Dynamics and the ozone distribution in the winter stratosphere : modelling the inter-hemispheric differences

Results from a two year simulation of a General Circulation Model are used to illustrate the main differences found in the lower stratosphere dynamics and the ozone distribution between the Southern and the Northern Hemispheres in winter.The model extends from ground to mesospheric levels with a spectral horizontal resolution up to isotropic wavenumber 42. It incorporates a fully interactive scheme for the ozone mixing ratio which accounts for photochemical sources and sinks, advection by the model winds and coupling with radiative calculations.The model reproduces the large scale inter hemispheric differences quite well, with a very stable and cold vortex in the Southern Hemisphere and a warmer vortex often distorted in the Northern Hemisphere. It is concluded that due to interactions between dynamics, polar stratospheric cloud formation and chemistry, there is a possibility that some stratospheric ozone depletion could be effective in late winter near the night terminator in the Northern Hemisphere, whereas significant ozone depletion only occurs in early spring in the Southern Hemisphere.The importance of synoptic scale dynamics on the ozone transport between the high latitudes and the equator is also stressed. The model develops tongues of ozone-rich air from the high latitudes which are irreversibly mixed at mid-latitudes with tongues of ozone-poor air from the low latitudes. Similar tongues or filaments are clearly visible in the TOMS satellite data. They result from the activity of medium scale-waves in the Southern Hemisphere, whereas in the Northern Hemisphere the larger scale planetary waves play a major role in their development, and their size and extension are larger. It is concluded that transport of the ozone depletion to the mid-latitudes could be more effective in the Northern than in the Southern Hemisphere.     

The role of stratospheric minor warmings in producing the total ozone deficiencies over Europe in 1992 and 1993

During 1992 and 1993, record low total ozone values were observed over the middle and high northern latitudes. The ozone data from the long-operating station at Belsk, Poland, have been used to examine their departures from climatological behaviour in 1992 and 1993. It seems that not only do the exceptionally low ozone amounts recorded over the northern mid-latitudes need an explanation but also their occurrence for two years in a row. One of the possible mechanisms which may be responsible for this event is suggested to be connected with the occurrence of stratospheric minor warmings. They occur without a breakdown of the polar vortex but only with the displacement of very cold air towards lower latitudes (as in January 1992 and February 1993). It is known that air masses in the polar vortex have been chemically disturbed and, when they arrive over the sunlit middle latitudes, chemical destruction of ozone is likely to occur. During the periods under study, the strongest negative total ozone deviations correspond to strong negative temperature deviations at 30 hPa and to large potential vorticity values; this points to the presence over Europe of air masses of polar vortex origin. It has been shown that the characteristics of mid-winter stratospheric warmings and the interannual variability of winter-spring total ozone averages at Belsk are associated with each other.     

Simulations of the behaviour of the polar thermosphere and ionosphere for northward IMF

The dynamics and structure of the polar thermosphere and ionosphere within the polar regions are strongly influenced by the magnetospheric electric field. The convection of ionospheric plasma imposed by this electric field generates a large-scale thermospheric circulation which tends to follow the pattern of the ionospheric circulation itself. The magnetospheric electric field pattern is strongly influenced by the magnitude and direction of the interplanetary magnetic field (IMF), and by the dynamic pressure of the solar wind. Previous numerical simulations of the thermospheric response to magnetospheric activity have used available models of auroral precipitation and magnetospheric electric fields appropriate for a southward-directed IMF. In this study, the UCL/Sheffield coupled thermosphere/ionosphere model has been used, including convection electric field models for a northward IMF configuration. During periods of persistent strong northward IMF B_z, regions of sunward thermospheric winds (up to 200 m s⁻¹) may occur deep within the polar cap, reversing the generally anti-sunward polar cap winds driven by low-latitude solar EUV heating and enhanced by geomagnetic forcing under all conditions of southward IMF B_z. The development of sunward polar cap winds requires persistent northward IMF and enhanced solar wind dynamic pressure for at least 2–4 h, and the magnitude of the northward IMF component should exceed approximately 5 nT. Sunward winds will occur preferentially on the dawn (dusk) side of the polar cap for IMF B_y negative (positive) in the northern hemisphere (reverse in the southern hemisphere). The magnitude of sunward polar cap winds will be significantly modulated by UT and season, reflecting E-and F-region plasma densities. For example, in northern mid-winter, sunward polar cap winds will tend to be a factor of two stronger around 1800 UT, when the geomagnetic polar cusp is sunlit, then at 0600 UT, when the entire polar cap is in darkness.     

臭氧浓度变化对馆藏银器文物腐蚀变色的影响

博物馆环境中的臭氧作为氧化剂,对文物起到氧化腐蚀的作用,会使馆藏银器文物腐蚀变色。本研究主要采用石英晶体微天平(QCM)反应性监测技术,监测电沉积金属银石英晶振片在不同臭氧浓度、不同相对湿度中的质量变化,从而推测银在臭氧中的腐蚀机理。研究结果表明,臭氧浓度对银的腐蚀具有较大的影响。随着臭氧浓度的升高,银表面质量不断增大,腐蚀程度加剧。银对相对湿度具有较小的敏感性,在不同的相对湿度下,银表面质量变化相差不大。据此推断银在臭氧中的腐蚀首先表现为银与臭氧分解产生的原子氧反应生成氧化银(Ag2O),随着Ag2O膜的不断变厚,氧化膜顶层生成过氧化银(AgO),AgO是由Ag2O与原子氧反应生成。     

On the Antarctic ozone hole

Some key observational evidence, that throws light on current ³ questions regarding the Antarctic ozone hole, is discussed. Together with dynamical theory and high-resolution numerical modelling results, the observed facts indicate that the problem has a very high degree of spatio-temporal structure in that sharp distinctions need to be made, for instance, between September and October behaviour, between behaviour near 50mb and near 100mb, and between soundings taken well inside, near the edge of, and outside the polar vortex. In winter and early spring the interior of the vortex probably behaves as an isolated material entity (at least on isentropic surfaces above about 70mb), while outside the vortex the ozone column is being increased in the classical, transport related manner, including the effect of relatively strong diabatic descent. Statistical constructs that blur these distinctions may miss important clues. The weight of evidence (as of August 1987) makes it difficult to escape the conclusion that substantial chemical destruction of ozone takes place inside the vortex in September, as originally postulated by Farmanet al.Nature315, 207 (1985).     

1987–1989 total ozone and ozone sounding observations in Northern Scandinavia and Antarctica,and the climatology of the lower stratosphere during 1965–1988 in Northern Finland

The total ozone observations of Tromsö (Northern Norway), Sodankylä (Northern Finland) and Murmansk (Northwestern Soviet Union) for 1987–1989 have been studied. Comparisons of the total ozone with stratospheric temperatures observed at Sodankylä have been made. These values have also been compared with the long-term mean total ozone at Tromsö and the long-term means of stratospheric temperatures at Sodankylä. No severe ozone depletions were observed. The exceptionally high total ozone values at these stations in February 1989 were connected to abnormally high stratospheric temperatures. The comparison of total ozone observed at roughly the same southern latitudes revealed great differences in the springtime.The 1989 ozone sounding observations of Sodankylä, Bear Island and Ny Ålesund (Spitzbergen) did not reveal any indications of pronounced ozone depletion. A comparative study of ozone, temperature and relative humidity indicated that the springtime variability of ozone in the lower stratosphere was clearly connected to meteorological variability. The lower tropospheric ozone had two distinct maxima, one in spring with large-scale photochemical causes and the other in summer connected with the emissions of hydrocarbons and oxides of nitrogen in Europe.Temperature observations made at Sodankylä over 24 yr revealed the existence of a potential for polar stratospheric cloud formation in the lower stratosphere in winter and early spring. A trend analysis of 50 hPa temperature revealed a negative trend of −0.16 K/yr in January and a positive trend of 0.15 K/yr in April; the annually-averaged trend was only −0.02 K/yr for this 24-yr period. When the January–February mean temperatures are separated according to the phase of the QBO in the tropical stratosphere, correlations between temperatures and sunspot numbers are found.     

Long-term tropospheric and lower stratospheric ozone variations from ozonesonde observations

An analysis is presented of the long-term mean pressure latitude seasonal distribution of tropospheric and lower stratospheric ozone for the four seasons covering, in part, over 20 years of ozonesonde data. The observed patterns show minimum ozone mixing ratios in the equatorial and tropical troposphere except in regions where net photochemical production is dominant. In the middle and upper troposphere, and low stratosphere to 50 mb, ozone increases from the tropics to subpolar latitudes of both hemispheres. In mid stratosphere, the ozone mixing ratio is a maximum over the tropics. The observed vertical ozone gradient is small in the troposphere but increases rapidly above the tropopause. The seasonal variation at a typical mid latitude station (Hohenpeissenberg) shows a summer maximum in the low to middle troposphere, shifting to a winter-spring maximum in the upper troposphere and lower stratosphere and spring -summer maximum at 10 mb. The amplitude of the annual variation increases from a minimum in the tropics to a maximum in polar regions. Also, the amplitude increases with height at all latitudes up to about 30 mb where the phase of the annual variation changes abruptly. The phase of the annual variation is during spring in the boundary layer, summer in mid troposphere, and spring in the upper troposphere and lower stratosphere. The annual long-term ozone trends are significantly positive at about + 1.2% yr in mid troposphere (500 mb) and significantly negative at about − 0.6% yr¹ in the lower stratosphere(50mb)     

Radiative perturbation due to the eruption of El Chichón: Effects on ozone

The ozone depletion over the Antarctic region is now attributed to processes involving heterogeneous chemistry on polar stratospheric clouds. Similar mechanisms are probably working also in the Northern hemisphere high latitudes [Douglass and Stolarski (1989) Geophys. Res. Lett. 16, 131] and may be important in explaining the secular trend of ozone in the last twenty years above 50° North [Pitari and Visconti (1991) J. geophys. Res. 96, 10,931]. Hofmann and Solomon [(1989) J. geophys. Res. 94, 5029] have shown that the local observed decrease in the ozone amount following the eruption of El Chichón could be explained in terms of heterogeneous chemistry on the volcanic aerosol surface. In this paper we use a two dimensional model to study the effects on ozone introduced by the El Chichón aerosols through a perturbation in the radiation field; both the temperature and the photolysis rates are affected. We show that up to half of the observed decrease may be attributed to radiative effects at mid latitudes.     

The middle atmospheric response to short and long term solar UV variations: analysis of observations and 2D model results

We have investigated the middle atmospheric response to the 27-day and 11-yr solar UV flux variations at low to middle latitudes using a two-dimensional photochemical model. The model reproduced most features of the observed 27-day sensitivity and phase lag of the profile ozone response in the upper stratosphere and lower mesosphere, with a maximum sensitivity of +0.51% per 1% change in 205 nm flux. The model also reproduced the observed transition to a negative phase lag above 2 mb, reflecting the increasing importance with height of the solar modulated HO_x chemistry on the ozone response above 45 km. The rnodel revealed the general anti-correlation of ozone and solar UV at 65–75 km, and simulated strong UV responses of water vapor and HO_x species in the mesosphere. Consistent with previous 1D model studies, the observed upper mesospheric positive ozone response averaged over ±40° was simulated only when the model water vapor concentrations above 75 km were significantly reduced relative to current observations. Including the observed temperature-UV response in the model to account for temperature-chemistry feedback improved the model agreement with observations in the middle mesosphere, but did not improve the overall agreement above 75 km or in the stratosphere for all time periods considered. Consistent with the short photochemical time scales in the upper stratosphere, the model computed ozone-UV sensitivity was similar for the 27-day and 11-yr variations in this region. However, unlike the 27-day variation, the model simulation of the 11-yr solar cycle revealed a positive ozone-UV response throughout the mesosphere due to the large depletion of water vapor and reduced HO_x-UV sensitivity. A small negative ozone response at 65–75 km was obtained in the 11-yr simulation when temperature-chemistry feedback was included,In agreement with observations, the model computed a low to middle latitude total ozone phase lag of +3 days and a sensitivity of +0.077% per 1% change in 205 nm flux for the 27-day solar variation, and a total ozone sensitivity of +0.27% for the 11-yr solar cycle. This factor of 3 sensitivity difference is indicative of the photochemical time constant for ozone in the lower stratosphere which is comparable to the 27-day solar rotation period but is much shorter than the 11-yr solar cycle.     

ALOMAR: atmospheric science using lidars,radars and ground based instruments

On top of the 379 m high Ramnan mountain on the island of Andøya (69°30′N, 16°01′E) in Northern Norway, the ALOMAR (The Arctic Lidar Observatory for Middle Atmospheric Research) will soon be in operation. Through measurements of different atmospheric parameters, ALOMAR will provide information on the dynamics of the middle and upper atmosphere using ground-based instrumentation. Routine measurements, including ozone observations, can be carried out more efficiently than currently possible. The observatory is currently using three LIDAR instruments, one radar and several ground-based instruments to measure density, temperature, wind profiles and aerosol densities over a height range of approximately 10 to 100 km. ALOMAR will provide scientists worldwide with the opportunity for year-round, in-depth studies of the polar middle atmosphere, concentrating on physics, chemistry and meteorology. The observatory will offer unique research opportunities, and its activities can be correlated using the Andøya Rocket Range (ARR), who operate the facility, and with other important research facilities such as the EISCAT radar, and the University of Tromsø observatories which are located nearby. There are many opportunities for additional cooperative scientific experiments using ground-based measurements and instruments carried by aircraft, balloons and sounding rockets.     

Measurements of odd oxygen in the polar region on 10 February 1984 during MAP/WINE

Abundances of atomic oxygen and ozone have been measured by various techniques over northern Scandinavia during the MAP/WINE campaign in the winter 1983–1984. On 10 February at Kiruna, Sweden, rocket experiments used resonance fluorescence and twin path absorption at 130 nm to measure [O]between 70 and 178 km. Rocket-borne measurements of nightglow at 557.7, 761.9 and 551.1 nm and at 1.27 μm have also been obtained and [O]values derived from the atmospheric band intensities. Ozone abundances between 50 and 90 km have been determined from rocket-borne measurements of the ν₃ 9.6 μm nightglow intensity from Andøya, Norway, and Kiruna. These have been compared with [O₃] measured on the same day from the Solar Mesospheric Explorer satellite, using measurements of dayglow at 1.27 μm, and with results from other rocket launchings in MAP/WINE. The results show evidence of low, perhaps exceedingly low, [O] and below normal [O₃] above the mesopause. Below 75 km at night [O₃] exceeded earlier and subsequent observations in the campaign. The measurements were made during a minor stratospheric warming, characterised by an offset polar vortex centred near the measurement zone.     

Rocket-borne measurements of atmospheric infrared fluxes

In the Energy Budget Campaign two rockets, one from Andøya Rocket Range, Norway, and one from Esrange, Sweden, each carrying a liquid helium cooled infrared spectrometer, were simultaneously launched as part of salvo B. The launches occurred during the recovery phase of the last of four auroral magnetic events after a Joule heating criteria was exceeded. At Andøya, zenith radiance altitude profiles were obtained of nitric oxide (NO) near 5.4μm from 70 to 185 km (rocket apogee), of ozone (O₃) near 9.6 μm from 70 to 105 km (instrument sensitivity) and of carbon dioxide (CO₂) near 15 μm from 70 to 150 km (instrument sensitivity). Measured CO₂ spectra at 72 km are shown to compare favorably to those calculated for local thermodynamic equilibrium conditions and instrument resolution. By comparing Andøya and Esrange CO₂ radiance profiles it is shown that there is evidence for spatial variation in the emission. Further, it is shown that the very disturbed conditions of salvo B prior to and during these launches appears to have significantly changed the O₃ 9.6μm radiance profiles compared to previous rocket measurements in polar disturbed and quiet atmospheres. Using the nitric oxide radiance profiles and spectrum, previous rocket results and computed models it is shown that no radiance increase could be detected from prompt auroral energy deposition. The results support the thesis that the NO density in auroral regions is significantly enhanced over mid-latitude values and that for weak auroras, the reaction NO(v = 0) + O → NO(r = 1) + O is the dominant radiation mechanism.     

Modelling of ozone reduction by stratospheric aircraft

Using a two-dimensional model of the atmospheric circulation and composition, different scenarios of the effects of stratospheric aircraft on ozone layer destruction were calculated. It is shown that the ozone loss depends strongly on the altitude and composition of engine emissions from high-speed civil transport aircraft. The inclusion in the two-dimensional model of the effects of chemical eddies results in significantly reduced ozone losses in the high latitudes of the northern hemisphere during wintertime, when the dynamics of the stratosphere are strongly disturbed by planetary waves. This result can be connected with the increase of stratosphere/troposphere exchange.     

Comment on connections between the 11-year solar cycle,the Q.B.O. and total ozone

Global total ozone does not show any evident connection during the period 1958–1984 with 10.7 cm solar flux (F_10.7). However, when the data are separated according to the east or west phase of the Quasi-Biennial-Oscillation (Q.B.O.) in the equatorial stratosphere, the following connection is found: when the Q.B.O. is in its west phase the global total ozone is positively correlated with the solar cycle; the opposite holds for the east phase of the Q.B.O.     

Ozone “miniholes” initiated by energetic solar protons

In this study, it is shown that during four Solar Proton Events (SPE), mostly of the Ground Level Event (GLE) type (May 1990, September and October 1989, and March 1989), inside the polar cap in the Arctic (or the Antarctic) short-term depletions were observed (up to 20%) in the ozone total content. These depletions or so-called ozone “miniholes” seem to be caused by energetic solar protons with energies of 150–300 MeV. For May 1990, the gas phase photochemical model includes only 1% ozone depletion compared with 18% observed at Barentsburg (Svalbard), and for none of the other events can homogeneous processes explain the observed depletion. The problem seems to be solved considering heterogeneous reactions in the presence of increased amounts of aerosol particles in the stratosphere which may be triggered by penetrating solar protons, or through an additional decrease of temperature, or through an increase of volume electric charge in the stratosphere (or even troposphere).     

Tropical behavior of mesospheric ozone as observed by SMM

The seasonal behavior of low latitude mesospheric ozone, as observed by the SMM satellite solar occultation experiment, is detailed for the 1985–1989 period. Annual as well as semi-annual waves are observed in the 50–70 km altitude region. In the latitude range of ±30 the ozone phase and amplitude are functions of temperature and seasonal changes in solar flux. Temperature is the controlling factor for the equatorial region and seasonal changes in solar flux become more dominant at latitudes outside the equatorial zone (greater than ±15). There is a hemispheric asymmetry in the ozone annual wave in the 20 30 region, with northern hemispheric ozone having a larger amplitude than southern hemispheric ozone. In this region temperature is nearly in phase with ozone in both hemispheres and is reduced in amplitude in the northern hemisphere. The equatorial region is characterized by a strong semi-annual wave in addition to the annual variation, and temperature is nearly out of phase with ozone. At all latitudes there is a larger ozone concentration at sunrise than at sunset. The sunrise sunset difference increases with increasing altitude     

Antarctic polar cap total electron content observations from Casey Station

In early 1990 a modified JMR-1 satellite receiver system was installed at Casey Station, Antarctica (g.g. 66.28°S, 110.54° E, -80.4°A, magnetic midnight 1816UT, L = 37.8), in order to monitor the differential phase between the 150 and 400 MHz signals from polar orbiting NNSS satellites. Total electron content (TEC) was calculated using the differential phase and Casey ionosonde foF2 data, and is presented here for near sunspot maximum in August 1990 and exactly one year later. The data are used to investigate long-lived ionization enhancements at invariant latitudes polewards of − 80° A, and the ‘polar hole’, a region from −70 to − 80° A on the nightside of the polar cap where reduced electron densitiy exists because of the long transport time of plasma from the dayside across the polar cap. A comparison is made between the Casey TEC data and the Utah State University Time Dependent Ionospheric Model (TDIM) which uses as variables the solar index (F 10.7), season (summer, winter or equinox), global magnetic index (K_p), IMF B_y direction, and universal time (UT) [sojkaet al. (1991) Adv. Space Res.11(10), 39].     

On the structure and circulation of the polar thermosphere under magnetically quiet conditions

A theoretical study is presented bearing on the thermospheric circulation and composition at polar latitudes. The observed motions and density perturbations in N₂, O and He have signatures which may be understood in terms of two different source mechanisms. We consider electric field momentum coupling and Joule heating as well as interactions between both processes. A spectral model in terms of vector spherical harmonics (with magnetic coordinates) is used, delineating the diurnal and mean (time independent) components. The important non-linearities are evaluated in configuration space. The electric field model of Volland and the global average density and temperature variations of Hedin (MSIS) are adopted as input. Our analysis leads to the following conclusions. (1) The vortex type double cell polar circulation (zonal wave number m = 1) is primarily driven by collisional momentum transfer from electric field induced ion convection. (2) Because of the thermospheric low pass filter, a large time independent component (zonal wave number m = 0) is produced by Joule heating; the heavier species (N₂) being concentrated where the lighter ones (O, He) are depleted, and vice versa due to wind induced diffusion (3) The electric field driven vortex circulation redistributes the mass and energy in the time independent density and temperature variations (from Joule heating), producing primarily diurnal variations (m = 1) in the temperature and composition near the pole and at auroral latitudes, again the heavier and lighter species varying out of phase. The above results are in substantial agreement with observations. It is worth noting that momentum rectification associated with the diurnally varying electric field and conductivity induces a weak zonally symmetric (single cell) prograde polar vortex. However, this motion is partially compensated by a retrograde vortex from geostrophic balance due to Joule heating, which dominates near the pole. These motions are small compared with the diurnally varying component in the polar circulation.     

A unified view on convection and field-aligned current patterns in the polar cap

During the last two decades measurements of polar cap ionospheric electric fields and currents, field-aligned currents, and global auroral forms have been made from ground-based and space-based platforms. An attempt is made to unify these observations into a large-scale view of polar phenomena. In this view, plasma convection patterns and the corresponding electrodynamics in the polar region can consistently be ordered by the orientation of the interplanetary magnetic field (IMF). The different patterns of the electric potential and of field-aligned currents depend on where the main interaction between the terrestrial and interplanetary fields occurs, on the morning or evening side of the central polar cap, or on the dayside portion of the ‘closed’ cusp region, or on the nightside portion of the ‘open’ cusp region. One of the essential elements of this unified view is that it is possible to account for various convection patterns ranging from the four-cell pattern (during periods of strong northward IMF and B_y ~ 0), to the three-cell pattern (B_z > 0 and |B_y| 2> 0), to the conventional two-cell pattern (B_z < 0) with its possible deformation into a convection throat near the dayside cusp (during southward IMF). We also discuss the way in which the complicated field-aligned current systems can consistently be accounted for in terms of these convection patterns.