Ozone Perturbations Research Articles

Stratosphere adjusted radiative forcing calculationsin a comprehensive climate model

A new technical procedure is introduced to determine the stratosphere adjusted radiative forcing at the tropopause in the framework of the 3-D climate model ECHAM4. However, the procedure appears to be appropriate for other GCMs as well. It allows to study in a straightforward way the problem of the general usefulness of radiative forcing as a reliable predictor of climate change. Some examples are given for illustration. It is, once again, confirmed that the climate sensitivity is practically equal for experiments with increased solar insolation and increased CO2 concentration. However, a higher climate sensitivity is obtained for ozone perturbations with a horizontally or vertically inhomogeneous distribution. The latter finding is in qualitative agreement with respective results reported in other studies, whereas the value of the climate sensitivity is exceptionally high in our model. The physical reasons for the unique model behaviour in the ozone experiments are currently not understood.

NOx Emissions from Aircraft: Its Impact on the Global Distribution of CH4 and O3 and on Radiative Forcing

Model studies of the impact of aircraft emissions on atmospheric ozone, and on the methane lifetime have been performed, using a global 3-D CTM (The Oslo CTMl). The calculated changes in the global distribution of ozone and methane have been used to calculate Radiative Forcing (RF) of the current and future fleets (2015 and 2050) of subsonic aircraft. The calculations show that ozone perturbation from aircraft emission occur predominantly in the Northern Hemisphere at mid and high latitudes. Maximum increase in ozone is found in the upper troposphere in the 10 to 12 km height region. The annual average future increases in the region of maximum increase, is estimated to be approximately 12 ppb in 2015. Scenarios for NOx emissions in 2050 are less certain, however; calculations give a near linear increase in global ozone perturbation for increasing NOx, in the future over the range of NOX, emissions assumed to be realistic until 2050. Calculations with NOx, emissions well beyond this range (50 % higher than the high NOx, emission) showed non-linear ozone response with less ozone increase per NOx increase than in the cases with lower NOx emission from aircraft. There is a reduction in the methane lifetime due to enhanced OH from NOx emissions. The change in the global methane lifetime is estimated to be -1.3 % in 1992 from aircraft emissions, increasing to -3.9 % in 2050. Estimates of RF from CH4 and O3 due to aircraft emissions are of opposite sign and of similar magnitude, which makes it difficult to give reliable estimates of the net impact. The regional patterns in RF for the two compounds are however highly different. It is therefore likely that the radiative impact from NOx, emissions could be larger than what can be obtained from global average RFs.

Radiative forcing and climate sensitivity: The ozone experience

AbstractThe climatic impact of ozone perturbations is studied with a general‐circulation model by considering both the radiative forcing at the tropopause and the change in the near‐surface temperature. In particular, we focus on the vertical sensitivity by imposing the perturbations in different vertical layers. the simulations are done in perpetual‐January mode and with climatological sea ice. the main part of the analysis is focused on global‐mean values and in particular on the surface energy budget. Moderate sensitivities related to weak positive feedbacks are found when the perturbation is applied to the troposphere or the lower stratosphere, while larger sensitivities are found for perturbations in the higher stratosphere. This difference in the feedbacks is argued to be related to the vertical partitioning of the radiative forcing determined by the relative strengths of the long‐wave and short‐wave forcings. While the larger part of the short‐wave forcing, which dominates for ozone perturbations in the higher stratosphere, penetrates to the surface, the long‐wave forcing is predominantly deposited in the free troposphere. When the model is forced at the surface, latent‐heat flux effectively transports energy to the free troposphere restoring the lapse rate to its unperturbed value. Conversely, when the forcing is located in the free troposphere transport to the surface is limited. For the experiments under consideration the radiative forcing and the climate sensitivity are found to be robust and only slightly sensitive to the definition of the tropopause. As a measure of climate change the forcing adjusted for thermal changes in the stratosphere is superior to both the instantaneous forcing and to the forcing obtained after dynamical changes in the stratosphere.

Climate effect of ozone changes caused by present and future air traffic

The potential of aircraft-induced ozone changes to force a substantial climate impact is investigated by means of simulations with an atmospheric general circulation model, coupled to a mixed layer ocean model. We present results from several numerical experiments that are based on ozone change patterns for 1992 aviation and on a future scenario for the year 2015. In both cases, the climate signal is statistically significant. The strength of the ozone impact is of comparable magnitude to that arising from aircraft CO2 emissions, thus meaning a non-negligible contribution to the total climate effect of aviation emissions. There are indications of a characteristic signature of the aircraft ozone related temperature response pattern, distinctly different from that associated with the increase of well-mixed greenhouse gases. Likewise, the climate sensitivity to non-uniform ozone changes including a strong concentration perturbation at the tropopause may be higher than the climate sensitivity to uniform changes of a greenhouse gas. In a hierarchy of experiments, for which the spatial structure of an aircraft-related ozone perturbation was left fixed, while the amplitude of the perturbation was artificially increased, the climate signal depends in a non-linear way on the radiative forcing.

The present-day and future impact of NO<sub>x</sub> emissions from subsonic aircraft on the atmosphere in relation to the impact of NO<sub>x</sub> surface sources

Abstract. The effect of present-day and future NOx emissions from aircraft on the NOx and ozone concentrations in the atmosphere and the corresponding radiative forcing were studied using a three-dimensional chemistry transport model (CTM) and a radiative model. The effects of the aircraft emissions were compared with the effects of the three most important anthropogenic NOx surface sources: road traffic, electricity generation and industrial combustion. From the model results, NOx emissions from aircraft are seen to cause an increase in the NOx and ozone concentrations in the upper troposphere and lower stratosphere, and a positive radiative forcing. For the reference year 1990, the aircraft emissions result in an increase in the NOx concentration at 250 hPa of about 20 ppt in January and 50 ppt in July over the eastern USA, the North Atlantic Flight Corridor and Western Europe, corresponding to a relative increase of about 50%. The maximum increase in the ozone concentrations due to the aircraft emissions is about 3-4 ppb in July over the northern mid-latitudes, corresponding to a relative increase of about 3-4%. The aircraft-induced ozone changes cause a global average radiative forcing of 0.025 W/m2 in July. According to the ANCAT projection for the year 2015, the aircraft NOx emissions in that year will be 90% higher than in the year 1990. As a consequence of this, the calculated NOx perturbation by aircraft emissions increases by about 90% between 1990 and 2015, and the ozone perturbation by about 50-70%. The global average radiative forcing due to the aircraft-induced ozone changes increases by about 50% between 1990 and 2015. In the year 2015, the effects of the aircraft emissions on the ozone burden and radiative forcing are clearly larger than the individual effects of the NOx surface sources. Taking chemical conversion in the aircraft plume into account in the CTM explicitly, by means of modified aircraft NOx emissions, a significant reduction of the aircraft-induced NOx and ozone perturbations is realised. The NOx perturbation decreases by about 40% and the ozone perturbation by about 30% in July over Western Europe, the eastern USA and the North Atlantic Flight Corridor.Keywords. Atmospheric composition and structure (troposphere · composition and chemistry) · Meteorology and atmospheric dynamics (radiative processes)

Impact of future subsonic aircraft NOx emissions on the atmospheric composition

The effect of future subsonic aircraft emissions on the chemical composition of the atmosphere is investigated by means of the coupled dynamic‐chemical circulation model ECHAM3/CHEM. Present (1992) and future (2015 and 2050) atmospheric compositions are simulated with three sets of time slice experiments. Expected changes of the sea surface temperatures (SST), surface emissions of CFCs, greenhouse gases (GHG), and NOx (=NO+NO2) are prescribed. Model results show future increases in NOx and O3 due to aircraft NOx emissions, which scale almost linearily with the magnitude of the emission. The calculated climate change reduces the NOx concentration as well as its increase due to aircraft emissions by more than 10% at flight altitude in 2015. However, the aircraft induced ozone perturbation decreases only slightly when climate change is regarded.

Effects of lightning and convection on changes in tropospheric ozone due to NOx emissions from aircraft

A global 3-dimensional chemical tracer model (CTM) has been used to calculate the impact on tropospheric ozone caused by NOx emissions from today’s fleet of subsonic aircraft (0.52 Tg(N)/ yr). Uncertainties in the magnitude and distribution in the ozone perturbation due to uncertainties in lightning and deep convection as sources of upper tropospheric NOx have been studied. Three sets of two CTM experiments have been performed, with and without emissions from aircraft. A reference set with normal convection and 12 Tg(N)/ yr from lightning, a set with reduced lightning source (5 Tg(N)/ yr), and one set with reduced convective activity (67% lower mass fluxes of air). A zonally homogeneous increase in upper tropospheric ozone north of 40°N, reaching a maximum of 5–6 ppbv during May was found in the reference case. The largest effect of lower NOx emissions from lightning was a 50–70% higher enhancement of ozone due to aircraft at northern mid- and high-latitudes during summer. This was caused by lower background concentrations of NOx and therefore more effcient ozone production. Reduced convective mixing lead to a 40% increased enhancement in aircraft induced ozone at northern mid-latitudes and 150% enhancement in the tropics. In this case background NOx levels were higher in the upper troposphere, giving a decreased ozone production efficiency of NOx from aircraft. This was however, more than compensated for by a decreased downward mixing of ozone produced by emissions from aircraft.

Estimates of radiative forcing due to modeled increases in tropospheric ozone

The GFDL R30 general circulation model (GCM) and a fixed dynamical heating model (FDHM) are used to assess the instantaneous and adjusted radiative forcing due to changes in tropospheric ozone caused by anthropogenic activity. Ozone perturbations from the GFDL global chemical transport model are applied to the GCM, and the instantaneous solar and terrestrial radiative forcings are calculated excluding and including clouds. The FDHM is used to calculate the adjusted radiative forcing at the tropopause. The net global annual mean adjusted radiative forcing, including clouds, ranges from +0.29 to +0.35 W m−2 with ∼80% of this forcing being in the terrestrial spectrum. If stratospheric adjustment is ignored, the forcing increases by ∼10%, and if clouds are ignored, the radiative forcing increases by a further 20–30%. These results are in reasonable agreement with earlier studies and suggest that changes in tropospheric ozone due to anthropogenic emissions exert a global mean radiative forcing that is of similar magnitude but of opposite sign to the direct forcing of sulfate aerosols.

The effects of sulfur emissions from HSCT aircraft: A 2‐D model intercomparison

Four independently formulated two‐dimensional chemical transport models with sulfate aerosol microphysics are used to evaluate the possible effects of sulfur emissions from high‐speed civil transport (HSCT) aircraft operating in the stratosphere in 2015. Emission scenarios studied are those from Baughcum and Henderson [1995], while assumptions regarding the form of emitted sulfur are similar to those of Weisenstein et al. [1996]. All models show much larger increases in aerosol surface area when aircraft sulfur is assumed to be emitted as particles of 10 nm radius rather than as gas phase SO2. If we assume an emission index (EI) for SO2 of 0.4 gm (kg fuel burned)−1 in 2015, maximum increases in stratospheric sulfate aerosol surface area range from 0.1 μm2 cm−3 to 0.5 μm2 cm−3 with sulfur emitted as SO2 gas and from 1.0 μm2 cm−3 to 2.5 μm2 cm−3 with sulfur emitted as particles. Model differences in calculated surface area are deemed to be due mainly to differences in model transport. Calculated annual average ozone perturbations due to aircraft emissions with EI(NOx) = 5, EI(H2O) = 1230, and EI(SO2) = 0.4 range from −0.1% to −0.6% at 45°N for sulfur emission as SO2 gas and from −0.4% to −1.5% with sulfur emission as 100% particles. The effect of zonal and temporal inhomogeneities in temperature on heterogeneous reactions rates is accounted for in the Atmospheric and Environmental Research model and the Université degli Studi L'Aquila model and significantly increases the calculated ozone depletion due to HSCT, particularly for the cases with concurrent increases in aerosol surface area. Sensitivities to polar stratospheric clouds, background chlorine amount, additional heterogeneous reactions, and background aerosol loading are also explored.

Aerosol particle evolution in an aircraft wake: Implications for the high‐speed civil transport fleet impact on ozone

Previous calculations of the ozone impact from a fleet of high‐speed civil transports (HSCTs) have been carried out by global two‐dimensional (2‐D) models [Bekki and PyIe, 1993; Pitari et al., 1993] which have not included explicit wake processing of sulfur species. This processing could be important for the global sulfate aerosol and ozone perturbations [Weisenstein et al., 1996]. For an HSCT scenario with emission indices of NOx and sulfur equal to 5 and 0.4, respectively, and a cruise speed of Mach 2.4 [Stolarski and Wesoky, 1993b], the Atmospheric and Environmental Research (AER) 2‐D model gives 0.50–1.1% as the range of the annually averaged O3 column depletion at 40°–50°N. This range is determined by the extreme assumption that emitted SO2 is diluted into the global model grid box either as gas or as 10 nm sulfate particles. A hierarchy of models is used here to investigate the impact of processes in the wake on the calculated global ozone response to sulfur emissions by a proposed HSCT fleet. We follow the evolution of aircraft emissions from the nozzle plane using three numerical models: the Standard Plume Flowfield‐II/Plume Nucleation and Condensation model (SPF‐II/PNC), an AER far wake model incorporating microphysics of aerosol particles, and the AER global 2‐D chemistry‐transport model. Particle measurements in the wake of the Concorde [Fahey et al., 1995a] are used to place constraints on sulfur oxidation processes in the engine and the near field. To explain the Concorde measurements, we consider cases with different fractions of SO3 (2%, 20%, and 40%) in the sulfur emissions at the nozzle plane and also the possibility of other unknown heterogeneous or homogeneous oxidation processes for SO2 in the wake. Assuming similar characteristics for the proposed HSCT fleet, the global ozone response is then calculated by the 2‐D model. Using the model‐calculated wake processing of sulfur emissions under the above assumptions and constrained by the Concorde particle measurements, the range of annually averaged O3 column depletion at 40°–50°N is reduced from 0.5–1.1% to 0.75–1.0%. Our analysis shows that the global ozone response is more sensitive to the assumed partitioning of sulfur emissions between SO2 and SO3 at the nozzle plane than to the wake dilution rate. Outstanding uncertainties and recommendations for further wake‐sampling experiments are also discussed.

Exploring the effects of forcing quasi‐biennial oscillations in a two‐dimensional model

Analytic forcing of the stratospheric quasi‐biennial oscillation (QBO) is introduced into a two‐dimensional middle atmosphere model containing interactive radiation, dynamics, photochemistry and climatological aerosols. The “WISCAR” model integrates the temperature equation in time, but diagnoses the meridional stream function and zonal wind. An analytic forcing function for the QBO is derived from zonal wind observations and employed in three different ways: (1) as a “thermal nudge” in the temperature equation, or in the diagnostic meridional stream function equation as (2) additional heating or (3) equivalent wave driving. A different amplification factor for each method is required to achieve good agreement with the observed QBO in column ozone. This lends insight into the relationship among thermal perturbations, heating, and vertical motion. The vertical variation of QBO amplitude leads to a vertical dependence of the phase relationship among vertical motion, temperature, and zonal wind. Equatorial upward motions range from nearly zero to twice the time mean. Feedbacks in the model result in different extratropical responses for the two hemispheres and modulation of the equatorial semiannual oscillation. Chemical feedbacks and phase relationships are explored for the two ozone regimes: photochemical control above 30 km and advective control beneath. Confirming other studies, QBO vertical motions alter the distribution of odd nitrogen species above 30 km, which, together with the temperature dependence of reaction rates, combine to exert a strong control on ozone perturbations.

A 3D chemistry transport model study of changes in atmospheric ozone due to aircraft NO x emissions

The effect of present day aircraft emissions of nitrogen oxides (NO x = NO + NO 2) on atmospheric NO x and ozone concentrations is investigated with the global three-dimensional chemistry transport model CTMK. This model uses 12-hourly meteorological data from the ECMWF analysis and includes parameterizations for subgrid scale processes such as convection. CTMK includes an ozone chemistry module containing the methane and carbon monoxide oxidation chain for the troposphere and lower stratosphere. It is found by using the ANCAT aircraft NO x emission invertory that aviation contributes to 20–100 pptv of the NO x at cruise altitudes in northern mid-latitudes, which corresponds to 30–80 and 20–50% of the background mixing ratios for January and July, respectively. This perturbation in NO x occurs mainly in the North Atlantic Flight Corridor and is transported eastwards. The resulting increase in upper tropospheric ozone is 2–3 ppbv (2%) in January and 5–10 ppbv (3%) in July. The ozone perturbation is almost zonally symmetric and attains maximum values at northern mid-latitudes in January and in the polar region in July. The calculated effect of aircraft emissions is found to be small (i.e. less than 5 pptv for NO x and less than 1 ppbv for 0 3) in the southern hemisphere. The perturbation of NO x by the aircraft emissions at cruise altitudes in northern mid-latitudes is large compared to the standard deviation. Therefore, it is expected that the effect of aviation on NO x is distinguishable from the contribution from other NO x sources. As the modelled natural variability of ozone is already about 30%, it will not be easy to detect the ozone perturbation due to aircraft NO x emissions.

On the response of a three‐dimensional general circulation model to imposed changes in the ozone distribution

A series of idealized ozone perturbations has been studied with the French community general circulation model Arpège. The perturbations are uniform 50% and 75% reductions and an ozone hole type reduction concentrated in the lower part of the stratosphere and the upper part of the troposphere. We compare the radiative drive, the temperature response under the fixed dynamical heating approximation, and the full dynamical response. In all experiments the main effect of the dynamics is a weakening of the diabatic meridional circulation and an associated latitudinal smoothing of the temperature response. The weakening of the diabatic meridional circulation is in agreement with a reduction of the wave forcing, associated with an extended tongue of low refractive index close to the tropopause. While the fixed dynamical heating approximation in general represents the pattern of the temperature response well, the strength of the response is altered when allowing dynamical effects. In the lower stratosphere this adjustment is 25%–50% of the fixed dynamical heating response. The mean zonal wind is very stable to uniform reductions, but the strength of the winter stratospheric jet increases drastically in the hole type experiment. The ozone perturbations decrease the variability in the winter hemisphere, which in the control experiment and the uniform reduction experiments is of oscillatory nature with a timescale of 50 to 100 days.

Modeling the spatial distribution of total ozone in northern hemisphere winter: 1979–1991

A form of the linearized steady state ozone continuity equation, expressed in terms of perturbations in lower stratospheric temperature and geopotential height, is used to model the spatial distribution (i.e., deviations from the zonal mean) of total ozone in northern hemisphere (NH) winter for the period 1979–1991. Lower stratospheric temperatures from channel 4 of the microwave sounding unit (MSU), geopotential height and temperature analyses provided by the Stratospheric Research Group at the Free University of Berlin (FUB), and climatological values of the zonal mean ozone mixing ratio are used to calculate the ozone mixing ratio perturbation for a given location at three levels in the lower stratosphere: 100, 50, and 30 mbar. These values are then summed together to provide an estimate of the total ozone perturbation at northern extratropical latitudes for the months of January and February. By applying this modeling approach to individual months on a year‐to‐year basis, much of the observed year‐to‐year variability in the spatial distribution of total ozone is approximately simulated. A significant decrease of the wave‐induced variance in the wintertime total ozone distribution over the 1979–1991 period is indicated, consistent with the observed longitude dependences of total ozone trends in NH winter.

Simulation of the Meridionally and Seasonally Varying Climate Response Caused by Changes in Ozone Concentration

Abstract Recent model studies have indicated that observed stratospheric ozone decline can have a cooling effect on climate. This study intends to investigate the climate response due to changes in the radiative fluxes caused by prescribed changes in stratospheric as well as tropospheric ozone. For this purpose the authors use a simplified climate model, basically consisting of an energy balance atmosphere model coupled to an advection–diffusion ocean model. The coupled climate model simulates the latitudinal and seasonal variations in zonal mean surface air temperature and the average lower (12–22 km) and higher (22–100 km) stratospheric temperatures. First, the quasi-equilibrium response of the model to various uniform ozone perturbations is examined. For instance, a uniform 50% reduction in lower stratospheric ozone results in a global average cooling of 3.5°C in the lower stratosphere with maximum values in the Tropics and of 0.46°C at the surface with maximum cooling in the polar winter. The latter i...

Radiative forcing and temperature trends from stratospheric ozone changes

Detailed shortwave and longwave radiative transfer models are used to calculate the radiative forcing and temperature trends due to stratospheric ozone depletion. These were calculated using the fixed dynamical heating approximation to adjust the stratospheric temperatures. Recent estimates of stratospheric ozone loss between 1979 and 1991 (from solar backscattered ultraviolet (SBUV) and stratospheric aerosol and gas experiment (SAGE) instruments) and updated radiative transfer schemes are used to obtain improved estimates of the radiative forcings. An annually and globally averaged radiative forcing of −0.13±0.02 W m−2 decade−1 (−0.22±0.03 W m−2 for the 1979–1996 period) was found from SBUV total column ozone trends, applying a constant percentage ozone depletion to a 7 km thick layer directly above the tropopause. SAGE ozone trends gave forcing estimates of −0.10±0.02 W m−2 decade−1 (−0.17±0.03 W m−2 for the 1979–1996 period), although assumptions needed to be made about the choice of the vertical profile of the ozone depletion below 17 km. Using Dobson instrument trends from 1964 to 1996, the total ozone forcing could be as negative as −0.26±0.05 W m−2. The quoted error bars derive from uncertainties in the total ozone trends. Using these values, the stratospheric ozone change may have offset about 30% of the forcing due to increases in well‐mixed greenhouse gases since 1979, and about 15% of the forcing since 1964, at least on a global and annual mean. The ozone forcings are shown to be nearly a linear function of the ozone amount depleted from the atmospheric column, provided the vertical profile of the depletion remains constant. As in previous studies, it was found that stratospheric adjustment altered the sign of the ozone forcing by strongly cooling the lower stratosphere. It is shown that (depending on the vertical structure of ozone depletion) coolings of up to 0.4 K decade−1 can be found at altitudes of 35 km; this provides a mechanism for cooling the stratosphere nearly as large as that from well‐mixed greenhouse gas increases, at altitudes where ozone changes were previously thought not to strongly affect stratospheric temperature trends. Previous studies have examined the effect of ozone trends by looking at the response of the surface temperature to a fixed absolute ozone change at different heights in the atmosphere. We argue that using absolute ozone perturbations places an unrealistically large emphasis on ozone changes near the tropopause. As the vertical profile of ozone change is more often reported as a percentage change, we present the sensitivity of the surface temperature to constant percentage, rather than absolute, changes in ozone.

An Estimation of the Climatic Effects of Stratospheric Ozone Losses during the 1980s

In order to study the potential climatic effects of the ozone hole more directly and to assess the validity of previous lower resolution model results, the latest high spatial resolution version of the Atmospheric and Environmental Research, Inc., seasonal radiative dynamical climate model is used to simulate the climatic effects of ozone changes relative to the other greenhouse gases. The steady-state climatic effect of a sustained decrease in lower stratospheric ozone, similar in magnitude to the observed 1979-90 decrease, is estimated by comparing three steady-state climate simulations: 1) 1979 greenhouse gas concentrations and 1979 ozone, II) 1990 greenhouse gas concentrations with 1979 ozone, and III) 1990 greenhouse gas concentrations with 1990 ozone. The simulated increase in surface air temperature resulting from nonozone greenhouse gases is 0.272 K. When changes in lower stratospheric ozone are included, the greenhouse warming is 0.165 K, which is approximately 39% lower than when ozone is fixed at the 1979 concentrations. Ozone perturbations at high latitudes result in a cooling of the surface-troposphere system that is greater (by a factor of 2.8) than that estimated from the change in radiative forcing resulting from ozone depiction and the model's 2 x CO, climate sensitivity. The results suggest that changes in meridional heat transport from low to high latitudes combined with the decrease in the infrared opacity of the lower stratosphere are very important in determining the steady-state response to high latitude ozone losses. The 39% compensation in greenhouse warming resulting from lower stratospheric ozone losses is also larger than the 28% compensation simulated previously by the lower resolution model. The higher resolution model is able to resolve the high latitude features of the assumed ozone perturbation, which are important in determining the overall climate sensitivity to these perturbations.

Equatorial Kelvin Waves: A UARS MLS View

Data from the Microwave Limb Sounder instrument on the Upper Atmosphere Research Satellite are used to compare two periods of Kelvin wave activity during different stages of the equatorial quasi-biennial oscillation. The analysis is carried out using an asynoptic mapping technique. A wide bandpass filter is used to isolate the frequency bands where Kelvin waves have been identified in previous studies. Time–height and time–latitude plots of the bandpassed data are used to identify Kelvin wave activity in the temperature and ozone fields. Frequency spectra of temperature and ozone amplitudes are constructed to further analyze the latitudinal and meridional distribution of Kelvin wave activity in zonal wavenumbers 1 and 2. The characteristics identified in these plots agree well with theoretical predictions and previous observations of middle atmosphere Kelvin waves. The time–height and time–latitude plots support the existence of Kelvin waves in discrete frequency bands; the slow, fast, and ultrafast Kelvin modes are all identified in the data. The characteristics of these modes do not vary much despite different mean flow conditions in the two periods examined. For the Kelvin wave–induced perturbations in ozone, the change from a transport-dominated regime below 10 hPa to a photochemically controlled regime above 10 hPa is clearly apparent in the height dependence of the phase difference between temperature and ozone. The ratios of the ozone perturbation amplitude to the temperature perturbation amplitude for the various observed Kelvin wave modes are in agreement with model estimates and LIMS (Limb Infrared Monitor of the Stratosphere) observations in the lower half of the region sampled but appear to be too large in the upper stratosphere and lower mesosphere.

Evolution of the Southern Hemisphere Subpolar Middle Atmosphere during Summer and Autumn

The evolution of zonal wind and zonal wavenumber one (wave 1) in the Southern Hemisphere subpolar middle atmosphere is described for the period December 1978 - May 1979 using temperature and ozone measurements from the Limb Infrared Monitor of the Stratosphere (LIMS) experiment. In late December maximum zonal easterlies of approx. -70 m/s are observed at 0.1 mb, 60 deg S. A zonal flow reversal occurs during late February and westerlies subsequently increase to 60-70 m/s in the upper stratosphere by April - May. LIMS zonal winds are compared with rocketsonde measurements and nadir sounder (derived) winds for summer and autumn. Although quantitative agreement is found at stratospheric levels, substantial discrepancies are evident in the mesosphere, most likely a reflection of sampling and resolution differences in the respective datasets. Stationary and traveling wave 1 temperature disturbances (amplitudes approx. 1 - 2 K at 60 deg S) are observed by LIMS during summer. The stationary wave is confined to the lower stratosphere near the level of zero zonal- mean wind flow, whereas the traveling wave is prominent in the middle stratosphere moves west at a rate similar to the zonal-mean wind, and exhibits a vertical - meridional structure similar to a P(sub 4)(sup 1) normal mode Rossby wave. A substantial intensification of wave 1 activity occurs during autumn (amplitudes approx. 5 - 10 K), which is found to be associated with an upward-directed Eliasse - Palm flux near the subpolar tropopause level. Evidence relating wave 1 activity in the lower - middle stratosphere to the occurrence of zonal ozone perturbations of 10% - 20% amplitude is presented for summer and autumn.

Rossby‐gravity waves in tropical total ozone

Randel [1992] has recently reported evidence for Rossby‐gravity waves in tropical data fields produced by the European Centre for Medium Range Weather Forecasts (ECMWF). The purpose of this paper is to show that similar features are observable in fields of total column ozone from the Total Ozone Mapping Spectrometer (TOMS) satellite instrument. The observed features are episodic, have zonal (east‐west) wavelengths of ∼ 6,000–10,000 km and oscillate with periods of 5–10 days. The modes exhibit westward phase progression and eastward group velocity. A simple linear model is used to estimate the magnitude of total ozone perturbations induced by Rossby‐gravity waves. The model is able to account for the magnitude of observed TOMS features as well as their asymmetry about the equator. The significance of finding Rossby‐gravity waves in total ozone fields is that (1) the report of similar features in ECMWF tropical fields is corroborated with an independent data set and (2) the TOMS data set is demonstrated to possess surprising versatility and sensitivity to relatively smaller scale tropical phenomena.