Winter Polar Vortex Research Articles

Global ozone trends in potential vorticity coordinates using TOMS and GOME intercompared against the Dobson network: 1978–1998

Global total column ozone trends were calculated by applying a least squares regression model to five homogenized satellite data sets from November 1978 to December 1998. Drifts and offsets in the satellite data were removed through comparisons of coincident satellite overpass and ground‐based Dobson spectrophotometer measurements and through intersatellite comparisons. The satellite data were remapped into a potential vorticity coordinate (equivalent latitude) before zonal means were calculated. This remapping preserves steep meridional gradients in ozone and ozone trends across the winter polar vortex boundary and thereby reveals statistically significant negative trends just poleward of the Antarctic vortex boundary (∼70°S equivalent latitude) during May (−0.51±0.42% yr−1 (2σ)), June (−0.65±0.44% yr−1) and July (−0.85±0.44% yr−1). This feature is indistinct in trends derived as a function of geographic latitude most likely as a result of the smoothing by zonal averaging across steep ozone gradients. Comparison with similar earlier analysis (November 1978 to May 1991) indicates that Antarctic ozone trends have weakened (from −3.71±1.80% yr−1 to −2.95±0.40% yr−1), most likely as a result of saturation, while Arctic ozone trends are now much larger (from −1.05±0.96%yr−1 to −1.93±0.40% yr−1) following severe wintertime Arctic ozone depletion in recent years. Midlatitude negative trends calculated over the longer time period are also slightly reduced.

A composite view of ozone evolution in the 1995–1996 northern winter polar vortex developed from airborne lidar and satellite observations

A three‐dimensional model using winds and temperatures from the Goddard Earth Observing System Data Assimilation System is used to simulate ozone evolution in the winter high‐latitude northern lower stratosphere. The simulation results are compared with ozone observations from three platforms: the differential absorption lidar (DIAL), the Microwave Limb Sounder (MLS), and the Polar Ozone and Aerosol Measurement (POAM II). Time series for the different data sets are consistent with each other, and diverge from model time series during December and January. The model ozone in December and January is much less sensitive to the model photochemistry than to the model vertical transport. Simulations with different initial conditions for ozone demonstrate sensivitity to the model ozone profile shape. The modeled ozone throughout December and January most closely resembles observed ozone when the vertical profiles between 12 and 20 km within the polar vortex closely match December DIAL observations. We make a quantitative estimate of the uncertainty in the vertical advection using diabatic trajectory calculations. The net transport uncertainty is significant and should be accounted for when comparing observations with model ozone. The observed and modeled ozone time series during December and January are not inconsistent when these transport uncertainties are taken into account.

A seasonal climatology of effective diffusivity in the stratosphere

A 7‐year (1992–1998) seasonal climatology of effective horizontal diffusivity is presented for the stratosphere (350–1900 K). As in previous studies, the diagnosis is based on the equivalent length of a test tracer advected on isentropic surfaces, in this case using the van Leer flux‐limiting scheme driven by the United Kingdom Meteorological Office assimilated winds. Although the magnitude of equivalent length is resolution dependent, its structure is shown to be reasonably robust for quantifying the inhomogeneous mixing in the stratosphere. The van Leer calculation agrees well with a more expensive spectral transform calculation at a comparable resolution, suggesting that the diagnostic is not sensitive to the advection scheme or subgrid representation of the models. A first attempt is also made at estimating the implicit numerical diffusion of the van Leer scheme, which is necessary to convert equivalent length to effective diffusivity. Compared with the spectral calculations with a constant diffusion coefficient, the van Leer results show notably greater diffusivity within the winter polar vortices despite the comparable equivalent lengths, suggesting that the scheme may be overdiffusing in the vicinities of mobile tracer extrema. The climatology of equivalent length is then presented, and the seasonal evolution is discussed in detail for the lower, middle, and upper stratosphere. The major mixing barriers (winter and summer polar vortices and tropical eddy transport barrier) are easily identified, as well as the strong mixing regions (surf zones). Generally, equivalent length tends to be small in regions of strong zonal wind and large in regions of weak wind. Large values of equivalent length are observed in the summer lower stratosphere and in the middle stratosphere during the breakup of the polar vortices. Interhemispheric asymmetry is observed both after the winter polar vortex breakup, where in the middle stratosphere the Southern Hemisphere shows much larger equivalent lengths, and in the summer tropical region, where larger equivalent lengths are observed in the Northern Hemisphere, likely related to the monsoon circulation. A monthly averaged equivalent length climatology is presented, and implications for two‐dimensional stratospheric chemistry‐transport modeling are discussed.

Troposphere‐stratosphere dynamic coupling under strong and weak polar vortex conditions

The relationship between Northern Hemisphere (NH) tropospheric and stratospheric wave‐like anomalies of spherical zonal wave number (ZWN) 1 is studied by applying Canonical Correlation Analysis (CCA). A lag‐correlation technique is used with 10‐day lowpass filtered daily time series of 50‐ and 500‐hPa geopotential heights. Generally stratospheric circulation is determined by ultralong tropospheric planetary waves. During winter seasons characterized either by an anomalously strong or weak polar winter vortex different propagation characteristics for waves of ZWN 1 are observed. The non‐linear perspective of the results have implications for medium range weather forecast and climate sensitivity experiments.

Properties of the 1979 SAM II Antarctic 1.0‐μm extinction coefficients: Implications of dehydration and seasonal evolution of the Antarctic polar vortex

The present study investigates the 1.0‐μm extinction coefficient measurements obtained in the Antarctic region in 1979 from the Stratospheric Aerosol Measurement (SAM) II, with particular focus on the background aerosol properties. Correlative meteorological information from the National Centers for Environmental Prediction is incorporated in this investigation. The results indicate that the data frequency distribution of the background aerosol extinction coefficient in the local summer and fall can be adequately modeled by using a single‐mode normal distribution, and that a binormal distribution is needed for modeling the distribution in the local winter and spring because of the different characteristics of the aerosols inside and outside the polar vortex. In general, the vertical distribution of the aerosol mean extinction coefficient exhibits two regions of different seasonal variation. Above 16 km the extinction coefficient is the highest during the local summer, and the lowest during the local spring inside the polar vortex. Below 16 km the aerosol seasonal variation is more complex, but the winter enhancement of the aerosol extinction coefficient inside the Antarctic polar vortex is clearly evident. As the season changes from winter to spring, the results inside the Antarctic polar vortex also indicate a reduction in aerosol optical depth in the stratosphere, but no significant changes in the upper troposphere. The present study further indicates that the bottom of the winter polar vortex in Antarctica is located at an altitude as low as 8 to 9 km, which is about 4 to 5 km lower than the bottom of the Arctic polar vortex. This difference may be attributable to the different strengths of the winter polar vortex and the planetary wave activities between the two hemispheres. In summary, the properties of the Antarctic background aerosol are very consistent with the effect of polar stratospheric clouds on the aerosol vertical distribution through their formation, sedimention, and evaporation, and with the seasonal evolution of the polar vortex. Finally, the result of the present study provides valuable opportunities for fully utilizing the multiyear SAM II tropospheric and stratospheric measurements to investigate the aerosol climatology and long‐term variations in the Arctic and Antarctic regions.

Decadal time scale trends and variability in the tropospheric circulation over the South Pole

This paper documents significant long‐term changes in the tropospheric circulation and progression of the seasonal cycle over the South Pole and relates aspects of the changes to the enhanced springtime stratospheric ozone depletion of recent decades. During the winter‐to‐summer transition over Antarctica, the formation of the thermal tropopause above 150 hPa is now delayed by a month compared to the early 1960s. A similar delay occurs in the breakdown of the winter polar stratospheric vortex, a prerequisite to the formation of the strong summertime tropopause near 300 hPa. A delay in tropopause formation implies delays in those aspects of the transition from winter to summer in the dynamics of the troposphere that are sensitive to its upper boundary condition, including the contraction of the Rossby radius of deformation during the shallow troposphere phase of summer. Consistent with this expectation, the normal springtime decline of baroclinic‐eddy activity in the lower troposphere now occurs later by a month. Similar delays occur in the time of an early summer minimum in total cloud fraction that coincides with the time of the maximum stratification between 150 and 100 hPa. In addition, the analyses reveal (1) an increase of 15 to 20% in spring and summer cloud fraction over the last four decades; (2) a separation of spring and summer cloudiness into two transport regimes with a distinct intervening minimum in cloud fraction; (3) a decadal modulation of springtime cloudiness that is coincident with decadal shifts in the directional mode of 300‐hPa winds following solar minima; (4) a long‐term increase in total inversion strength, inversion depth, and 650‐ to 300‐hPa thickness; and (5) a long‐term decrease in wintertime surface wind speed, surface temperature, and near‐surface lapse rate. The study also introduces a covariance technique that exploits the control by continental‐scale orography of synoptic and surface flow regimes, increasing the value of lower tropospheric observations for the study of climate change over Antarctica. Finally, a cautionary note arises from the apparent effect of the Weddell Polynyas and the reduction of the maximum extent of sea ice of the middle to late 1970s, with increased tropospheric and lower stratospheric temperatures and an earlier time of formation of the tropopause. This observation suggests that the occurrence of decadal and subdecadal variability in the ocean and the atmosphere of the Southern Hemisphere may obscure the detection of long‐term trends in the circulation over Antarctica.

Simulation of stratospheric tracers using an improved empirically based two‐dimensional model transport formulation

We have developed a new empirically based transport formulation for use in our Goddard Space Flight Center (GSFC) two‐dimensional chemistry and transport model. In this formulation, we consider much of the information about atmospheric transport processes available from existing data sets. This includes zonal mean temperature, zonal wind, net heating rates, and Eliassen‐Palm flux diagnostics for planetary and synoptic‐scale waves. We also account for the effects of gravity waves and equatorial Kelvin waves by utilizing previously developed parameterizations in which the zonal mean flow is constrained to observations. This scheme utilizes significantly more information compared to our previous formulation and results in simulations that are in substantially better agreement with observations. The new model transport captures much of the qualitative structure and seasonal variability observed in stratospheric long lived tracers, such as isolation of the tropics and the southern hemisphere winter polar vortex, the well‐mixed surf‐zone region of the winter subtropics and midlatitudes, and the latitudinal and seasonal variations of total ozone. Model simulations of carbon 14 and strontium 90 are in good agreement with observations, capturing the peak in mixing ratio at 20–25 km and the decrease with altitude in mixing ratio above 25 km. We also find mostly good agreement between modeled and observed age of air determined from SF6 outside of the northern hemisphere polar vortex. However, inside the vortex, the model simulates significantly younger air compared to observations. This is consistent with the model deficiencies in simulating CH4 in this region and illustrates the limitations of the current climatological zonal mean model formulation. The model correctly propagates the phase of the lower stratospheric seasonal cycles in 2CH4+H2O and CO2. The model also qualitatively captures the observed decrease in the amplitude of the stratospheric CO2 seasonal cycle between the tropics and midlatitudes. However, the simulated seasonal amplitudes were attenuated too rapidly with altitude in the tropics. The generally good model‐measurement agreement of these tracer simulations demonstrate that a successful formulation of zonal mean transport processes can be constructed from currently available atmospheric data sets.

Atmospheric radical production by excitation of vibrational overtones via absorption of visible light

We present calculations using a radiative transfer model which predict that in the lower stratosphere at high zenith angles, significant enhancements to the photodissociation rates of HNO3 and HNO4 can result from visible wavelength excitation of OH overtone vibrations containing sufficient energy to cleave the O‐O and N‐O bonds. The results indicate that atmospheric chromophores such as HONO2, HO2NO2 and H2O2, could make a potentially significant contribution to the production of HOx and NOx. Calculating the relative importance of their effect requires better knowledge of the absolute absorption cross sections, both for vibrational overtones and in the near UV. Stratospheric air masses in which this process could be important are those that experience lengthy exposure at high solar zenith angles: the outer regions of the polar winter vortex and the polar summer anticyclone. We note that the general mechanism may have application elsewhere, such as in the atmospheres of other planets and in generating the diffuse interstellar bands associated with molecular clouds.

A Very High Resolution General Circulation Model Simulation of the Global Circulation in Austral Winter

This paper discusses a simulation obtained with the Geophysical Fluid Dynamics Laboratory “SKYHI” troposphere–stratosphere–mesosphere general circulation model run at very high horizontal resolution (∼60-km grid spacing) and without any parameterization of subgrid-scale gravity wave drag. The results are for a period around the austral winter solstice, and the emphasis is on the simulated Southern Hemisphere (SH) winter circulation. Comparisons are made with results obtained from lower horizontal resolution versions of the same model. The focus in this paper is on two particularly striking features of the high-resolution simulation: the extratropical surface winds and the winter polar middle atmospheric vortex. In the extratropical SH, the simulated surface westerlies and meridional surface pressure gradients in the high-resolution model are considerably stronger than observed and are stronger than those simulated at lower horizontal resolution. In the middle atmosphere, the high-resolution model produces a simulation of the zonal mean winter polar vortex that is considerably improved over that found with lower resolution models (although it is still significantly affected by the usual cold pole bias). Neither the improvement of the middle atmospheric polar vortex simulation nor the deterioration of the simulation of surface winds with increased model resolution shows a clear convergence, even at the ∼60-km grid spacing employed here.

Seasonal variation of middle atmospheric CH4 and H2O with a new chemical‐dynamical model

A new zonally averaged, chemical‐dynamical model of the middle atmosphere is used to study the processes which control the distributions and seasonal variability of CH4 and H2O. This model incorporates a nondiffusive, nondispersive advection scheme, a time‐dependent linear model of planetary wave drag and horizontal mixing (Kyy), a new parameterization of gravity wave drag and vertical mixing (Kzz), and an explicit treatment of LTE (local thermodynamic equilibrium) and non‐LTE IR cooling. Model chemistry is calculated using a Newton‐Raphson iterative scheme, which allows consistent simulations of species with highly nonuniform chemical lifetimes. In this study we focus on the sensitivity of model CH4 and H2O to the magnitude of tropospheric latent heat release, planetary wave and gravity wave activity, and the methane oxidation rate. Model results show that in the tropical stratosphere their vertical distributions are strong functions of both the methane oxidation rate and the ascent rate, the latter driven by a combination of tropospheric latent heat release and atmospheric drag. At low latitudes HALOE observations and model results both show conservation of “potential H2” (2×CH4+H2O) below ∼50 km. However, the conservation of potential H2 from HALOE observations breaks down above ∼55 km, while the model shows conservation well into the middle mesosphere (∼70 km). This may suggest serious inadequacies in our understanding of the photochemistry of water vapor and mesospheric HOx, in particular those processes which control the partitioning of H2 and H2O. At high latitudes, H2O model/data comparisons suggest that horizontal mixing is important in determining the observed latitudinal gradient in mesospheric water vapor. We also find that inside the polar winter vortex, while the strength of tropical latent heat forcing and planetary wave drag influence the descent rate, both horizontal mixing and the methane photochemistry play important roles in determining the CH4 mixing ratio. Finally, we suggest that the observed interhemispheric asymmetry in the seasonal cycle of mesospheric H2O may be linked to larger values of Kzz in the southern winter mesosphere. This represents a key difference between mesospheric and stratospheric tracer transport. In the stratosphere, greater net unmixed descent in the southern hemisphere directly translates into lower tracer values relative to the northern hemisphere, while mesospheric tracer transport shows the opposite behavior.

Polar Vortex Meandering and Stratospheric Aerosol Distribution: Lidar Measurements at Fairbanks, Alaska.

Lidar measurements at Fairbanks, Alaska (64°49' N, 147°52' W) and Ny-Aalesund, Norway (78°54'N, 11°53'E) in December 1991 and January, 1994, respectively, was made to monitor distribution of stratospheric aerosols. The location of Fairbanks lidar site is advantageous to see the effect of polar vortex wall to stratospheric aerosol density distribution since the lidar site is near the polar vortex wall, and sometimes inside and sometimes outside of the polar vortex owing meandering motion of polar vortex. Lidar site of Ny-Aalesund is usually inside of the polar vortex in winter. Comparison of measurements at Fairbanks and Ny-Aalesund showed that polar vortex meandering disturbed profiles of stratospheric aerosol content. Comparing measurements at Fairbanks and Toyokawa, Japan (34°45' N, 137°24' E) the measurements at Fairbanks are well corresponding to the long tens trend observed at Toyokawa, and noticeable day-to-day variations in aerosol content, integrated backscattering coefficient of stratospheric particulate matter, in winter of 1993/1994, which possibly due to the polar atmospheric effect, are detected in the Fairbanks data. Above about 20 km there was little aerosol content when the lidar station was inside of polar vortex. Large enhancement of aerosol load near the local tropopause was observed, which may associate with aerosol descending from the stratosphere to the troposphere near the wall of polar vortex.

Variations of the total ozone trend over the northern hemisphere midlatitudes in winter‐spring seasons 1963–1964 through 1992–1993

Monthly means of total ozone for winter‐spring seasons (December through the next March) since 1963–1964 through 1992–1993 from a network of 19 Dobson stations are averaged to form a composite time series of the winter‐spring midlatitudinal total ozone. A multiple regression statistical model, comprising overall mean level, linear trend, quasi‐biennial, El Niño, solar (including both the 11‐year and its double length “geomagnetic” oscillations), and dynamics of the winter polar vortex terms, is applied to estimate interannual variations of total ozone over the northern hemisphere midlatitudes in winter‐spring seasons. The model used explains about 88% of total variance of the winter‐spring total ozone and gives the value of the anthropogenic trend in the northern hemisphere midlatitudinal total ozone of the order of −3.5%±0.4% per decade since winter‐spring of 1969–1970. The model's results support the hypothesis that the increase of the ozone loss during the 1980s and 1990s over the northern hemispheric midlatitudes may be partially caused by a superposition of “natural” long‐term oscillations in total ozone related to the climatic changes in the atmospheric dynamics. The initial evidences for a strong manifestation of the interannual changes of the polar vortex in the winter‐spring midlatitudinal total ozone data are found. The 22‐year oscillation in the total ozone data is disclosed; however, because of the limited length of the ozone data, one should be cautious to relate this fluctuation uniquely to the solar input.

Lagrangian Transport Calculations UsingUARSData. Part I. Passive Tracers

The transport of passive tracers observed by the Upper Atmosphere Research Satellite is simulated using computed three-dimensional trajectories of ≈ 100 000 air parcels initialized on a stratosphere grid, with horizontal winds provided by the United Kingdom Meteorological Office data assimilation system, and vertical (cross isentropic) velocities computed using a fast radiation code. The conservative evolution of trace constituent fields is estimated over 20–30-day periods by assigning to each parcel the observed mixing ratio of the long-lived trace gases N20 and CH4 observed by the Cryogenic Limb Army Etalon Spectrometer (CLAES) and H2O observed by the Microwave Limb Sounder (MLS) on the initialization date. Agreement between calculated and observed fields is best inside the polar vortex and is better in the Arctic than in the Antarctic. Although there is not always detailed agreement outside the vortex, the trajectory calculations still reproduce the average large-scale characteristics of passive tracer evolution in midlatitudes. In late winter, synoptic maps from trajectory calculations reproduce all major features of the observations, including large tongues or blobs of material drawn from low latitudes into the region of the anticyclone during February–March 1993. Comparison of lower-stratospheric observations of the CLAES tracers with the calculations suggests that discontinuities seen in CLAES data in the Antarctic late winter lower stratosphere are inconsistent with passive tracer behavior. In the Arctic, and in the Antarctic late winter, MLS H20 observations show behavior that is inconsistent with calculations and with that expected for passive tracers inside the polar vortex in the middle-to-upper stratosphere. Diabatic descent rates in the Arctic lower stratosphere deduced from data are consistent with those from the calculations. In the Antarctic lower stratosphere, the calculations appear to underestimate the diabatic descent. The agreement between large-scale features of calculated and observed tracer fields supports the utility of these calculations in diagnosing trace species transport in the winter polar vortex.

Air mass exchange across the polar vortex edge during a simulated major stratospheric warming

Abstract. The dynamics of the polar vortex in winter and spring play an important role in explaining observed low ozone values. A quantification of physical and chemical processes is necessary to obtain information about natural and anthropogenic causes of fluctuations of ozone. This paper aims to contribute to answering the question of how permeable the polar vortex is. The transport into and out of the vortex ("degree of isolation") remains the subject of considerable debate. Based on the results of a three-dimensional mechanistic model of the middle atmosphere, the possibility of exchange of air masses across the polar vortex edge is investigated. Additionally the horizontal and vertical structure of the polar vortex is examined. The model simulation used for this study is related to the major stratospheric warming observed in February 1989. The model results show fair agreement with observed features of the major warming of 1989. Complex structures of the simulated polar vortex are illustrated by horizontal and vertical cross sections of potential vorticity and inert tracer. A three-dimensional view of the polar vortex enables a description of the vortex as a whole. During the simulation two vortices and an anticyclone, grouped together in a very stable tripolar structure, and a weaker, more amorphous anticyclone are formed. This leads to the generation of small-scale features. The results also indicate that the permeability of the vortex edges is low because the interior of the vortices remain isolated during the simulation.

Descent of long‐lived trace gases in the winter polar vortex

Recent observations of CH4 and HF from the UARS Halogen Limb Occultation Experiment (HALOE) suggest that vigorous descent occurs within the polar winter vortex with “mesospheric” values of CH4 evident down to 30 mbar. This study shows that a highly accurate two‐dimensional model advection scheme coupled with a modern radiation scheme, parameterized planetary and gravity wave drag algorithms can produce tracer distributions consistent with HALOE observations. The modeled tracer distribution within the polar vortex is found to be principally dependent on the strength of dynamical drag in the middle atmosphere and the strength of the planetary wave forcing. However, the strong downward transport of tracers at the poles during winter can be disrupted in midwinter by planetary wave mixing. Thus the weaker planetary wave forcing in the southern hemisphere winter allows for a more coherent descent of long‐lived tracers from the mesosphere than during the northern hemisphere winter. Multiple‐year integrations of the model reveal a general circulation of the stratosphere which lofts tracers to mesospheric altitudes. Material removed from the mesosphere returns to the stratosphere principally within the polar regions. Upward vertical transport of material is found to be enhanced by horizontal planetary wave mixing.

TOMS total ozone trends in potential vorticity coordinates

Global total ozone measurements from the Nimbus 7 Total Ozone Mapping Spectrometer (TOMS) are analyzed using potential vorticity (PV) as an approximate vortex‐following coordinate. We analyze the time period November 1978–May 1991, prior to the volcanic eruption of Mt. Pinatubo. The TOMS data are remapped into PV coordinates and trends are calculated, thereby characterizing ozone losses inside and outside the winter polar vortices. These analyses show large regions of ozone loss outside of the vortex in both hemispheres. Furthermore, these data suggest that midlatitude losses in the NH during winter‐spring do not result solely from the transport of ozone depleted air from inside to outside the vortex.

Interannual Variability in the Northern Hemisphere Winter Middle Atmosphere in Control and Perturbed Experiments with the GFDL SKYHI General Circulation Model

Abstract This paper reports on interannual variability of the Northern Hemisphere winter stratospheric circulation as simulated by the 40-level GFDL “SKYHI” general circulation model. A 31-year control simulation was performed using a climatological annual cycle of sea surface temperatures. The interannual variability of the stratospheric circulation in this model has some realistic features. In particular, the simulated variance of monthly mean, zonal-man temperature and wind in the. extratropical Northern Hemisphere agrees fairly well with observations. The day-to-day variability of the circulation also appears to be rather well simulated, with midwinter warmings of realistic intensity and suddenness appearing in the polar regions. The major deficiency is the absence of a realistic quasi-biennial oscillation (QBO) in the simulated winds in the tropical lower stratosphere. There is also an indication of long period (∼10 year) variability in the winter polar vortex. This appears not to be related to any o...

Isentropic Mass Exchange between the Tropics and Extratropics in the Stratosphere

The isentropic mass exchange between the Tropics and extratropics in the stratosphere is investigated with a semi-Lagrangian transport model for the periods from 1 June to 31 October 1992 and from 1 December 1992 to 30 April 1993 using winds from the U.K. Meteorological Office data assimilation system. Calculations with an idealized, initially zonally symmetric tracer show that in the middle and upper stratosphere the bulk of tropical air is transported into the midlatitudes of the winter hemisphere although there exist quasi-permeable barriers in the subtropics. The transport takes place in the form of planetary-scale 'tongues' of material that are drawn poleward in association with the episodic amplification of planetary-scale waves in high latitudes of the winter hemisphere. Once air of tropical origin is transported to the midlatitudes it is irreversibly mixed with the midlatitude air in the 'surf zone.' Air of tropical origin can, however, hardly penetrate into the interior of the winter polar vortex until the breakdown of the vortex. Transport of tropical air into the midlatitudes of the summer hemisphere is strongly inhibited. In the lower stratosphere, tropical air is transported into the northern and southern midlatitudes. During the period from 1 June to 31 October 1992, the amount of tropical air transported into the Northern Hemisphere is, however, much smaller than that transported into the Southern Hemisphere, and there exist strong gradients in the tracer field in the equatorial region, indicating that there is a quasi-permeable barrier to cross-equator mass exchange. During the period from 1 December 1992 to 30 April 1993, on the other hand, roughly the same amounts of tropical air are transported into the Southern Hemisphere, and there exist strong gradients in the tracer field in the equatorial region, indicating that there is a quasi-permeable barrier to cross-equator mass exchange. During the period from 1 December 1992 to 30 April 1993, on the other hand, roughly the same amounts of tropical air are transported into the northern midlatitudes and into the southern midlatitudes, and there exist no significant transport barriers in the equatorial area.

Penetration of Mt. Pinatubo aerosols into the north polar vortex

Observations of the Mount Pinatubo cloud as it moved northward and into the winter polar vortex were made with a balloon borne two‐wavelength backscattersonde. Some volcanic debris had arrived at far northerly latitudes below 20 km by October and was apparently incorporated into the initial vortex. Subsequent measurements did not show a significant increase in the central vortex aerosol until after a mid January disturbance, at which time the backscatter profiles began to increase. Above 20 km and near the center of the vortex there was no significant increase in aerosol through mid March. The column stratospheric aerosol mass loading as calculated from individual soundings at the wall of the vortex during March 1992 ranged from 18 to 24 megatonnes per unit area where the unit of area is that of the earth. This indicates that a significant amount of material was transported northward by the end of the winter.

Spring polar ozone behavior

It has been recognized since the commencement of Antarctic ozone measurements during the IGY that spring southern polar total ozone amount is less than spring northern polar total ozone amount. More importantly, since 1980 there has been a decline in the minimum spring total ozone value, from 250 DU in 1980 to 125 DU in 1987 and below 120 in 1991. This decline occurs within the winter polar vortex, which acts as a containment vessel preventing polar ozone from escaping to lower latitudes and excluding ozone-rich air from the polar region. Ozone decrease can be explained in terms of heterogeneous reactions of chlorine and nitrogen reservoir molecules on polar stratospheric clouds. These clouds form in the lower polar stratosphere during winter when temperatures in the Antarctic are sufficiently low to create water ice clouds. Clouds involving nitric acid form at higher temperatures. Chlorine reservoirs such as HCl are converted to Cl 2, which is photodissociated in the presence of sunlight. The resulting Cl reacts with O 3 to form ClO. Measurements of ClO and other species give agreement of theory and experiment within the uncertainties of the measurement. Heterogeneous chemistry accounts for most of the ozone hole. A small amount of ozone loss is also observed above the polar stratospheric cloud level, implying another mechanism, either chemical or dynamical. Above 25 km, formation of ozone-destroying odd nitrogen in the upper stratosphere by energetic electrons and the existence of any trend is still an open question. There is much less ozone depletion in the Arctic. This is the result of a less stable polar vortex and warmer temperatures, which reduce polar stratospheric cloud formation. There is strong evidence that tropospheric forcing within or just outside the vortex leads to adiabatic cooling with resulting cloud formation. During such events ozone-poor tropospheric air is transported into the stratosphere. In the Arctic this can result in the transport of long-lived hydrocarbons. Subsequent reactions lead to the formation of HCl, reducing the effect of Cl. There is also production of HO 2, which accelerates ozone loss due to chlorine. There are also small areas of large and rapid ozone depletion termed miniholes. Ozone-poor air from these regions can propagate to lower latitudes, as can the air from within the vortex, when it disintegrates in late spring. Data from the BUV ozone-measuring instrument on the Nimbus 4 satellite indicate the existence of October 1970 Antarctic ozone of only 250 DU. This is evidence of the existence of ozone loss with only CH 3Cl and low concentrations of CFCs as chlorine sources.