Equatorial Lower Stratosphere Research Articles

鉛直シアー流中での内部重力波の水平温度フラックス

The equations of motion describing two-dimensional internal gravity waves were analyzed to derive an expression for the horizontal buoyancy flux-i.e., the correlation between buoyancy (or temperature) and horizontal velocity-in various cases involving vertical shear. It is shown that, although this correlation vanishes at zeroth order for a single wave, its first order contribution is nonzero due to shear, even for steady, conservative, incompressible waves. Departures from steady, conservative motion or quasi-compressibility also cause a nonzero correlation. Several cases were analyzed and some numerical results obtained for waves approaching a critical layer of reduced intrinsic phase speed. With weak shear, the buoyancy flux is small relative to vertical momentum flux, as expected from the perturbation theory. Strong vertical shear enhances the buoyancy flux within the shear zone and causes partial reflection beneath, producing a nonzero correlation (at zeroth order) in this region. These effects may explain recent observations of zonal wind and temperature cospectra in the equatorial lower stratosphere.

1/5セクター・3次元モデルに現れたQBO的な振動について

A QBO-like oscillation has been obtained in a simulation using the 1/5 sector three-dimensional model, with moist convective adjustment, derived from a preliminary version of the Center for Climate System Research/National Institute for Environmental Studies (CCSR/NIES) general circulation model. The basic structure of the oscillation is similar to that of the observed quasi-biennial oscillation (QBO). The amplitude of the oscillation is rather strong, about 30ms-1, which is comparable to the observed oscillation (25ms-1). However, the period of the oscillation is about 400 days, or half as long as that of the observed QBO in the lower equatorial stratosphere. Further, the height of the simulated oscillation is about 15km higher than that of the observed QBO.From a preliminary analysis of the wave behavior, it is suggested that gravity waves are important in the simulation of the QBO-like oscillation.

A Three-Dimensional Modeling Study of the Extratropical Quasi-Biennial Oscillation in Ozone

Abstract The two phases of the quasi-biennial oscillation in ozone are simulated using winds generated by a three-dimensional mechanistic stratospheric model input into an off-line ozone transport model. Ozone chemistry is parameterized in the off-line model. The mechanistic model is run with either easterly or westerly zonal winds in the lower equatorial stratosphere, so as to model the equatorial wind structure during the two phases of the equatorial quasi-biennial oscillation (QBO). When forcing is applied at the lower model boundary in the winter hemisphere, the mechanistic model simulates differences in the global circulation between the easterly and westerly phases of the QBO. The resulting modeled total ozone is larger in the polar regions during the easterly phase of the QBO than during the westerly phase, in agreement with observations. Using the residual-mean formalism the authors find that the difference in the modeled budget of ozone between the two phases of the QBO is due to a modulation of ...

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.

赤道下部成層圏における約2日の周期帯のじょう乱による西風運動量の上向き輸送

赤道下部成層圏における約2日の周期帯のじょう乱による西風運動量の上向き輸送

赤道下部成層圏の短周期擾乱

Temperature and horizontal wind fluctuations with periods shorter than 3 days in the equatorial lower stratosphere are examined by using data from routine rawinsonde observations at Singapore (1N, 104E) during 1978-1993. Internal wave-like structures having a period of about 2 days and a short vertical wavelength of 5 km are occasionally observed both in temperature and wind fluctuations. The result of power spectral analysis indicates that short-period fluctuations have significant energy, separated from Kelvin waves and mixed Rossby-gravity waves. Long-term variations of the spectral characteristics are investigated in relation to the quasi-biennial oscillation (ABO). Zonal wind and temperature variances due to the short-period fluctuations are generally large and particularly enhanced in the ABO phase when the direction of mean wind changes from easterly to westerly. Co-spectra of temperature and zonal wind fluctuations at short periods are significantly larger than the quadrature spectra, and the sign of co-spectra changes according to the phases of the ABO. These results indicate strong connection of the short-period disturbances with the ABO

A model for tropical‐extratropical transport of volcanic ash in the lower stratosphere

Large nonspherical volcanic ash particles up to 30 micrometer in size were collected between 17–19 km altitude over the northern hemisphere at high latitudes between October 1988 and April 1990. These particles may be derived from minor Plinian eruptions in the tropics rather than from localised volcanic activity close to the collection region. Ash particles were injected into the lower equatorial stratosphere where they entered a regime of efficient transport just above the tropopause from the tropical region towards the northern extratropical region. Transport is enhanced by stable autorotation that generates a sufficient lift force to loft nonspherical ash with a rough surface during transport, and by the gradually decreasing altitude of the tropopause from the tropics to the polar regions.

赤道域下部成層圏におけるケルビン波の活動性と準2年周期振動

赤道域下部成層圏におけるケルビン波の活動性と準2年周期振動

The interannual variability of trace gases in the stratosphere: A comparative study of the LIMS and UARS measurement periods

The inter‐annual variability of trace gases in the equatorial lower stratosphere is examined in order to explore the usefulness of comparisons between measurements from LIMS and the UARS satellite instruments. The quasi biennial oscillation (QBO) is simulated in a two dimensional model by relaxing the modelled equatorial winds towards observed values. It is shown that during the LIMS data period the QBO was in the opposite phase to that expected during the first winter of the UARS measurements. The model predicts that LIMS measurements of ozone, NO2, and HNO3 may have been up to 30% less than the long‐term average in the equatorial lower stratosphere while the first few months of UARS measurements may observe abundances up to 30% more than the long‐term average. The importance of taking this variability into account and the desirability of several complete cycles of the QBO in the derivation of climatological values is noted.

The Mean Zonal Flow Response to Rossby Wave and Gravity Wave Forcing in the Equatorial Lower Stratosphere: Relationship to the QBO

Observations show that the westerly acceleration of the equatorial quasi-biennial oscillation (QBO) can be accounted for by Kelvin waves, but that there is a deficiency in the easterly acceleration due to Rossby-gravity waves. Rossby waves and westward propagating gravity waves have been suggested as alternative sources for the easterly acceleration. The possible role of these two wave modes has been tested in a two-dimensional model of the QBO. When the easterly acceleration is due to Rossby waves, the zonal-mean response is steady; when it is due to gravity waves, an oscillation with some features similar to the QBO occurs, but it is of short period and weak amplitude. A similar result occurs when a standing-wave forcing pattern is imposed. These results suggest that Rossby waves play only a minor role in the QBO, and that while the Rossby-gravity mode is essential, other gravity modes may also be important for the easterly phase.

Intensity variation and coherence of 3–6 day equatorial waves

Meridional wind oscillations of 3–6 day period are common in the equatorial lower stratosphere. Upper air data obtained from 32 equatorial stations, 1973–87, show that 70 mb wave intensity over the Maritime continent varied annually with largest amplitude in NH late winter and spring, but the seasonal cycle of intensity was not uniform in the tropics. Time series of meridional velocity and its spatial coherence suggest that 70 mb wave packets were local in longitude and episodic in time.

An estimate of the Antarctic ozone modulation by the QBO

The possible effects of the QBO on the ozone distribution have been studied including in a 2D model a parameterization of Kelvin and Rossby‐gravity wave forcing in the lower equatorial stratosphere. A chemical code complete with heterogeneous reactions allows a simulation of the ozone depletion due to the increase of stratospheric chlorine. With this model we study the possible modulation of the secular trend in the Antarctic ozone hole by the QBO. When heterogeneous chemistry is not included, the model shows a polar ozone oscillation (±6 Dobson Units) comparable to that deduced from early measurements (1970–1975). When heterogeneous reactions are taken into account, the model predicts a larger ozone oscillation in the Southern Hemisphere (±12 Dobson Units) comparable to that obtained from recent observations. This behavior seems to point out a QBO induced temperature effect and its feedback on PSC with the activation of heterogeneous chemistry.

A Multiwave Model of the Quasi-biennial Oscillation

Abstract A simple one-dimensional model of the quasi-biennial oscillation is discussed. Our model is essentially a generalization of the Holton–Lindzen models. We consider a large number of vertically propagating internal waves interacting simultaneously with the mean flow. Effects of both wave damping and critical level absorption are included, but wave–wave interaction is neglected. The effects of momentum advection due to the Hadley circulation are also parameterized. This model is used to study how the mean flow in the equatorial lower stratosphere would respond to forcing by a tropospheric wave spectrum with a significant amount of momentum flux at slow horizontal phase speeds. We find that a “continuous” wave spectrum forces mean flow oscillations in a manner quite similar to a “discrete” two-wave spectrum. But the factors that control the period and amplitude of the oscillations, in the case of a continuous spectrum, seem to be quite different. Our results also suggest that mean rising motion in th...

赤道下部成層圏における季節変化を除いた平均流加速の年周期変動

The quasi-biennial oscillation (QBO) in the equatorial lower stratosphere appears to be influenced by the seasonal cycle, as phase transitions at 50 mb occur primarily in the northern spring/summer season (April-August). Descent of east wind regimes varies widely from one QBO cycle to another. Most of this variation occurs because easterly shears slow down or 'stall' in their descent sometime between July and February. Minimum mean flow accelerations at 50 mb occur in the northern winter season, slightly before the annual minimum in equatorial tropopause temperature. Although a weak effect of the semiannual oscillation can be detected near 10 mb, the seasonal effect over most of the QBO region is annual. The seasonal cycle apparently modulates the onset of QBO phases, and slightly enhances the ability to predict the QBO, but is of insufficient strength or consistency to exactly synchronize the quasi-biennial oscillation with the seasonal cycle.

Laterally‐propagating Rossby waves in the easterly acceleration phase of the quasi‐biennial oscillation

Abstract It is shown that oscillating mean flow solutions exist in the one‐dimensional Holton‐Lindzen (1972) model in the presence of a single Kelvin wave, mean flow diffusion, and an easterly zonal force per unit mass that is constant in height and time except at those points in the time‐height cross‐section where the latitudinally‐integrated mean flow is less than some prescribed easterly value. The latter forcing is intended to crudely represent the absorption of quasi‐stationary planetary Rossby waves at the tropical zero‐wind line. Our results suggest an alternative, and somewhat simpler, possible interpretation of the quasi‐biennial mean zonal wind oscillation in the equatorial lower stratosphere.

Dynamical models of the middle atmosphere for tracer studies

The distribution of ozone and other tracers depends on chemical factors and on the atmospheric circulations, which themselves depend, through heat sources and sinks, on the ozone amount. Examples of the sensitivity of ozone to variations in the dynamics are given from a two dimensional model. The total ozone depends greatly on the heating rate in the equatorial low er stratosphere; changes of tens of milli atmosphere centimetres f arise from alteration of only a few tens of kelvins per day in heating. The effects of variations in m om entum flux and mixing-surface slope are also discussed. T he current ability to model dynamics for tracer studies is reviewed. Only three dimensional models provide a consistent formulation; they simulate many observed atmospheric phenomena broadly b u t not in detail. However, the amount of computing needed precludes all but the simplest treatment of chem istry. Two dimensional models offer a valuable com promise. Although these are rather empirical and not adequately based on sound physical principles, good agreement can be found in practice between observed and modelled ozone, winds and temperatures. The calculation of model-dependent meridional circulations in two dimensional models is shown to be important, for which a parametrization of the momentum flux is an essential prerequisite. Some observations are given tha t suggest that a parametrization based on potential vorticity fluxes may be possible. This would be consistent with the usual formulation of tracer transport.

Quasi-biennial oscillation and summer southwest monsoon

Stratospheric circulation in the height range of 16 to 60 km in the equatorial region, has been studied using the rocket wind data of Thumba. It is inferred that the quasi-biennial oscillation in the lower and middle stratosphere are out of, phase by six months. The dominant feature in the lower stratosphere is the appearance of the monsoon easterlies during the period June to September. The westerlies or easterlies in the lower stratosphere over the equator extend to about 10°N in the period November to May. In the middle stratosphere, the quasi-biennial oscillation follows a regular alternation of easterlies and westerlies with a downward phase propogation of one km per month, while the quasi-biennial oscillation in the lower stratosphere interacts with the monsoon circulation. The four possible wind phases W, W IE, E and EIW during March to May in the equatorial lower stratosphere are linked with the onset of southwest monsoon near the normal date and normal rains over India. Appearance of the easterlies in the lower stratosphere over Gan Island in May is an indication of the onset of monsoon over Kerala a month later. The Berson westerlies are a poleward extension of the equatorial westerlies, rather than a descent from the upper stratosphere. The out of phase relationship between QBO in the lower and middle, stratospheres suggests an interaction between the QBO and monsoon circulation. The phase of the QBO, over the equator appears to give a prior indication of the onset of the southwest monsoon over Kerala and also the rainfall in the central parts of the country.

The upper portion of the Hadley cell circulation as deduced from the 1968 French and Chinese nuclear tests

French and Chinese nuclear testing in 1968 provided an opportunity for studying stratospheric transport processes. The French tests of August and September 1968 and the Chinese test of December 1968 provided radioactive tracers to follow air motions in the lower stratosphere of each hemisphere. The two sources are readily distinguishable by fission product ratios (e.g., Zr-95/Ce-144). Information on interhemispheric exchange in the upper troposphere and lower stratosphere has been developed by the analysis of air-filter samples obtained by aircraft and balloons. The meridional transport of radioactive debris into the lower equatorial stratosphere from two sources is qualitatively explained by a mean meridional circulation superimposed on large-scale mixing processes. It is further suggested that the mean circulation is an extension of the tropospheric Hadley Cell into the lower stratosphere.

赤道下部成層圏の大規模擾乱による垂直輸送

赤道下部成層圏の大規模擾乱による垂直輸送

冬期成層圏における水平運動量輸送の年々変化

Year to year variations of horizontal flux of momentum due to the planetary-scale disturbances in the winter stratosphere are studied based on the 3-month mean statistics throughout five winters from 1963 to 1967 by the use of the gridpoint data of 30mb isobaric heights over the Northern Hemisphere. Computations are made to separate the total momentum transport into the standing eddy flux and the transient eddy flux, as well as into the zonal wave numbers, for the purpose of clarifying the nature of planetary waves.It is likely that the poleward flux of momentum observed in the lower latitudes is caused by the penetration of planetary waves from the middle latitudes into the low-latitude stratosphere. It is also found that, in contrast to Wallace and Newell (1966), the year to year variation of total momentum transport does not necessarily show a direct relation to the quasi-biennial oscillation of the zonal wind in the equatorial lower stratosphere, but that planetary wave activity in the winter stratospheric circulation has, in a broad sense, a close relation to the zonal wind in the tropics.