Minor Stratospheric Warming Research Articles

High and Equatorial Mesospheric Dynamical Response to the Minor Stratospheric Warming of 2014/15: Comparison with major SSW Events 2005/06 and 2008/09

High and Equatorial Mesospheric Dynamical Response to the Minor Stratospheric Warming of 2014/15: Comparison with major SSW Events 2005/06 and 2008/09

Seasonal evolution of winds, atmospheric tides, and Reynolds stress components in the Southern Hemisphere mesosphere–lower thermosphere in 2019

Abstract. In this study we explore the seasonal variability of the mean winds and diurnal and semidiurnal tidal amplitude and phases, as well as the Reynolds stress components during 2019, utilizing meteor radars at six Southern Hemisphere locations ranging from midlatitudes to polar latitudes. These include Tierra del Fuego, King Edward Point on South Georgia island, King Sejong Station, Rothera, Davis, and McMurdo stations. The year 2019 was exceptional in the Southern Hemisphere, due to the occurrence of a rare minor stratospheric warming in September. Our results show a substantial longitudinal and latitudinal seasonal variability of mean winds and tides, pointing towards a wobbling and asymmetric polar vortex. Furthermore, the derived momentum fluxes and wind variances, utilizing a recently developed algorithm, reveal a characteristic seasonal pattern at each location included in this study. The longitudinal and latitudinal variability of vertical flux of zonal and meridional momentum is discussed in the context of polar vortex asymmetry, spatial and temporal variability, and the longitude and latitude dependence of the vertical propagation conditions of gravity waves. The horizontal momentum fluxes exhibit a rather consistent seasonal structure between the stations, while the wind variances indicate a clear seasonal behavior and altitude dependence, showing the largest values at higher altitudes during the hemispheric winter and two variance minima during the equinoxes. Also the hemispheric summer mesopause and the zonal wind reversal can be identified in the wind variances.

Detection of Large Fluctuations in Ozone Content in the Middle Atmosphere during Sudden Stratospheric Warmings and Subpolar Latitudes of the Arctic

The results of microwave radiometry studies of the ozone-content dynamics in the middle atmosphere above Peterhof during stratospheric warmings of two winters, 2015–2016 and 2016–2017, are presented. Ground-based observations employed mobile microwave ozone-measuring instrument (operating frequency is 110.8 GHz). The vertical ozone distribution in the altitude range of 22–60 km is estimated. The results are compared to satellite-borne data on the total ozone content, to vertical profiles of the ozone and temperature in the middle atmosphere, and to data from an ozone-measuring sounder. In the middle atmosphere above Peterhof, there have been significant variations (by several times) in the ozone content at heights of 40–60 km due to minor stratospheric warming.

Observations of OH airglow from ground, aircraft, and satellite: investigation of wave-like structures before a minor stratospheric warming

Abstract. In January and February 2016, the OH airglow camera system FAIM (Fast Airglow Imager) measured during six flights on board the research aircraft FALCON in northern Scandinavia. Flight 1 (14 January 2016) covering the same ground track in several flight legs and flight 5 (28 January 2016) along the shoreline of Norway are discussed in detail in this study. The images of the OH airglow intensity are analysed with a two-dimensional FFT regarding horizontal periodic structures between 3 and 26 km horizontal wavelength and their direction of propagation. Two ground-based spectrometers (GRIPS, Ground-based Infrared P-branch Spectrometer) provided OH airglow temperatures. One was placed at ALOMAR, Northern Norway (Arctic Lidar Observatory for Middle Atmosphere Research; 69.28∘ N, 16.01∘ E) and the other one at Kiruna, northern Sweden (67.86∘ N, 20.24∘ E). Especially during the last third of January 2016, the weather conditions at Kiruna were good enough for the computation of nightly means of gravity wave potential energy density. Coincident TIMED-SABER (Thermosphere Ionosphere Mesosphere Energetics Dynamics–Sounding of the Atmosphere using Broadband Emission Radiometry) measurements complete the data set. They allow for the derivation of information about the Brunt–Väisälä frequency and about the height of the OH airglow layer as well as its thickness. The data are analysed with respect to the temporal and spatial evolution of mesopause gravity wave activity just before a minor stratospheric warming at the end of January 2016. Wave events with periods longer (shorter) than 60 min might mainly be generated in the troposphere (at or above the height of the stratospheric jet). Special emphasis is placed on small-scale signatures, i.e. on ripples, which may be signatures of local instability and which may be related to a step in a wave-breaking process. The most mountainous regions are characterized by the highest occurrence rate of wave-like structures in both flights.

Gravity waves excited during a minor sudden stratospheric warming

Abstract. An exceptionally deep upper-air sounding launched from Kiruna airport (67.82∘ N, 20.33∘ E) on 30 January 2016 stimulated the current investigation of internal gravity waves excited during a minor sudden stratospheric warming (SSW) in the Arctic winter 2015/16. The analysis of the radiosonde profile revealed large kinetic and potential energies in the upper stratosphere without any simultaneous enhancement of upper tropospheric and lower stratospheric values. Upward-propagating inertia-gravity waves in the upper stratosphere and downward-propagating modes in the lower stratosphere indicated a region of gravity wave generation in the stratosphere. Two-dimensional wavelet analysis was applied to vertical time series of temperature fluctuations in order to determine the vertical propagation direction of the stratospheric gravity waves in 1-hourly high-resolution meteorological analyses and short-term forecasts. The separation of upward- and downward-propagating waves provided further evidence for a stratospheric source of gravity waves. The scale-dependent decomposition of the flow into a balanced component and inertia-gravity waves showed that coherent wave packets preferentially occurred at the inner edge of the Arctic polar vortex where a sub-vortex formed during the minor SSW.

Frequency spectra and vertical profiles of wind fluctuations in the summer Antarctic mesosphere revealed by MST radar observations

AbstractContinuous observations of polar mesosphere summer echoes at heights from 81–93 km were performed using the first Mesosphere‐Stratosphere‐Troposphere/Incoherent Scatter radar in the Antarctic over the three summer periods of 2013/2014, 2014/2015, and 2015/2016. Power spectra of horizontal and vertical wind fluctuations, and momentum flux spectra in a wide‐frequency range from (8 min)−1 to (20 days) −1 were first estimated for the Antarctic summer mesosphere. The horizontal (vertical) wind power spectra obey a power law with an exponent of approximately −2 (−1) at frequencies higher than the inertial frequency of (13 h)−1 and have isolated peaks at about 1 day and a half day. In addition, an isolated peak of a quasi‐2 day period is observed in the horizontal wind spectra but is absent from the vertical wind spectra, which is consistent with the characteristics of a normal‐mode Rossby‐gravity wave. Zonal (meridional) momentum flux spectra are mainly positive (negative), and large fluxes are observed in a relatively low‐frequency range from (1 day)−1 to (1 h)−1. A case study was performed to investigate vertical profiles of momentum fluxes associated with gravity waves and time mean winds on and around 3 January 2015 when a minor stratospheric warming occurred in the Northern Hemisphere. A significant momentum flux convergence corresponding to an eastward acceleration of ~200 m s−1 d−1 was observed before the warming and became stronger after the warming when mean zonal wind weakened. The strong wave forcing roughly accorded with the Coriolis force of mean meridional winds.

The Impact of GCM Dynamical Cores on Idealized Sudden Stratospheric Warmings and Their QBO Interactions

Abstract The paper demonstrates that sudden stratospheric warmings (SSWs) can be simulated in an ensemble of dry dynamical cores that miss the typical SSW forcing mechanisms like moist processes, land–sea contrasts, or topography. These idealized general circulation model (GCM) simulations are driven by a simple Held–Suarez–Williamson (HSW) temperature relaxation and low-level Rayleigh friction. In particular, the four dynamical cores of NCAR’s Community Atmosphere Model, version 5 (CAM5), are used, which are the semi-Lagrangian (SLD) and Eulerian (EUL) spectral-transform models and the finite-volume (FV) and the spectral element (SE) models. Three research themes are discussed. First, it is shown that SSW events in such idealized simulations have very realistic flow characteristics that are analyzed via the SLD model. A single vortex-split event is highlighted that is driven by wavenumber-1 and -2 wave–mean flow interactions. Second, the SLD simulations are compared to the EUL, FV, and SE dynamical cores, which sheds light on the impact of the numerical schemes on the circulation. Only SLD produces major SSWs, while others only exhibit minor stratospheric warmings. These differences are caused by SLD’s more vigorous wave–mean flow interactions in addition to a warm pole bias, which leads to relatively weak polar jets in SLD. Third, it is shown that tropical quasi-biennial oscillation (QBO)–like oscillations and SSWs can coexist in such idealized HSW simulations. They are present in the SLD dynamical core that is used to analyze the QBO–SSW interactions via a transformed Eulerian-mean (TEM) analysis. The TEM results provide support for the Holton–Tan effect.

Mesospheric signatures observed during 2010 minor stratospheric warming at King Sejong Station (62°S, 59°W)

A minor stratospheric sudden warming (SSW) event was noticed in the southern hemisphere (SH) during September (day 259) 2010 along with two episodic warmings in early August (day 212) and late October (day 300) 2010. Among the three warming events, the signature of mesosphere response was detected only for the September event in the mesospheric wind dataset from both meteor radar and MF radar located at King Sejong Station (62°S, 59°W) and Rothera (68°S, 68°W), Antarctica, respectively. The zonal winds in the mesosphere reversed approximately a week before the September SSW event, as has been observed in the 2002 major SSW. Signatures of mesospheric cooling (MC) in association with stratospheric warmings are found in temperatures measured by the Microwave Limb Sounder (MLS). Simulations of specified dynamics version of Whole Atmosphere Community Climate Model (SD-WACCM) are able to reproduce these observed features. The mesospheric wind field was found to differ significantly from that of normal years probably due to enhanced planetary wave (PW) activity before the SSW. From the wavelet analysis of wind data of both stations, we find that strong 14–16 day PWs prevailed prior to the SSW and disappeared suddenly after the SSW in the mesosphere. Our study provides evidence that minor SSWs in SH can result in significant effects on the mesospheric dynamics as in the northern hemisphere.

Comparing Sudden Stratospheric Warming Definitions in Reanalysis Data*

Abstract Sudden stratospheric warmings (SSWs) are characterized by a pronounced increase of the stratospheric polar temperature during the winter season. Different definitions have been used in the literature to diagnose the occurrence of SSWs, yielding discrepancies in the detected events. The aim of this paper is to compare the SSW climatologies obtained by different methods using reanalysis data. The occurrences of Northern Hemisphere SSWs during the extended-winter season and the 1958–2014 period have been identified for a suite of eight representative definitions and three different reanalyses. Overall, and despite the differences in the number and exact dates of occurrence of SSWs, the main climatological signatures of SSWs are not sensitive to the considered reanalysis. The mean frequency of SSWs is 6.7 events decade−1, but it ranges from 4 to 10 events, depending on the method. The seasonal cycle of events is statistically indistinguishable across definitions, with a common peak in January. However, the multidecadal variability is method dependent, with only two definitions displaying minimum frequencies in the 1990s. An analysis of the mean signatures of SSWs in the stratosphere revealed negligible differences among methods compared to the large case-to-case variability within a given definition. The stronger and more coherent tropospheric signals before and after SSWs are associated with major events, which are detected by most methods. The tropospheric signals of minor SSWs are less robust, representing the largest source of discrepancy across definitions. Therefore, to obtain robust results, future studies on stratosphere–troposphere coupling should aim to minimize the detection of minor warmings.

Subseasonal variability of water vapor in the upper stratosphere/lower mesosphere over Northern Europe in winter 2009/2010

For the upper stratosphere and lower mesosphere (USLM) we used microwave spectrometer measurements of water vapor to investigate the cause of subseasonal variability over Northern Europe: at ALOMAR (Andenes, 69.3° N, 16.1° E), Northern Norway, and at the Leibniz Institute of Atmospheric Physics (Kühlungsborn, 54.2° N, 11.8° E), Northern Germany, for winter 2009/2010. The MERRA data set of NASA is applied to study the dynamical link between the local variability of H2O and transport in the USLM.Besides a slow increase in January and a stronger decrease in February and March 2010, episodes of significant increase and decrease of H2O were found over Northern Germany. These structural changes are in good agreement with MERRA which show similar patterns induced by a dominant conservative horizontal transport of H2O. Due to the strong negative meridional gradient and mixing barrier, higher values of water vapor have been observed outside the polar vortex and lower values inside. We found that the complex polar vortex evolution over Northern Germany during the minor stratospheric sudden warming (mSSW) in the beginning of December 2009 and the major warming (MSSW) at the end of January 2010, as well as between the two, fits well into this relationship. An episode of strong increase in water vapor over ALOMAR at about 55–60km altitude was observed during the MSSW on the 27th of January 2010 resulting in a significant double peak in altitude. Based on MERRA data we show that this dual peak was caused by a relatively strong regional northward propagation of more moist air in the lower mesosphere. In the lower mesosphere strong polar intrusion of warm and moist air occurred mainly over Northern Europe resulting in a well-mixed polar anticyclone on the 30th of January. In comparison with observations the local maxima of H2O in MERRA are underestimated by approximately 1–2ppmv. After the MSSW, the vertical descent rate of the MERRA reanalysis is half as much as the observed one over Kühlungsborn.Nevertheless the high coherence between local water vapor measurements over Northern Germany and Northern Norway and MERRA supports the extended use of MERRA for subseasonal transport studies in the USLM. We conclude that the observed strong subseasonal variability of H2O in the USLM over Northern Europe, especially over Kühlungsborn and over ALOMAR, during the winter 2009/2010 was caused by the passage of the edge of polar vortex and linked transport of H2O.

Observation of polar stratospheric clouds over Obninsk in December 2012

In late December 2012 a blocking anticyclone followed by the event of minor stratospheric warming, set in the troposphere over West Siberia and, after that, over the European part of Russia. As a result of the deformation of a polar stratospheric vortex, the temperature in the lower stratosphere over Obninsk dropped below the threshold of the formation of polar stratospheric clouds. The lidar measurements of temperature, ozone values, and aerosol characteristics in the middle atmosphere were carried out at the lidar station during this atmospheric event. In three cases, polar stratospheric clouds (PSCs) referred to NAT Ia type according to the sounding results, were registered at the height of about 20 km. No considerable decrease in the ozone concentration in the area of PSC formation was revealed in these measurements.

Resonant interaction between two planetary waves as a precursor for stratospheric warmings?

Stratospheric warmings are attributed to an enhanced planetary wave activity, occurring nearly each winter – at least in the northern hemisphere – with different strengths. The generation of stratospheric warmings is not totally understood. One of the most promising explanations is the interaction of planetary waves: in many cases, the amplitude of the quasi-stationary planetary wave 1 builds up, until it transmits its momentum and energy to the background wind field. The role of wave 2 is usually considered to be less important. Based on ERA-40 and DYANA temperature data (January–February 1990), we found evidence that a resonant wave–wave interaction between a travelling and a stationary wave 2 was responsible for a minor stratospheric warming in February 1990. The interaction being observed during four weeks can eventually be used as an indication for an upcoming stratospheric warming.

Mesosphere and lower thermosphere temperature anomalies during the 2002 Antarctic stratospheric warming event

Abstract. We present kinetic temperatures at ~87 km and ~94 km altitudes inferred from OH (6,2) and O2 At(0,1) airglow observations, respectively, at South Pole (90° S), Antarctica in the austral winter of 2002. These OH and O2 rotational temperatures measurements show mesosphere and lower thermosphere (MLT) temperature anomalies prior to the 2002 Southern Hemisphere Sudden Stratospheric Warming (SSW) . In this paper we focus on the first of the three minor stratospheric warmings which preceded the major SSW event. Temperature anomalies observed in the MLT region show sudden cooling (ΔT=~30 K) in OH temperatures accompanied by warming (ΔT=~15 K) in O2 temperatures preceding the onset of SSW event by about three to four weeks. This shallow vertical extent of mesospheric cooling is in agreement with the numerical simulation of Coy et al. (2005), however, the model cooling was centered well below the mesopause level. The other observed feature of the South Pole MLT temperature dataset is the intensification of planetary wave activity prior to the onset of SSW event. Fourier analyses of both OH and O2 temperatures show amplification of planetary wave activity in the 5–10 day range prior to the onset of SSW. The timing of wave amplification seen in the wave spectra coincides with the peak in OH and O2 temperature anomalies.

Latitudinal and longitudinal variability of mesospheric winds and temperatures during stratospheric warming events

Continuous MF and meteor radar observations allow detailed studies of winds in the mesosphere and lower thermosphere (MLT) as well as temperatures around the mesopause. This height region is characterized by a strong variability in winter due to enhanced planetary wave activity and related stratospheric warming events, which are distinct coupling processes between lower, middle and upper atmosphere. Here the variability of mesospheric winds and temperatures is discussed in relation with major and minor stratospheric warmings as observed during winter 2005/06 in comparison with results during winter 1998/99. Our studies are based on MF radar wind measurements at Andenes (69°N, 16°E), Poker Flat (65°N, 147°W) and Juliusruh (55°N, 13°E) as well as on meteor radar observations of winds and temperatures at Resolute Bay (75°N, 95°W), Andenes (69°N, 16°E) and Kühlungsborn (54°N, 12°E). Additionally, energy dissipation rates have been estimated from spectral width measurements using a 3 MHz Doppler radar near Andenes. Particular attention is directed to the changes of winds, turbulence and the gravity wave activity in the mesosphere in relation to the planetary wave activity in the stratosphere. Observations indicate an enhancement of planetary wave 1 activity in the mesosphere at high latitudes during major stratospheric warmings. Daily mean temperatures derived from meteor decay times indicate that strong warming events are connected with a cooling of the 90 km region by about 10–20 K. The onset of these cooling processes and the reversals of the mesospheric circulation to easterly winds occur some days before the changes of the zonal circulation in the stratosphere start indicating a downward propagation of the circulation disturbances from the MLT region to the stratosphere and troposphere during the stratospheric warming events. The short-term reversal of the mesospheric winds is followed by a period of strong westerly winds connected with enhanced turbulence rates and an increase of gravity wave activity in the altitude range 70–85 km.

The atmospheric background situation in northern Scandinavia during January/February 2003 in the context of the MaCWAVE campaign

Abstract. The atmosphere background wind field controls the propagation of gravity waves from the troposphere through the stratosphere into the mesosphere. During January 2003 the MaCWAVE campaign took place at Esrange, with the purpose of observing vertically ascending waves induced by orography. Temperature data from the U. Bonn lidar at Esrange (68° N/21° E) and the ALOMAR RMR lidar (69° N/16° E), wind data from Esrange MST radar ESRAD, as well as wind data from the ECMWF T106 model, are used to analyse the atmospheric background situation and its effect on mountain wave propagation during January/February 2003. Critical levels lead to dissipation of vertically ascending waves, thus mountain waves are not observable above those levels. In the first half of January a minor as well as a major stratospheric warming dominated the meteorological background situation. These warmings led to a wind reversal, thus to critical level filtering and consequently prevented gravity waves from propagating to high altitudes. While the troposphere was not transparent for stationary gravity waves most of the time, there was a period of eight days following the major warming with a transparent stratosphere, with conditions allowing gravity waves generated in the lower troposphere to penetrate the stratosphere up to the stratopause and sometimes even into the lower mesosphere. In the middle of February a minor stratospheric warming occurred, which again led to critical levels such that gravity waves were not able to ascend above the middle stratosphere. Due to the unfavourable troposphere and lower stratosphere conditions for gravity wave excitation and propagation, the source of the observed waves in the middle atmosphere is probably different from orography.

Modeling the August 2002 minor warming event

Hindcasts of the Southern Hemisphere minor stratospheric warming and mesospheric cooling event of August 2002, made with a new high altitude version of the Navy's operational forecast model, are compared with temperatures acquired by SABER (Sounding of the Atmosphere using Broadband Emission Radiometry). Results show realistic hemispheric evolution of both the stratospheric warming and mesospheric cooling over a 10‐day time period. Use of Rayleigh friction to model mesospheric gravity wave drag shows improvement in the upper mesosphere over a hindcast without Rayleigh friction. The limited vertical extent of the main mesospheric cooling signature disagrees with the Liu and Roble (2002) model results but is supported by SABER temperature observations (Siskind et al., 2005). Examination of 3D EP‐flux vectors over the 10‐day forecast suggests that the planetary wave responsible for the warming/cooling event originated from a horizontally localized region of the troposphere.

Model Simulations of the Impact of the 2002 Antarctic Ozone Hole on the Midlatitudes

Abstract The 2002 Antarctic winter was characterized by unusually strong wave activity. The frequency and intensity of the anomalies increased in August and early September with a series of minor stratospheric warmings and culminated in a major stratospheric warming in late September. A three-dimensional high-resolution chemical transport model is used to estimate the effect of the exceptional 2002 Antarctic winter on chemical ozone loss in the midlatitudes and in polar regions. An ozone budget analysis is performed using a range of geographical and chemical ozone tracers. To highlight the unusual behavior of the 2002 winter, the same analysis is performed for the more typical 2001 winter. The ability of the model to reproduce the evolution of polar and midlatitude ozone during these two contrasted winters is first evaluated against ozonesonde measurements at middle and high latitudes. The evolution of the model-calculated 2002 ozone loss within the deep vortex core is found to be somewhat similar to that seen in the 2001 simulation until November, which is consistent with a lower-stratospheric vortex core remaining more or less isolated even during the major warming. However, the simulations suggest that the wave activity anomalies in 2002 enhanced mixing well before the major warming within the usually weakly mixed vortex edge region and, to a lesser extent, within the surrounding extravortex region. As a result of the increased permeability of the vortex edge, the export of chemically activated vortex air is more efficient during the winter in 2002 than in 2001. This has a very noticeable impact on the model-calculated midlatitude ozone loss, with destruction rates being about 2 times higher during August and September in 2002 compared to 2001. If the meteorological conditions of 2002 were to become more prevalent in the future, Antarctic polar ozone depletion would certainly be reduced, especially in the vortex edge region. However, it is also likely that polar chemical activation would affect midlatitude ozone earlier in the winter.

Rayleigh lidar observations of temperature over Tsukuba: winter thermal structure and comparison studies

Routine lidar observations are in progress at the National Institute for Environmental Studies (NIES), Tsukuba, Japan (36°N, 140°E), providing vertical profiles of ozone and temperature in the stratosphere and lower mesosphere. The present study focuses the winter thermal structure over Tsukuba and validation/comparison of the lidar derived temperature profiles. Atmospheric temperatures for the altitude region 30–75 km are determined from the neutral density profiles. 33 temperature observations recorded during the winter months of 1995 and 1996 are used in the study. Winter thermal structure is characterized with its variability with an observed stratopause temperature of about 260 K. Although evidence of minor stratospheric warming is seen on some days, there appears to be no direct linkage with major mid winter warmings at the poles. Temperature profiles show evidence of mesospheric temperature inversion layers. For the purpose of validation/comparison of the lidar temperature profiles, other datasets based on rocketsondes, National Meteorological Center (NMC) analyses, Solar Mesosphere Explorer (SME) spacecraft measurements, and CIRA 86 have been used. Encouraging agreements between the lidar and rocket profiles are evident, which confirms the potential of lidar technique for deriving the absolute temperatures in the atmosphere. Climatological comparisons also reveal satisfactory agreement between the lidar and other datasets.

Impact of stratospheric dynamics and chemistry on northern hemisphere midlatitude ozone loss

The importance of dynamics for stratospheric ozone distribution in the northern hemisphere is investigated by using multiannual simulations of the coupled dynamic‐chemical general circulation model ECHAM3/CHEM. This model includes a parameterization for heterogeneous reactions on the surfaces of polar stratospheric clouds (PSCs) and on sulfate aerosols. A warm and a cold stratospheric winter are examined to estimate the range of chemical ozone loss in the model due to heterogeneous reactions on PSCs. Ozone depletion in the model mainly occurs inside the polar vortex. An additional ozone reduction due to heterogeneous reactions on PSCs is found outside the polar vortex. Secondary vortex formation and vortex contraction after an elongation lead to a transport of air masses with chemically reduced ozone values out of the vortex. Except for such events the edge of the modeled polar vortex acts as a barrier to transport. During the formation of secondary vortices no additional heterogeneous reactions occur therein. Other dynamic events, such as the elongation of the polar vortex and its displacement to lower latitudes, lead to an intense ozone depletion. A minor stratospheric warming in the model causes a total deactivation of chlorine compounds and prevents further ozone depletion. In midlatitudes, the amplitude of short‐term variations of total ozone is amplified by PSC heterogeneous chemical ozone reduction.

N2O transport in a three‐dimensional model driven by U. K. Meteorological Office winds

A three‐dimensional spectral chemical transport model truncated at T21 is employed to simulate N2O transport. The wind and vertical motion fields are taken from the U.K. Meteorological Office four‐dimensional assimilation data set. UARS cryogenic limb array etalon spectrometer (CLAES) N2O measurements are used to initialize the model in late August 1992. Model results are shown to simulate the CLAES measurements quite well over the first few months: N2O variability is similar at extratropical latitudes in the southern hemisphere over the period September 2–17, 1992, at 4.6 and 10 mbar, and there is good agreement in the synoptic maps of minor warmings during this period. Prior to a large warming event on September 30, minor stratospheric warmings are shown to produce negligible changes in the vortex below 4.6 mbar, but considerable mixing of air from the vortex edge and subtropical air is indicated. This results in a steepening of the N2O gradient at the vortex edge. During a warming event when the vortex center moves away from the pole, downward transport by the residual circulation can be large. This is offset by eddy transport effects, but these terms reverse during the recovery from the warming. From September 2 to 17, there is evidence of continuous mixing at midlatitudes at 4.6 mbar in contrast to more discontinuous, warming‐associated mixing at 10 mbar. The breakup of the vortex is initiated by the September 30 warming, and a warming on October 13 has a strong influence on the breakup. The breakup propagates downward. The climatological distribution of N2O in the tropics follows the seasonal variation of the solar radiation with a maximum, which is determined by the strength of the upward residual motion, shifting towards the summer hemisphere by 10°–15° latitude. The surf zone in both the model and the observations at the middle latitudes is well defined, but the gradients of N2O at the edge of the tropics and at the edge of the vortex are smaller in the model than in the observations. This is probably being caused by excessive mixing in the model.