Equatorial Stratosphere Research Articles

On the Development of the Theory of the QBO

The events that led to the discovery of the quasi-biennial oscillation (QBO) of the equatorial stratosphere are described together with the development of current QBO theory. Three independent areas of inquiry led to this theory: (1) the observational and theoretical studies of equatorial waves, (2) the theoretical study of the behavior of mountain waves at critical levels where the mean flow speed went to zero, and (3) the semiempirical study of the momentum budget of the QBO. The efforts in each of these three areas are discussed.

The Ozone Hole, Dynamically Speaking

The Ozone Hole, Dynamically Speaking

On the Variation in Period and Amplitude of the Quasi-biennial Oscillation in the Equatorial Stratosphere, 1951–85

The quasi-biennial variation in zonal-wind period and amplitude is examined for Balboa and Ascension at 50, 30 and 10 mb, and for Singapore at 50 and 30 mb, through July 1985. The increase in zonal-wind period following the Agung eruption in 1963 is shown to be due almost entirely to an increase in west-wind half period at 50 mb but east-wind half period at 10 mb. There have been average correlation coefficients of 0.96 and 0.71 between zonal-wind half periods at 50 mb and the next downcoming half period of opposite phase at 10 mb. There is little indication of a relation between zonal-wind period or amplitude and equatorial sea-surface temperature (El Niño), and the nearly inverse relation between zonal-wind period and sunspot number is not significant at the 5% level taking into account serial correlations. The 30 mb west-wind amplitude was relatively large at Balboa and small at Ascension after Agung, but there is little evidence of an appreciable change in amplitude or period following the EX Chichón eruption in 1982. Consequently, the impact of volcanic eruptions on the quasi-biennial oscillation is uncertain at this time.

Longitudinal differences and inter-annual variations of zonal wind in the tropical stratosphere and troposphere

A quantitative assessment has been made of the longitude-dependent differences and the interannual variations of the zonal wind components in the equatorial stratosphere and troposphere, from the analysis of rocket and balloon data for 1979 and 1980 for three stations near ±8.5° latitude (Ascension Island at 14.4°W, Thumba at 76.9°E and Kwajalein at 67.7°E) and two stations near 21.5° latitude (Barking Sands at 159.6°W and Balasore at 86.9°E). The longitude-dependent differences are found to be about 10–20 m s −1 (amounting to 50–200% in some cases) for the semi-annual oscillation (SAO) and the annual oscillation (AO) amplitudes, depending upon the altitude and latitude. Inter-annual variations of about 10 m s −1 also exist in both oscillations. The phase of the SAO exhibits an almost 180° shift at Kwajalein compared to that at the other two stations near 8.5°, while the phase of the AO is independent of longitude, in the stratosphere. The amplitude and phase of the quasi-biennial oscillation (QBO) are found to be almost independent of longitude in the 18–38 km range, but above 40 km height the QBO amplitude and phase have different values in different longitude sectors for the three stations near ±8.5° latitude. The mean zonal wind shows no change from 1979 to 1980, but in the troposphere at 8.5° latitude strong easterlies prevail in the Indian zone, in contrast to the westerlies at the Atlantic and Pacific stations.

Sudden mesospheric warming over equatorial region

Using M-100 meteorological rocketsonde data collected over Thumba Equatorial Rocket Launching Station (8°32′N, 76°52′E), Trivandrum, India for the period January to May 1972, a sudden mesospheric warming followed by an almost equal cooling has been detected over this region on 1st March 1972. The Thumba equatorial mesospheric temperature showed an increase of about 50 to 60 °C in an altitude region between 60 and 70 km from 23 February to 1 March 1972 which was followed by a cooling of about 50 to 70 °C from 1 to 15 March in that altitude region. Sudden warmings in the stratosphere at high latitudes during winter time are well known. However, sudden mesospheric warnings over equatorial regions have not been observed too frequently and are thus relatively less studied compared to high latitude warnings. The sudden warming observed in the equatorial mesosphere on 1st March 1972 is quite rare and unique event which followed minor midwinter stratospheric warmings at high latitudes in the Northern Hemisphere, although the equatorial stratosphere remained practically undisturbed in its thermal structure during this period.

Quasi-biennial Oscillation and Its Analog under the Assumption of Wave Self-Acceleration

We have investigated numerically, using a slowly varying model, how the behavior of the quasi-biennial oscillation (QBO) in the equatorial stratosphere and its analog in the laboratory water tank are modified when wave phase velocities are allowed to be variable in time and space. Associated modifications of the wave characteristics were examined as well. The differences between the wave self-acceleration assumption and a constant phase velocity assumption are significant for the equatorial QBO, probably due to strong deformation of the mixed Rossby–gravity waves. The oscillation period of the equatorial QBO is extended approximately 25%, which is too large to be simply neglected in obtaining a correct oscillation period. Easterly wind speed is more accelerated than expected and exceeds the initial value of the mixed Rossby–gravity wave phase velocity. At the same time, westward phase velocity of the mixed Rossby–gravity wave is drastically accelerated at almost twice the initial value. Eastward phase velocity acceleration of the Kelvin wave is restricted to within only 15%. Phase velocity of the mixed Rossby–gravity wave decreases to zero in the westerly regime. Phase velocity acceleration (or deceleration) of the mixed Rossby–gravity wave is predominant in the upper region of the QBO phenomenon and amplitude of the wave must be small there due to strong damping experienced Phase velocities of the internal gravity waves in the case of the QBO analog reproduced in a water tank are accelerated (or decelerated) very little (within 10% of the initial phase velocity) except in the early transient stage. Increase of the oscillation period associated with incorporation of wave self-acceleration is also restricted to within 10%. The reason that the internal gravity waves, together with the Kelvin wave, do not experience significant wave self-acceleration may be principally due to the smaller group velocities.

A review of synoptic scale wave perturbations in the equatorial stratosphere

We review the observations of waves with periods in the range from roughly 1 day to 1month in the equatorial stratosphere and briefly compare these to the theoretical predictions. It is well established that the Kelvin and mixed Rossby-gravity waves dominate synoptic wave perturbations in the tropical stratosphere. However, there are still many open questions about the sources responsible for generating these waves, their interaction with the large-scale environment, including momentum and energy transport, the rate of decrease of the wave amplitude away from the equator, and the effect of the large-scale flow on the observed periods and vertical structure. Observations of Rossby waves have also been claimed, though usually only as the residual of the wind field once the Kelvin and mixed Rossby-gravity wave contributions have been subtracted. Theoretical predictions show that the amplitude of the meridional wind perturbation associated with the two-day wave, a specific Rossby mode, should be significant above 30 km at the equator. So far, the difficulty of observing the upper stratosphere has precluded confirmation of the theory. Recent high altitude-resolution wind measurements by balloon and radar have shown quasi-inertial waves to be a ubiquitous feature of the altitude regime covering the lowest 5–10 km of the stratosphere. The wave amplitudes and observed dissipation imply redistribution of significant amounts of energy and momentum. Also, quasi-inertial waves appear to be responsible for generating thin, long-lived turbulent layers, which are a common feature in the lower stratosphere.

Climatology of the Equatorial Lower Stratosphere

Twenty years of radiosonde data have been analyzed in an attempt to develop a latitudinal structure climatology of winds, temperature and geopotential at 30 and 50 mb in the equatorial stratosphere. The fine latitudinal resolution provided by the WMO station network reveals several interesting features in the latitudinal structure of the annual and quasi-biennial cycles which dominate this region. For example, the westerly and easterly acceleration phases of the quasi-biennial oscillation are markedly different. Westerly accelerations appear first at the equator, spreading outward with time to higher latitudes, and are more intense, on average, than the easterly accelerations. The easterly accelerations are more uniform in latitude, but less uniform in time, sometimes occurring in two stages. The quasi-biennial wind and temperature oscillations are symmetric about the equator, while the annual harmonic in zonal wind is antisymmetric about the equator, but is not proportional to the Coriolis parameters. Monthly mean zonal wind and temperature appear to be in thermal wind balance at the equator. Some brief remarks are also made concerning variability of the quasi-biennial oscillation and the effects of El Chichon.

A rocket‐borne photometric measurement of O3 and NO2 in the equatorial stratosphere

A determination of the NO2 and O3 mixing ratios has been made by measuring the attenuation of sunlight at 409 nm and 254, 288, 316 nm from an ascending rocket launched from Kourou, French Guiana (5°N) during the morning twilight of March 21, 1974. A peak NO2 volume mixing ratio of 4.4. x was measured at 30 km altitude.

Middle atmosphere dynamics

The dynamics of the middle atmosphere, the stratosphere, and the mesosphere are considered. After a brief overview of the structure of the middle atmosphere, a circulation equation is derived and used to understand the mean conditions. In particular, the vertical circulation that results from localized forcing is shown to affect a deep layer. Moreover, this framework provides a view of the meridional overturning circulation as a wave-driven pump due to planetary wavebreaking. Extreme examples of such wavebreaking, which lead to sudden stratospheric warming, are discussed. Waves of the equatorial stratosphere are reviewed and their roles in the quasi-biennial oscillation (QBO) are discussed.

Wave Transience in a Compressible Atmosphere. Part II: Transient Equatorial Waves in the Quasi-Biennial Oscillation

Abstract The Holton-Lindzen (1972) theory of the quasi-biennial oscillation is reevaluated in the light of some recent developments in the theory of wave, mean-flow interaction due to Andrews and Mclntyre (1976a, b). These developments suggest that wave transience should be regarded as the primary cause of the oscillation, in a chronological sense, with wave absorption providing an essential, but chronologically secondary cause. The spontaneous formation and descent of shear zones anticipated in the analytic theory of Part I is here applied to the theory of the oscillation, by extending the numerical calculations of that paper to include equatorial waves of the type observed in the equatorial stratosphere. Some remarks are also made concerning probable causes of the observed QBO variability.

Wave Transience in a Compressible Atmosphere. Part I: Transient Internal Wave, Mean-Flow Interaction

Abstract Vertically propagating internal waves give rise to mean flow accelerations in an atmosphere due to the effects of wave transience resulting from compressibility and vertical group velocity feedback. Such accelerations appear to culminate in the spontaneous formation and descent of regions of strong mean wind shear. Both analytical and numerical solutions are obtained in an approximate quasi-linear model which describes this effect. The numerical solutions display mean flow accelerations due to Kelvin waves in the equatorial stratosphere. Wave absorption alters the transience mechanism in some significant respects, particularly in causing the upper atmospheric mean flow acceleration to be very sensitive to the precise magnitude and distribution of the damping mechanisms. Part II of this series discusses numerical simulations of transient equatorial waves in the quasi-biennial oscillation. These results are of sufficient qualitative interest to merit attention in this paper, and this is done with t...

Atmospheric Quasi-Biennial Oscillations

Abstract A detailed analysis has been made of a quasi-biennial oscillation (QBO) in the tropospheric ultralong waves of the Southern Hemisphere and interrelationships with the QBO in zonal mean westerly winds of the equatorial stratosphere. In spite of the fact that highly significant spectral peaks at a period close to 26 months occur in both phenomena, results of cross-spectral analysis reveal that the two QBO's are unrelated. The manner in which these QBO's relate to a recently discovered QBO in the 500 mb zonal mean westerly winds throughout the Southern Hemisphere is also considered.

A Lagrangian mean theory of wave, mean‐Flow interaction with applications to nonacceleration and its breakdown

A review is given of a new Lagrangian mean theory of wave transport. Attention is focused on the so‐called ‘nonacceleration’ theorem, and it is shown that such a theorem arises naturally in the Lagrangian mean framework. Also discussed is a simple example of the Stokes drift, a concept which is central to nonacceleration. The Lagrangian mean theory substantially simplifies and unifies our understanding of wave driving in cases where nonacceleration is violated because of wave transience and dissipation. Moreover, the theory has given new insights in one particular case, that of Rossby gravity wave, meanflow interaction. These insights have successfully explained some hitherto unresolved paradoxes in the theory of the quasi‐biennial oscillation of zonal wind in the equatorial stratosphere. Some brief remarks are also made concerning some of the outstanding difficulties of the theory in need of future investigation.

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.

Interannual Variability of the 500 mb Zonal Mean Flow in the Southern Hemisphere

Abstract An analysis has been made of the means and variability of the 500 mb field in the Southern Hemisphere, with accent on the zonal means of geopotential height and westerly wind. Long-term means for May 1972-January 1978 are significantly different from previous analyses and reveal very large and significant trends in geopotential height, especially over Antarctica. Standard deviations of zonal winds and heights are larger than in the Northern Hemisphere and, in contrast to the Northern Hemisphere, are lowest in winter. Temporal variations in the zonal mean component of the flow are very pronounced and some aspects of the very anomalous flow in December 1976 are documented. Interannual variations show a remarkable quasi-biennial fluctuation in the zonal mean fields with a systematic progression of the anomalies from low to high latitudes. These are correlated with the quasi-biennial oscillation in the equatorial stratosphere.

Equatorial Wave-Mean Flow Interaction: A Numerical Study of the Role of Latitudinal Shear

A time-dependent primitive equation model for an equatorial channel is used to assess the interaction of equatorial Kelvin and mixed Rossby-gravity waves with the mean flow. The proposed model involves a semiimplicit time-differencing scheme and a finite-difference grid in the meridional plane. It is shown that forced equatorial waves interact with mean flow to produce equatorial jets characterized by downward-moving westerly (Kelvin wave forcing) and easterly (mixed Rossby-gravity wave forcing) shear zones, respectively. For parameters characteristic of the observed waves in the equatorial stratosphere, the wave-mean flow interaction process always reduces the amplitude of any initial cross-equatorial mean wind shear. The mean flow profile tends to become symmetric about the equator as the interaction process continues.

Evidence for Vertical Motions in the Equatorial Middle Atmosphere

The Diurnal Experiment, which took place in March 1974, offers a unique data set for use in studying vertical velocities in the equatorial stratosphere and mesosphere. Application of the standard thermodynamic vertical velocity equation along with a generalized thermal wind relation reveals magnitudes of vertical velocities ranging from 1-2 cm/s in the lower stratosphere to 10-15 cm/s in the lower mesosphere. Diurnal and semidiurnal tidal modes are quite apparent and, in agreement with tidal theory, the amplitudes of the oscillations increase with height. Inspection of time cross sections and use of Fourier analysis indicates vertical wavelengths on the order of 8-16 km. These results suggest that modification of temperature and wind profiles in the stratosphere and lower mesosphere on an hourly basis can be quite significant. The analysis also suggests the presence of unresolved vertical winds on both longer and shorter time scales than the principal tidal components.

Radiative equilibrium temperatures in the stratosphere and mesosphere ‐ A comparison for the stellar occultation and BUV ozone data

Recently published ozone densities in the equatorial stratosphere and mesosphere from satellite stellar occultation measurements are appreciably larger than previous independent midlatitude measurements and ozone densities predicted by photochemical theory. We have calculated radiative equilibrium temperature profiles with a stratosphere‐mesosphere optimized model for each of these ozone profiles and find that the stellar occultation studies cannot be discounted on energy balance grounds.

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.