer due to chemical destruction. In the middle stratosphere, the calculated annual ozone variation is largely due to the annual variation in the odd-oxygen production rate, and in the upper stratosphere, the computed annual ozone variation is caused by the large calculated annual oscillation in temperature. Comparison between the model and observations suggests that the equatorial semi-annual oscillation above 10 mb is caused mainly by the semi-annual temperature and wind oscillation (SAO). Below 10 mb the computed equatorial ozone variation is caused by the increased rates of odd-oxygen production associated with the semi-annual zenith crossings of the Sun. Finally, the calculated polar semi-annual ozone oscillations are found to be caused by modulation of the radiatively driven middle-stratospheric ozone variation by temperature dependent chemical destruction processes.     

Annual and semi-annual zonal wind components and corresponding temperature and density variations, 60–130 km

From rocket and radar-meteor wind observations, annual and semi-annual components of the zonal flow are derived for latitudes N at heights between 60 and 130 km. Height regions of maximum and minimum amplitude are described with reference to changes in phase. The annual components decrease with height throughout the mesosphere and, after a reversal of phase, enhance to 25 m/sec at 100 ± 5 km. The semi-annual components have maximum amplitudes of 25 m/sec over a wide range of latitude in two height regions at 90 and 120 km and in a limited range of latitude (near 50°) at 65 km.

Calculated temperatures and log densities are discussed in terms of amplitude and phase as functions of height and latitude. Below 100 km a comparison is made with temperature amplitudes derived from independent temperature data. Above 100 km the annual temperature variation maximizes at 115 km and is particularly large at high latitudes (exceeding 50°K). On the other hand, the semi-annual component increases rapidly with height between 110 and 120 km at all latitudes maximizing at the 120 km level, where amplitudes exceed 25°K at high and low latitudes and 10°K at mid-latitudes. The annual component of log density, like the temperature variation, is largest at high latitudes up to 125 km. The semi-annual variation has a minimum at 110–115 km, above which amplitudes increase with height, reaching 5–12 per cent at 130 km according to latitude. The phases at and near 130 km for the annual and semi-annual density variations are very close to those found at greater heights from satellite orbits and amplitudes could be readily extrapolated to agree with those in the satellite region.     

A numerical response of the middle atmosphere to the 11-year solar cycle

A two-dimensional numerical model with coupled photochemistry and dynamics has been used to investigate the response of the middle atmosphere (16–116 km) to changes in solar activity over the 11-year solar cycle. Model inputs that vary with solar cycle include solar radiation, cosmic ray and auroral ionization rates and the flux of NO_x at the model's upper boundary.In this study, the results of model runs for solar cycle minimum and maximum conditions are compared. In the stratosphere, using currently accepted estimates of changes in solar radiation at wavelengths longer than 180 nm, only small responses in ozone, temperature and zonal winds are obtained. On the other hand, changes at shorter wavelengths, and the effects of particle precipitation, lead to large variations in the abundances of trace species in the thermosphere and upper mesosphere. In particular, very large abundances of NO_x are produced above 90 km by auroral particle precipitation. Considerable amounts of NO_x are transported subsequently to the stratosphere by the global mean meridional circulation. It is shown that this excess NO_x can lead to significant decreases in ozone concentrations at high latitudes and that it may explain observations of nitrate deposition in Antarctic snow.     

Chemical response of the middle atmosphere to changes in the ultraviolet solar flux

Time variations in the solar flux between 1000 and 4000 Å induce changes in the concentrations of minor constituents in the upper stratosphere and mesosphere. The response of mesospheric ozone to variations in the Lyman α line over the course of several solar rotations may be of measurable magnitude. Large Lyman α fluxes lead to small O₃ densities above 65 km due to the enhanced dissociation of H₂O and resultant destruction of odd oxygen by odd hydrogen. An increase in continuum and Lyman α fluxes causes a slight enhancement in both the odd oxygen and hydrogen concentrations in the upper stratosphere.     

Transport of thermospheric NO to the upper stratosphere?

The rate of production of NO in the thermosphere is expected to vary greatly over the course of an 11-year solar cycle because the fluxes of both extreme ultraviolet radiation and auroral particles are known to increase substantially from solar minimum to solar maximum. In the stratosphere, NO participates in a catalytic cycle which constitutes the dominant photochemical destruction mechanism for stratospheric ozone. If appreciable long range transport of NO from the thermosphere to the upper stratosphere occurs, its effects should therefore be manifested in upper atmospheric ozone density variations over the 11-year solar cycle. In this paper, model predictions of the seasonal and latitudinal variations in upper stratospheric O₃ associated with NO transport for different levels of solar activity are compared to satellite observations of upper stratospheric ozone abundances.     

An overview of sage I and II ozone measurements

The Stratospheric Aerosol and Gas Experiments (SAGE) I and II measure Mie, Rayleigh, and gaseous extinction profiles using the solar occultation technique. These global measurements yield ozone profiles with a vertical resolution of 1 km which have been routinely obtained for the periods from February 1979 to November 1981 (SAGE I) and October 1984 to the present (SAGE II). The long-term periodic behavior of the measured ozone is presented as well as case studies of the observed short-term spatial and temporal variability.

A linear regression shows annual, semi-annual, and quasi-biennial oscillation (QBO) features at various altitudes and latitudes which, in general, agree with past work. Also, ozone, aerosol, and water vapor data are described for the Antarctic springtime showing large variation relative to the vortex. Cross-sections in latitude and altitude and polar plots at various altitudes clearly delineate the ozone hole vertically and areally. Comparisons of vertical profiles are made from 1979 to 1988.

Although there is a three-year gap between the SAGE I and II measurements, the two data sets have been used to determine long-term changes in ozone. The intercomparison generally shows decreases in the upper stratosphere (25–50 km) of 4% or less from 1980 to 1986.     

Discussion of the 60 year total ozone record at Arosa based on measurements of the vertical distribution and a meteorological parameter

On G. M. B. Dobson's initiative, ozone measurements were started by Götz at Arosa, Switzerland, in 1926, which led to the longest total ozone record in the world over 60 years. Later these measurements were supplemented by Umkehr observations, also at Arosa, and by ozone soundings at Payerne, Switzerland, yielding the concurrent vertical distribution which allows among other things to distinguish between regional and hemispheric scale processes influencing total ozone. Using a meteorogical parameter a method is developed which allows the extension of this distinction to the early period without measurements of the vertical distribution, although with somewhat larger uncertainty. On this basis the ozone variations over Arosa during the past 60 years are discussed.

The ozone loss around the level of the ozone maximum contributed most to the strong decline in the total amount observed between 1970 and 1988. High loss rates were further detected in the upper stratosphere, extending down into the middle stratosphere. While the upper stratospheric decrease is very probably largely a consequence of the CFC input into the atmosphere the loss at the lower level seems to be only partly due to that reason, but in its main contribution to be produced by circulation changes. About one-third of the stratospheric ozone decline is compensated by rapidly rising concentrations in the troposphere caused by increasing air pollution.     

Wind variations in Jupiter's equatorial atmosphere: A QQO counterpart?

Jupiter's equatorial atmosphere, much like the Earth's, is known to show quasi-periodic variations in temperature, particularly in the stratosphere, but variations in other jovian atmospheric tracers have not been studied for any correlations to these oscillations. Data taken at NASA's Infrared Telescope Facility (IRTF) from 1979 to 2000 were used to obtain temperatures at two levels in the atmosphere, corresponding to the upper troposphere (250 mbar) and to the stratosphere (20 mbar). We find that the data show periodic signals at latitudes corresponding to the troposphere zonal wind jets, with periods ranging from 4.4 (stratosphere, 95% confidence at 4° S planetographic latitude) to 7.7 years (troposphere, 97% confidence at 6° N). We also discuss evidence that at some latitudes the troposphere temperature variations are out of phase from the stratosphere variations, even where no periodicity is evident. Hubble Space Telescope images were used, in conjunction with Voyager and Cassini data, to track small changes in the troposphere zonal winds from 20° N to 20° S latitude over the 1994-2000 time period. Oscillations with a period of 4.5 years are found near 7°-8° S, with 80-85% significance. Further, the strongest evidence for a QQO-induced tropospheric wind change tied to stratospheric temperature change occurs near these latitudes, though tropospheric temperatures show little periodicity here. Comparison of thermal winds and measured zonal winds for three dates indicate that cloud features at other latitudes are likely tracked at pressures that can vary by up to a few hundred millibar, but the cloud altitude change required is too large to explain the wind changes measured at 7° S.     

An analysis of the strong zonal circulation within the stratosphere of Venus

A dynamical model is presented for the observed strong zonal circulation within the stratosphere of Venus. The model neglects rotational effects and considers a compressible and radiating atmosphere. It is shown that diurnal radiative heating is negligible within the lower stratosphere, a region below 85km, while observational evidence for the strong zonal circulation pertains to the lower stratosphere within which a direct thermal driving for the circulation is absent. The analysis, however, suggests that propagating internal gravity waves generated by diurnal solar heating of the upper stratosphere induce mean zonal velocities within the upper and lower stratosphere.Considering the linearized equations of motion and energy, and following Stern's (1971) analysis for an analogous problem, it is shown that the zonal velocity induced by internal gravity waves is retrograde in direction, a result which is in agreement with observation. The nonlinear equations of motion and energy are then solved by an approximate analytical method to determine the magnitude of the zonal velocity. This velocity increases from zero at the tropopause to about 200 msec^?1 at the 85 km level. The velocity near the uv-cloud level compares favorably with the observed value of 100 msec^?1.     

Ozone variability

The data base for the ozone total amount and the vertical distribution is considered. On examination it becomes evident that the analysis of the global data set of relatively short duration must be combined with that of single stations with long series. It is found that the strong positive trend during the sixties could be attributable to the effect of changes in the general circulation and the associated influence on ozone transport. However, the NO_x injection into the stratosphere by the bomb tests in the late fifties and the early sixties cannot be ruled out as a cause either. The ozone content at different levels in the stratosphere is influenced by circulation patterns on different scales and thus gives different types of information. A solar cycle influence on the ozone content in the upper stratosphere seems—if the contribution of volcanic disturbance is also taken into consideration—compatible with the observational series at Arosa. However, due to the noisy character of the ozone concentration course at that level it is not possible to derive any reliable long-term trend.     

The temperature variations in the troposphere,stratosphere, and mesosphere of the Northern Hemisphere, 1965–1981

We have used temperature data obtained from radiosondes and rocketsondes for the time interval 1965–1981 to estimate the interconnection of mean-annual temperature fluctuations at the various layers from the surface to the lower mesosphere of the Northern Hemisphere. Profiles of coefficients of correlation of the mean-annual temperature at each layer with mean-annual temperature at higher layers are shown for locations in the low, middle, and high latitudes. It is suggested that the mean-annual temperature variations at high latitudes of the troposphere are related with mean-annual temperature variations of the high latitudes of the lower stratosphere. Also, the mean-annual temperature variations at the high latitudes of the lower stratosphere are connected with mean-annual temperature variations at the high latitudes of the upper stratosphere. Furthermore, the mean-annual temperature variations of the upper stratosphere have an impressive correlation with mean-annual temperature variations of the lower mesosphere for whole northern hemisphere.     

Stratospheric ozone: Impact of human activity

Current knowledge of the chemistry of the stratosphere is reviewed using measurements from the Atmospheric Trace Molecule Spectroscopy (ATMOS) experiment to test the accuracy of our treatment of processes at mid-latitudes, and results from the Airborne Antarctic Ozone Experiment (AAOE) to examine our understanding of processes for the polar environment. It is shown that, except for some difficulties with N₂O₅ and possibly ClNO₃, gas phase models for nitrogen and chlorine species at 30°N in spring are in excellent agreement with the data from ATMOS. Heterogeneous processes may have an influence on the concentrations of NO₂, N₂O₅, HNO₃, and ClNO₃ for the lower stratosphere at 48°S in fall. Comparison of model and observed concentrations of O₃ indicate good agreement at 30°N, with less satisfactory results at 48°S. The discrepancy between the loss rate of O₃ observed over the course of the AAOE mission in 1987 and loss rates calculated using measured concentrations of ClO and BrO is found to be even larger than that reported by Anderson et al. (1989, J. geophys. Res. 94, 11480). There appear to be loss processes for removal of O₃ additional to the HOC1 mechanism proposed by Solomon et al. (1986, Nature 321, 755), the ClO-BrO scheme favored by McElroy et al. (1986, Nature 321, 759), and the ClO dimer mechanism introduced by Molina and Molina (1987, J. phys. Chem. 91, 433). There is little doubt that industrial halocarbons have a significant impact on stratospheric O₃. Controls on emissions more stringent than those defined by the Montreal Protocol will be required if the Antarctic Ozone Hole is not to persist as a permanent feature of the stratosphere.     

Three-dimensional simulations of ozone in the stratosphere and comparison with UARS data

A 3-D Atmospheric Chemical Transport model has been developed and used to simulate the present-day ozone distributions in the troposphere and stratosphere. A 5-year-long steady-state model run using 1995 boundary conditions and circulation fields derived from the 24-layer University of Illinois at Urban a-Champaign (UIUC) Atmospheric General Circulation model has been carried out. The simulated distribution of ozone is compared with available observations made by the HALOE, CLAES and MLS instruments onboard the LIARS satellite. The comparison is carried out for the monthly zonal-mean climatology of the ozone distribution. The correlations between the monthly zonal-mean ozone derived from the simulated and measured data are calculated. The results of this comparison show reasonable agreement (within 30%) of the simulated and measured monthly zonal-mean ozone distributions, although the location of the simulated maximum in the ozone distribution is generally lower by about 2–3 km than shown by the satellite data. The model overestimates the ozone mixing ratio in the lower stratosphere and slightly underestimates it in the upper stratosphere. A better overall agreement was found between the simulated ozone and the ozone measured by HALOE than by CLAES and MLS.     

O3 Profiles from GOME satellite data—II: Observations in the Arctic Spring 1997 and 1998

Ozone observations by the Global Ozone Monitoring Experiment (GOME) on board the ERS-2 satellite during the Arctic spring periods 1997 and 1998 are presented. From the derived ozone vertical distributions, extensive regions of low ozone total column were observed and it is shown that the major decrease is dominating in the lower and middle stratosphere inside the polar vortex. The winter 1997/98 was warmer than the year before and less ozone depletion was observed. In spring 1998 an ozone mini-hole event was observed by GOME and ozone profiles under minihole conditions were derived for the first time.     

North-south asymmetries of solar particle events in upper stratospheric ozone

Stratospheric ozone depressions, following intense solar particle events (SPE) observed by the backscattered ultraviolet (BUV) experiment on the Nimbus-4 satellite, indicate the existence of distinct asymmetries between the Northern and Southern Hemispheres. These asymmetries are observed in the magnitude of the depressions above the 5-mb level, their temporal variations, and the spatial (i.e., latitude and longitude) dependence of these variations. Possible causes of asymmetries, shown by two events on 4 August 1972 and 25 January 1971, can be attributed to: (1) tilt of the interplanetary magnetic field (IMF) with respect to the Earth's dipole magnetic field which influences the precipitation of energetic solar particles into the polar atmospheres; (2) differences in ozone chemistry caused by the large change in atmospheric temperature between summer and winter hemispheres; (3) seasonal differences of the stratosphere's dynamic states which are affected by upward propagating planetary waves in winter in contrast to the relatively undisturbed zonal flow in summer; (4) topographic asymmetry between Northern and Southern Hemispheres.These effects are shown by three-dimensional plots of the events in geographic coordinates and by color contour plots of the stratospheric ozone distributions in geomagnetic and geographic polar coordinates, respectively.     

Cyclic Variations of CME Velocity

The semiannual mean CME velocities for the time interval of 1979–1989 have been analyzed to reveal a complex cyclic variation with a peak at the solar cycle maximum and a secondary peak at the minimum of the cycle. The growth of the mean CME width is accompanied by a growth of the mean CME velocity. It is shown that the cyclic variations of the mean CME velocity and the mean CME width are associated with the cyclic variations of the large-scale magnetic field structure and that the secondary peak of the semiannual mean CME velocity in 1985–1986 is due to a significant contribution of fast CMEs with a width of 100° at the minimum of the cycle. This peak is supposed to be due to the increasing role of the global large-scale magnetic field system with a characteristic size of cells of 70°–100° at the minimum of the cycle and the respective particularities of the large-scale magnetic field configuration in the corona.     

Origin of the sunspot modulation of ozone: Its implications for stratospheric NO injection

The measured modulation of cosmic rays deposited in the stratosphere over a sunspot cycle produces an oscillating source of stratospheric NO with an 11-yr (quasi) period. The resulting modulation of ozone over this period is calculated and is shown to give good agreement with available measurements of the time lag, the latitude dependence and the magnitude of cyclic variations of ozone. This correlated modulation is then used to discuss the effect on ozone of the injection of NO into the stratosphere from artificial sources, viz. a fleet of supersonic transports and nuclear bomb explosions in the atmosphere.     

Diagnostics of Titan's stratospheric dynamics using Cassini/CIRS data and the 2-dimensional IPSL circulation model

The dynamics of Titan's stratosphere is discussed in this study, based on a comparison between observations by the CIRS instrument on board the Cassini spacecraft, and results of the 2-dimensional circulation model developed at the Institute Pierre-Simon Laplace, available at http://www.lmd.jussieu.fr/titanDbase [Rannou, P., Lebonnois, S., Hourdin, F., Luz, D., 2005. Adv. Space Res. 36, 2194-2198]. The comparison aims at both evaluating the model's capabilities and interpreting the observations concerning: (1) dynamical and thermal structure using temperature retrievals from Cassini/CIRS and the vertical profile of zonal wind at the Huygens landing site obtained by Huygens/DWE; and (2) vertical and latitudinal profiles of stratospheric gases deduced from Cassini/CIRS data. The modeled thermal structure is similar to that inferred from observations (Cassini/CIRS and Earth-based observations). However, the upper stratosphere (above 0.05 mbar) is systematically too hot in the 2D-CM, and therefore the stratopause region is not well represented. This bias may be related to the haze structure and to misrepresented radiative effects in this region, such as the cooling effect of hydrogen cyanide (HCN). The 2D-CM produces a strong atmospheric superrotation, with zonal winds reaching 200 m s⁻¹ at high winter latitudes between 200 and 300 km altitude (0.1-1 mbar). The modeled zonal winds are in good agreement with retrieved wind fields from occultation observations, Cassini/CIRS and Huygens/DWE. Changes to the thermal structure are coupled to changes in the meridional circulation and polar vortex extension, and therefore affect chemical distributions, especially in winter polar regions. When a higher altitude haze production source is used, the resulting modeled meridional circulation is weaker and the vertical and horizontal mixing due to the polar vortex is less extended in latitude. There is an overall good agreement between modeled chemical distributions and observations in equatorial regions. The difference in observed vertical gradients of C₂H₂ and HCN may be an indicator of the relative strength of circulation and chemical loss of HCN. The negative vertical gradient of ethylene in the low stratosphere at 15° S, cannot be modeled with simple 1-dimensional models, where a strong photochemical sink in the middle stratosphere would be necessary. It is explained here by dynamical advection from the winter pole towards the equator in the low stratosphere and by the fact that ethylene does not condense. Near the winter pole (80° N), some compounds (C₄H₂, C₃H₄) exhibit an (interior) minimum in the observed abundance vertical profiles, whereas 2D-CM profiles are well mixed all along the atmospheric column. This minimum can be a diagnostic of the strength of the meridional circulation, and of the spatial extension of the winter polar vortex where strong descending motions are present. In the summer hemisphere, observed stratospheric abundances are uniform in latitude, whereas the model maintains a residual enrichment over the summer pole from the spring cell due to a secondary meridional overturning between 1 and 50 mbar, at latitudes south of 40-50° S. The strength, as well as spatial and temporal extensions of this structure are a difficulty, that may be linked to possible misrepresentation of horizontally mixing processes, due to the restricted 2-dimensional nature of the model. This restriction should also be kept in mind as a possible source of other discrepancies.     

Temporal variations of Titan’s middle-atmospheric temperatures from 2004 to 2009 observed by Cassini/CIRS

We use five and one-half years of limb- and nadir-viewing temperature mapping observations by the Composite Infrared Radiometer-Spectrometer (CIRS) on the Cassini Saturn orbiter, taken between July 2004 and December 2009 (L_S from 293° to 4°; northern mid-winter to just after northern spring equinox), to monitor temperature changes in the upper stratosphere and lower mesosphere of Titan. The largest changes are in the northern (winter) polar stratopause, which has declined in temperature by over 20 K between 2005 and 2009. Throughout the rest of the mid to upper stratosphere and lower mesosphere, temperature changes are less than 5 K. In the southern hemisphere, temperatures in the middle stratosphere near 1 mbar increased by 1-2 K from 2004 through early 2007, then declined by 2-4 K throughout 2008 and 2009, with the changes being larger at more polar latitudes. Middle stratospheric temperatures at mid-northern latitudes show a small 1-2 K increase from 2005 through 2009. At north polar latitudes within the polar vortex, temperatures in the middle stratosphere show a ∼4 K increase during 2007, followed by a comparable decrease in temperatures in 2008 and into early 2009. The observed temperature changes in the north polar region are consistent with a weakening of the subsidence within the descending branch of the middle atmosphere meridional circulation.     

Median ionospheric height variations over a sunspot cycle in the Australian-Japanese longitudinal sector

The monthly median virtual height (h′F) of the F-region was studied for a period of 6 years (1980–1985) from sunspot maximum to minimum, using data from 11 ionosonde stations in the Japanese-Australian longitudinal sector, in an invariant latitude range: 37°N to 54°S. The night-time maximum in the median height progressively decreases equatorwards, particularly in the local winter and spring, while a reverse weak tendency is observed in summer. The median height reaches peak in both hemispheres from 1 to 2 years after sunspot maximum then decreases towards sunspot minimum. A second diurnal maximum in h′F, preceded by a well-defined minimum, was consistently observed over the solar cycle close to the sunrise time at the F-region, mainly at low invariant latitudes (9–20°). The second maximum has a distinct seasonal variation, being most pronounced in winter and diminishing in summer. It is envisaged that the second peak in h′F is associated with the wave disturbance generated by the supersonic motion of the sunrise terminator. Possible effects of the background height variations on the propagation of the magnetic storm-induced travelling ionospheric disturbances are discussed.