Quasi-biennial Oscillation Winds Research Articles

Interannual Variability of Trace Gases in the Subtropical Winter Stratosphere

Abstract Measurements of water vapor and methane from the Halogen Occultation Experiment instrument on board the Upper Atmosphere Research Satellite are used to study the interannual variability of trace gas distributions in the atmosphere. Particular attention is paid to the mechanisms influencing trace gas distributions in the subtropics. The study highlights the quasi-biennial oscillation (QBO) dependence of subtropical tracer distributions more clearly than in previous studies. There is a strong correlation between the equatorial wind QBO and the slope of the tracer isolines in the Northern Hemisphere subtropics, with steeper subtropical isoline slopes in the easterly phase compared with the westerly phase. This is particularly so in the lower stratosphere. Two possible mechanisms for the QBO signal in subtropical isoline slopes are identified: advection by the mean circulation and isentropic mixing. A comparison between the QBO signal in the slope of the tracer isolines and the isentropic tracer grad...

Global QBO Circulation Derived from UKMO Stratospheric Analyses

Global circulation anomalies associated with the stratospheric quasi-biennial oscillation (QBO) are analyzed based on U.K. Meteorological Office (UKMO) assimilated wind and temperature fields. Zonal winds and temperatures from the assimilation are compared with Singapore rawinsonde data (the standard QBO reference time series), showing reasonable agreement but an underestimate of maxima in the UKMO analyses. Global structure of the QBO in zonal wind, temperature, and residual mean meridional circulation (derived from thermodynamic balance and mass continuity) is isolated, showing coherent tropical and midlatitude components. Important aspects of the QBO revealed in these data include 1) out of phase maxima in temperature (and vertical velocity) between the lower and upper stratosphere, and 2) strong seasonal synchronization of midlatitude anomalies. These characteristics are also evident in long records of satellite radiance measurements.

Answer to Comments on “Space–Time Structure of Monsoon Interannual Variability”

The conceptual model proposed by Khandekar aims at explaining the occurrence of good or bad Indian summer monsoons (ISM). He argues that good or bad monsoon years can be explained by three independent factors: El Nino–Southern Oscillation (ENSO), Eurasian snow cover, and the stratospheric wind quasi-biennial oscillation (QBO). On the other hand, the results in my paper (Terray 1995) suggest that ISM interannual variability is dominated by two low-frequency modes, the biennial (B) mode with a period very close to 2 yr and the lowfrequency (LF) mode with a period close to 4–6 yr. The LF mode contains more energy than the B mode, but the ocean–atmosphere coupling seems to be stronger at the B timescale over the Indian Ocean. Finally, the occurrence and intensity of exceptional ISMs depend upon the coupling or the decoupling of the B and LF modes during recent decades. It is true that Khandekar’s model for drought and flood in the Indian monsoon has some connections with my results and can be, among others, a working hypothesis for explaining them. However, there are still some issues that need to be clarified in Khandekar’s model from my point of view.

The latitudinal structure of the quasi‐biennial oscillation

AbstractThe theory of symmetric time‐dependent meridional circulations in a radiatively damped atmosphere shows that, when the frequency σ of the time variation of an applied body force is smaller than the radiative damping rate α, there are two distinct forms of response to the force. At high latitudes, the response appears primarily as a quasi‐steady meridional circulation, whilst at low latitudes it appears primarily as an acceleration. The transition from ‘low‐latitude’ to ‘high‐latitude’ response occurs at a distance from the equator equation image where N is the buoyancy frequency, D is the depth scale of the force and β is the horizontal gradient of the Coriolis parameter at the equator.It is argued here that this latitudinal scale may also be that of the quasi‐biennial oscillation (QBO), in that the width of the region where there is a substantial QBO in the zonal wind may be determined by the radiative damping rate and the frequency of the oscillation at the equator, rather than by the latitudinal scale of the wave‐momentum fluxes (provided that this latter scale is sufficiently broad). Numerical simulations of QBO‐like oscillations are carried out in a two‐dimensional (height and latitude) model with wave‐momentum fluxes imposed independently at each latitude. Variation of the background rotation rate and the thermal damping rate between these simulations helps to elucidate the effect of these two parameters on the latitudinal structure of the oscillations. In particular, it is shown that, provided that there is weak lateral transport of momentum, the width of the oscillation does indeed correspond to the low‐latitude/high‐latitude transition scale given above. For this mechanism of equatorial confinement, the depth scale D is, in effect, set by the vertical phase variation of the oscillation, which is highly unrealistic in many simple QBO models.

The first simulation of an ozone QBO in a general circulation model

The quasi‐biennial oscillation (QBO) of ozone has been simulated for the first time in a general circulation model (GCM). The model employed is the first version GCM developed at the Center for Climate System Research / National Institute for Environmental Studies (CCSR/NIES). The model has a horizontal resolution of T21 and 60 layers in the vertical. Modified Chapman reactions of ozone photochemistry are included in the model, allowing the ozone to be fully interactive with the radiative and dynamical fields. The simulation is, for the most part, successful except that a deficiency of about 40 DU exists in the total ozone in comparison with observations. The simulated zonal wind QBO has a rather short period (about 1.5 years) and somewhat weak easterlies. The lower stratospheric ozone QBO over the equator is qualitatively similar to that observed, although the simulated amplitude of about 0.3 ppmv is somewhat less than that observed. The phase relationship between the equatorial upper and lower stratospheric ozone QBOs is successfully reproduced, although the phase of the modeled ozone QBO slightly precedes the observed phase in the upper stratosphere. The mid‐latitude ozone QBO is out of phase with the equatorial ozone QBO. The phase propagation of planetary‐scale structure in ozone from the mid‐latitudes to equatorial regions is more pronounced during the westerly than in the easterly phase of the stratospheric QBO, especially in the Northern Hemisphere winter.

Role of the quasi‐biennial oscillation in the transport of aerosols from the tropical stratospheric reservoir to midlatitudes

The temporal evolution of the stratospheric aerosol distribution in the tropical stratospheric reservoir after the eruption of Mount Pinatubo was observed from 1992 to 1995 by the HALOE instrument on the UARS satellite. Since the spatial gradient of aerosol loading is large at the boundaries of the tropical stratospheric reservoir due to the volcanic aerosols, the effect of the meridional circulation on the distribution is seen clearly. The mechanism for dispersal of aerosol in the lower stratosphere from the tropics into midlatitudes strongly depends on the phase of the equatorial zonal wind. The time‐latitude crosssections of the normalized distribution of aerosol on isentropic surfaces are used to observe the equatorial variation as well as change in meridional dispersal during the quasibiennial period. Observed tropical stratospheric winds are used with a simple analytical dynamical model to examine transport processes of tracers from the tropics during several phases of the quasibiennial oscillation (QBO) from 1992 to 1995. The Lagrangian meridional circulation in the tropics is consistent with the vertical and meridional velocities correlated with the QBO in the zonal wind. We find that vertical motion plays a crucial role in vertical and subsequent meridional transport. The pattern of meridional divergence derived from the vertical velocity is closely related to the observed HALOE aerosol distributions and their temporal development in the equatorial region. The westerly (easterly) shear phase of the QBO is associated with sinking (rising) motions at the equator and subsequent poleward (equatorward) transport in the lower stratosphere.

An Analytical Study of Ozone Feedbacks on Kelvin and Rossby–Gravity Waves: Effects on the QBO

An equatorial beta-plane model of the middle atmosphere is used to analytically examine the effects of radiative cooling and ozone heating on the spatial and temporal evolution of the quasi-biennial oscillation (QBO). Under the assumption that the diabatic heating is weak and the background fields of wind, temperature, and ozone are slowly varying, a perturbation analysis yields expressions describing the vertical spatial modulation of Kelvin and Rossby–gravity waves in the presence of ozone. These expressions show that wave-induced changes in the diabatic heating arising from the advection of basic-state ozone reduce the local radiative damping rate by up to 15% below 35 km. In a one-dimensional model of the QBO, eddy ozone heating increases the amplitude of the zonal wind QBO by 1–2 m s−1 and increases the oscillation period by about two months. The significance of these results to the observed QBO is discussed.

Kelvin waves in the equatorial lower stratosphere as revealed by cryogenic limb array etalon spectrometer temperature data

Space‐time variations of atmospheric temperature in the equatorial lower stratosphere are investigated using temperature data derived from the cryogenic limb array etalon spectrometer (CLAES) instrument onboard the Upper Atmosphere Research Satellite (UARS) for January 1992 to May 1993. During this period, clear evidence is observed for eastward progression of zonal wavenumber 1 temperature anomalies. This variation has a period of about 2 weeks and a maximum amplitude at the equator. The vertical scale of this variation is about 10 km and has an eastward phase tilt with increasing height. Consequently, the phase structure propagates downward with time. From these observed features it is concluded that the temperature variation seen in the equatorial lower stratosphere is due to so‐called “slow” Kelvin waves. This observational study using satellite measurements clearly shows much more detail on the global structure of “slow” Kelvin waves than previous studies. Space‐time spectral analysis of temperature variations for the entire CLAES observation period were also performed. During the period January 1992 to May 1993 the background zonal winds (derived from radiosonde observations at Singapore) change from easterlies to westerlies in agreement with the quasi‐biennial oscillation (QBO) in the equatorial lower stratosphere. The slow Kelvin wave activity (measured by the power spectral density averaged over a period of about 2 weeks) is vigorous during the easterly regime but rapidly decreases in the westerly regime of the QBO. This clearly indicates the different transmission characteristics of Kelvin waves in the two QBO wind regimes.

Lower stratosphere coupling with the mesosphere through the QBO

Abstract Long term variations of 2–3 yr in ionospheric absorption data observed at 2.132 MHz over Delhi (28.6°N, 77.2°E) were investigated. The association of this oscillation with that of the stratospheric quasi-biennial oscillation (QBO) in wind data, was examined. The anti-correlation coefficient between the QBO in radio absorption data in the lower ionosphere and wind data at 28 km altitude was found to be 0.8, significant at the 99% confidence level. One of the causes of the presence of the QBO in absorption data seems to be controlled by the QBO in lower stratosphere winds.

Observation of a sunspot penetrating a larger spot of the same polarity : Liu Qing-zhong & Wu Qin-di Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing 210008

Corotating coronal holes of the Sun induce fluctuations of the solar wind speed in the vicinity of the Earth. The fluctuations of solar wind speed are closely correlated with geomagnetic activity. Solar wind speed has been monitored by satellites since the mid 1960s. The long-term series of solar wind speed show enhanced amplitudes at the solar rotation period 27.3 days and at its harmonics 13.6 and 9.1 days. The amplitude series are modulated by a quasi-biennial oscillation (QBO) with a period of 1.75a (21 months) as bispectral analysis reveals. A 1.75a QBO component is also present in the equatorial, zonal wind of the stratosphere at 30 hPa, in addition to the well-known QBO component at the period 2.4a (29 months). The solar wind QBO may influence the stratospheric QBO, the global electric circuit, and cloud cover by modulation of ionospheric electric fields, cosmic ray flux, and particle precipitation. For a further analysis, the series of solar wind speed fluctuations are bandpass-filtered at the period 1.75a. The filtered series provide the amplitude of the solar wind QBO as function of time. The maxima of the solar wind QBO series are correlated with those of the ENSO index. The analysis indicate that the solar wind QBO may trigger the ENSO activity. This result is speculative at the moment. However, the focus of the study is on the investigation of the long-term modulations of the short-term (4–45 days) oscillations of the solar wind speed which are quite unexplored yet.

The influence of the quasi‐biennial oscillation on global constituent distributions

Long‐lived stratospheric constituents observed by the Upper Atmosphere Research Satellite are compared between late northern winters 1991–1992 and 1992–1993. The phase of the quasi‐biennial oscillation (QBO) was such that the equatorial zonal wind was easterly (westerly) in the lower stratosphere during the first (second) winter. In the tropics, winter‐to‐winter differences in the latitudeheight distribution of zonally averaged N2O and H2O are associated with differences in large‐scale upwelling, the QBO's mean meridional circulations, and lateral mixing by incident extratropical Rossby waves. In the extratropics, significant differences in zonally averaged N2O and H2O are also found which are believed to represent the extratropical constituent QBO. The transport processes creating the extratropical constituent QBO are tentatively identified: In the winter extratropics, QBO modulation of the residual circulation appears to dominate in the upper stratosphere, while QBO‐modulated Rossby‐wave isentropic mixing dominates below about 10 mbar. Analysis of Rossby waves at low latitudes, as deduced from isobaric constituent distribution maps, suggests that the meridional distribution of Rossby wave breaking depends on the tropical zonal winds. Easterly QBO winds prevent Rossby‐wave radiation into the tropics, so that a strong constituent concentration gradient is generated in the winter subtropics. Westerly QBO winds allow some wave radiation into the tropics, accompanied by isentropic mixing in the winter subtropics, cross‐equatorial propagation, and occasional mixing in the summer subtropics. These effects depend on altitude and appear to vary in accord with the descending phases of the QBO.

Modeling quasi‐biennial variability in the semiannual double peak

The interannual variability of the stratopause semiannual double peak as seen in methane measurements made by the Halogen Occultation Experiment (HALOE) is investigated using a two‐dimensional isentropic model. It is found that the observed variability is reproduced when the quasi‐biennial oscillation (QBO) of the tropical lower stratosphere is introduced into the model and that there are two distinct means by which the QBO affects stratopause tracer values. First, the QBO modulates the westerly phase of the stratopause semiannual oscillation (SAO) by the selective damping of the parameterized waves responsible for producing this phase of the SAO in the model. This results in a modulation of the strength and duration of the SAO circulation, with it being stronger when the QBO is easterly than when it is westerly. However, when the QBO is easterly, the effect of stronger downwelling in the tropics is negated by subsequent upwelling; the overall effect is to reduce or eliminate the trough of the double peak. For a westerly QBO phase the downwelling, although weaker, is also more persistent, giving a deeper trough. Second, the tropical QBO influences the propagation of planetary waves in the winter hemisphere, thereby producing an extratropical QBO in the winter stratosphere and mesosphere. This alters the circulation in this region, giving more upwelling in the winter extratropics above 0.5 hPa when the QBO (defined at 15 hPa) is westerly. This causes the winter lobe of the double peak to form only when the QBO is westerly. The combined result of the two processes is to produce a double peak when the QBO winds are westerly but a single peak in the summer hemisphere when the QBO winds are easterly. This modeled result agrees well with the interannual variability displayed by the HALOE methane observations.

Vertical structure of the extratropical quasi‐biennial oscillation in ozone, temperature, and wind derived from ozonesonde data

An analysis of the extratropical ozone sounding data shows that ozone, temperature, and wind as well as tropopause height reveal interannual variability with timescales in the range 24–30 months. This quasi‐biennial variability is noticed, as a rule, in a whole range of ozone sounding heights (0–35 km) and connected with the equatorial quasi‐biennial oscillation (QBO). The QBOs in ozone and temperature are revealed differently in the middle and lower stratosphere as well as in the troposphere. In the middle stratosphere a downward phase progression of the ozone QBO is noticed similar to that of the equatorial dynamical QBOs. The rate of the phase descent of the QBO depends on the geographical region and varies from 0.5 km/month to 1 km/month. In the northern hemisphere the rate is greatest in the polar region but lowest in the subtropics. Local maxima are noticed in the vertical structures of the ozone QBOs. In the northern midlatitudes these maxima are centered at heights of about 12, 20, and 30 km. In the lower stratosphere the QBO in ozone is in phase with that in temperature. Mean amplitudes of the temperature QBO in the troposphere are close to those in the stratosphere. The downward phase progression of the temperature QBO is noticed in the northern midlatitude stratosphere. There is an abrupt phase shift between the QBOs in ozone and temperature in the lower stratosphere and those in the middle troposphere. Over the North American stations the phase shift is that the temperature QBOs in the troposphere are in opposite phase with those in the stratosphere. In zonal and meridional winds the QBO are revealed in the northern midlatitudes. In contrast with the QBOs in ozone and temperature, the wind QBOs reveal the small rate of the phase changing with height. There are no significant phase shifts between the wind QBO in the lower stratosphere and that in the troposphere. The maximum amplitudes of the wind QBO are noticed in the vicinity of the tropopause position and at heights of 25–30 km. Amplitudes of the QBO in zonal wind are close to those in meridional wind. The QBO of tropopause height exposed in the northern hemisphere has the greatest amplitude over Japan. The tropopause height QBO is in opposite phase (in phase) with the QBOs in ozone and temperature in the lower stratosphere (troposphere). Vertical structures of the QBOs obtained for ozone, temperature, and wind in the stratosphere as well as the corresponding QBO of tropopause height support a hypothesis that the extratropical QBO is caused by quasi‐biennial forcing of the mean diabatic meridional circulation by planetary‐scale waves with the resulting modulation of vertical advection.

The QBO modulation of the occurrence of the counter electrojet

Continuous hourly measurements of horizontal component of geomagnetic field obtained at Trivandrum, India, have been used to study the occurrence of the Counter Electrojet (CEJ). Because the data cover nearly three solar activity cycles, they can be used to investigate long‐term variations of the CEJ. After the data have been processed and detrented to reduce the influence of solar cycles and annual variations, they reveal a quasi‐biennial oscillation (QBO) of the occurrence of the CEJ. When compared with the QBO wind phase in the stratosphere, the occurrence of the CEJ is found to increase in east phase and to decrease in west one. This relationship suggests a possible mechanism for the production of the CEJ. Under certain conditions the interaction between the upward propagating planetary waves and the solar or lunar semidiurnal tides may cause the semidiurnal tides to be increased in amplitude and shifted in phase, which ultimately results in the occurrence of the CEJ.

On the phase propagation of extratropical ozone quasi‐biennial oscillation in observational data

Global column ozone data from total ozone mapping spectrometer (TOMS), backscattered ultraviolet (BUV) and Dobson stations are analyzed to determine the pattern and phase property of the ozone quasi‐biennial oscillation (QBO) signal. It is found that the ozone QBO signal is strongest in middle and high latitudes and is present mainly in the winter‐spring season in both hemispheres. The extratropical ozone QBO signal is out of phase with the equatorial ozone QBO, which is itself in phase with the QBO in equatorial zonal wind. There are three distinctive regions, namely tropical, midlatitudinal, and polar regions, in each of which the ozone QBO signal has a fairly constant phase with respect to latitude. There is a phase reversal (sign change) between the equatorial and the extratropical regions associated with the return branch of the equatorial QBO secondary circulation, and this sign reversal occurs at ±12° of latitude symmetric about the equator. In the northern hemisphere between the midlatitudinal and polar regions, there is another possible phase reversal in some (but not all) years possibly in connection with the presence or absence of midwinter sudden warming, which creates a positive or negative anomaly relative to the region outside the polar vortex. In the southern hemisphere polar latitudes, the ozone QBO signal is usually delayed until spring in connection with the final warming. These properties are found in all data sets analyzed by the same method. Evidence does not support a gradual phase propagation from the subtropical region to the high‐latitude region. Previous reported evidence for phase propagation is reexamined and is found to be artifacts of data processing.

Seasonal dependence of stratospheric temperature variations

Abstract. Stratospheric temperatures show distinct trends, not necessarily monotonically upward or downward. At the North Pole, trends were large only during winter and spring and were different for different months; downward for November, December, mixed for January and upward for February, March and April. For the 10°-90°N belt, the trends were variable, viz., downward during 1971-1975, upward during 1975-1978 and downward again from 1978 onwards up to date, opposite to the upward trend of ground temperature in the Northern hemisphere in recent years. Twelve-monthly running averages revealed strong QBO (quasi-biennial oscillation). For the North Pole, the QBO showed colder (lower) temperatures during 50-mb wind QBO westerly phase maxima. For the 10°-90°N belt, the QBO was similar for 30 mb and 50 mb but the QBO phases did not match well with 50-mb wind QBO phases.

Quasi‐biennial oscillation and tropical waves in total ozone

Westward and eastward propagating tropical waves in total ozone are investigated in 13 years (1979–1991) of version 6 total column ozone data from the Nimbus 7 total ozone mapping spectrometer (TOMS) satellite instrument. A clear synchronization between the stratospheric quasi‐biennial oscillation (QBO) zonal winds and the fast (periods < 15 days) propagating waves in tropical TOMS data is detailed. Largest total ozone wave amplitudes (∼3–6 Dobson units) occur when their phase propagation direction is primarily opposite the Singapore QBO lower‐stratospheric winds. This effect is most apparent in meridionally symmetric components. Examination of specific episodes, including cross‐spectral calculations with Singapore rawinsonde wind data (10–70 hPa), reveals signatures of tropically confined eastward propagating Kelvin waves of zonal wavenumbers 1–2 during the descending eastward QBO phase, consistent with acceleration of that QBO phase by Kelvin waves. The TOMS results are also consistent with possible forcing of the westward QBO wind phase by episodes of both meridionally symmetric and asymmetric westward waves (W' events in the text). However, in contrast to the case of eastward (Kelvin) waves the strongest westward events (W events in the text) appear to be filtered by, rather than forcing, the westward phase of the stratospheric QBO wind. These dominant westward episodes are interpreted as meridionally symmetric westward global normal modes and tropically confined equatorial‐Rossby waves 2–6. The events exhibit phase and group speeds characteristic of wave dynamics rather than simple wind advection. These results underscore the utility of the long time series and excellent horizontal coverage of TOMS data for dynamical investigations in the relatively observation‐poor tropical stratosphere.

Stratospheric ozone variations in the equatorial region as seen in Stratospheric Aerosol and Gas Experiment data

An analysis is made of equatorial ozone variations for 5 years, 1984–1989, using the ozone profile data derived from the Stratospheric Aerosol and Gas Experiment II (SAGE II) instrument. Attention is focused on the annual cycle and also on interannual variability, particularly the quasi‐biennial oscillation (QBO) and El Niño‐Southern Oscillation (ENSO) variations in the lower stratosphere, where the largest contribution to total column ozone takes place. The annual variation in zonal mean total ozone around the equator is composed of symmetric and asymmetric modes with respect to the equator, with maximum contributions being around 19 km for the symmetric mode and around 25 km for the asymmetric mode. The persistent zonal wavenumber 1 structure observed by the total ozone mapping spectrometer over the equator is almost missing in the SAGE‐derived column amounts integrated in the stratosphere, suggesting a significant contribution from tropospheric ozone. Interannual variations in the equatorial ozone are dominated by the QBO above 20 km and the ENSO‐related variation below 20 km. The ozone QBO is characterized by zonally uniform phase changes in association with the zonal wind QBO in the equatorial lower stratosphere. The ENSO‐related ozone variation consists of both the east‐west vacillation and the zonally uniform phase variation. During the El Niño event, the east‐west contrast with positive (negative) deviations in the eastern (western) hemisphere is conspicuous, while the decreasing tendency of the zonal mean values is maximum at the same time.

Quasi-Biennial Oscillations of Ozone and Diabatic Circulation in the Equatorial Stratosphere

The quasi-biennial oscillation (QBO) in ozone in the equatorial stratosphere is obtained by analyzing the Stratospheric Aerosol and Gas Experiment (SAGE) data from 1984 to 1989. The phase of the ozone QBO in the lower stratosphere is found to precede the zonal wind QBO by several months as opposed to the theoretically expected in-phase relationship between the two. A mechanistic model is developed to explore possible reasons for this disagreement. The model is capable of simulating the actual time evolution of the ozone QBO by introducing the observed zonal wind profile as input. The modeled results confirm the conventional view that the ozone QBO is generated by the vertical ozone advection that is driven to maintain the temperature structure against radiative damping. However, a series of experiments emphasizes the importance of the feedback of the ozone QBO to the diabatic heating through the absorption of solar radiation. Due to this effect, the phase of the ozone QBO shifts up to a quarter cycle ahead and approaches that of the temperature QBO. Because of this inphase relationship, the feedback of the ozone QBO to the diabatic heating acts to compensate for the radiative damping of the temperature structure, thus reducing the magnitude of the induced diabatic circulation. Because the reduction of the magnitude of the vertical motion facilitates downward transport of easterly momentum by the mean flow, this feedback process can help to resolve the insufficiency of the easterly momentum in driving the dynamical QBO in general circulation models (GCMs). It should be emphasized that more sophisticated models that allow for full interaction between the chemical species and radiative and dynamical processes should be developed to improve our understanding of both dynamical and ozone QBOs.

The Modeled Latitudinal Distribution of the Ozone Quasi-Biennial Oscillation Using Observed Equatorial Winds

A simulation of precise years of the quasi-biennial oscillation (QBO) is achieved in a two-dimensional model by relaxing the modeled equatorial winds in the lower stratosphere toward radiosonde observations. The model has been run for the period 1971-90. A QBO signal in column ozone is produced in the model that agrees reasonably well with observational data from the BUV, TOMS, and SAGE II satellite datasets. The model results confirm previous indications of the importance of the interaction of the QBO with the annual cycle in the determination of the subtropical ozone anomaly. The low-frequency modulation of the subtropical ozone anomaly is now particularly clear. The low-frequency modulation of the subtropical ozone anomaly in the model arises as a result of the interaction of the QBO with the annual cycle in the vertical advection by the Hadley circulation. The possibility of a further, similar modulation arising from the interaction of the equatorial wind QBO and the annual cycle in midlatitude eddy activity is discussed, with particular emphasis on the implications for the eddy transfer of ozone to high latitudes and on the ability to predict the severity of the Antarctic ozone hole. A link is proposed between the QBO signal inmore » the severity of the Antarctic ozone hole and the amount of ozone observed in the subtropical/midlatitude springtime maximum in the Southern Hemisphere. On the basis of this relationship, the reliability of the model as a predictor of the severity of the ozone hole is explored. A conclusion of the study is that a reliable predictor of the severity of the ozone hole must take into account the timing of the descent of the equatorial wind QBO at the equator with respect to the annual cycle and that the use, as in previous studies, of a single parameter, such as the sign of the 50-mb equatorial wind, will not be entirely reliable because it cannot do this. 31 refs., 11 figs.« less