Quasi-biennial Oscillation Period Research Articles

Sensitivity of the quasi‐biennial oscillation simulated in WACCM to the phase speed spectrum and the settings in an inertial gravity wave parameterization

AbstractThe application of inertial gravity wave parameterization has allowed for the spontaneous generation of quasi‐biennial oscillation (QBO) in the Whole Atmosphere Community Climate Model (WACCM), although there is some mismatch when comparing with observations. The parameterization is based on Lindzen's linear saturation theory, modified to describe inertia‐gravity waves (IGW) by considering the Coriolis effect. In this work, we improve the parameterization by importing a more realistic IGW phase speed spectrum that exhibits a double peak Gaussian distribution calculated from tropical radiosonde observations. A series of numeric simulations are performed to test the sensitivity of QBO‐like oscillation features to the phase speed spectrum and the settings of parameterized IGW. All these simulations are capable of generating equatorial wind oscillations in the stratosphere based on standard spatial resolution settings. Central phase speeds of the “double‐Gaussian parameterization” affect QBO magnitudes and periods, and the momentum flux of IGW determines the acceleration rate of zonal wind. Furthermore, stronger IGW forcing can lead to a propagation of the QBO‐like oscillation to lower altitude. The intermittency factor of the parameterization also prominently affects the QBO period. Stratospheric QBO‐like oscillation with obvious improvements is generated using the new IGW parameterization in a long‐time simulation.

Seasonal, inter-annual and solar cycle variability of the quasi two day wave in the low-latitude mesosphere and lower thermosphere

We analyzed 17 years (1993–2009) of horizontal winds measured by the medium frequency (MF) radar located at Tirunelveli (8.7°N, 77.8°E) and 10 years (2005–2014) of horizontal winds measured by a meteor radar located at Thumba (8.5°N, 77°E) to examine the seasonal, inter-annual, and solar cycle variability of the Quasi-Two Day Wave (QTDW) in the mesosphere and lower thermosphere region. These two radars are nearly co-located, but differ in their measurement technique. Comparison of the estimated QTDW amplitudes by the two radars shows that the amplitudes are larger in the meteor radar than those in the MF radar. The difference between the amplitudes is larger in May in the zonal component and in April and September in the meridional one. Furthermore, the differences are larger in the meridional component. The QTDWs in both the radars show a strong semi-annual oscillation (SAO). In addition, the meridional QTDW amplitudes of both the MF and meteor radars show a distinct enhancement in the month of October. While the whole spectra of QTDWs contribute to the SAO amplitudes, only 45–50h waves contribute to the October enhancement. The amplitudes of the QTDWs, in general, show large inter-annual variability. The QTDW amplitudes from both the radars show modulation at period of quasi-biennial oscillation. The QTDWs of the MF radar show a small negative correlation with solar activity while those of meteor radar do not show any correlation. The above aspects are discussed in the light of current understanding of the QTDWs.

The QBO, gravity waves forced by tropical convection, and ENSO

AbstractBy means of theory, a simplified cartoon illustrating wave forcing of the stratospheric quasi‐biennial oscillation (QBO), and general circulation modeling of the QBO, it is argued that the period of the QBO is mainly controlled by the magnitude of the gravity wave (GW) vertical fluxes of horizontal momentum (GWMF) forcing the QBO, while the QBO amplitude is mainly determined by the phase speeds of the GWs that make up this momentum flux. It is furthermore argued that it is the zonally averaged GWMF that principally determines the QBO period irrespective of the longitudinal distribution of this GW momentum flux. These concepts are used to develop a hypothesis for the cause of a previously reported El Niño–Southern Oscillation (ENSO) modulation of QBO periods and amplitudes. Some observational evidence is reported for the ENSO modulation of QBO amplitudes to have been different before the 1980s than after about 1990. A hypothesis is also given to explain this in terms of the different ENSO modulation of tropical deep convection that took place before the 1980s from that which occurred after about 1990. The observational evidence, while consistent with our hypotheses, does not prove that our hypotheses are correct given the small number of El Niños and La Niñas that occurred in the early and later periods. Further research is needed to support or refute our hypotheses.

Modeling the QBO-Improvements resulting from higher-model vertical resolution.

Using the NASA Goddard Institute for Space Studies (GISS) climate model, it is shown that with proper choice of the gravity wave momentum flux entering the stratosphere and relatively fine vertical layering of at least 500 m in the upper troposphere‐lower stratosphere (UTLS), a realistic stratospheric quasi‐biennial oscillation (QBO) is modeled with the proper period, amplitude, and structure down to tropopause levels. It is furthermore shown that the specified gravity wave momentum flux controls the QBO period whereas the width of the gravity wave momentum flux phase speed spectrum controls the QBO amplitude. Fine vertical layering is required for the proper downward extension to tropopause levels as this permits wave‐mean flow interactions in the UTLS region to be resolved in the model. When vertical resolution is increased from 1000 to 500 m, the modeled QBO modulation of the tropical tropopause temperatures increasingly approach that from observations, and the “tape recorder” of stratospheric water vapor also approaches the observed. The transport characteristics of our GISS models are assessed using age‐of‐air and N2O diagnostics, and it is shown that some of the deficiencies in model transport that have been noted in previous GISS models are greatly improved for all of our tested model vertical resolutions. More realistic tropical‐extratropical transport isolation, commonly referred to as the “tropical pipe,” results from the finer vertical model layering required to generate a realistic QBO.

Stratospheric Aerosols from Major Volcanic Eruptions: A Composition-Climate Model Study of the Aerosol Cloud Dispersal and e-folding Time

Large explosive volcanic eruptions are capable of injecting considerable amounts of particles and sulfur gases above the tropopause, causing large increases in stratospheric aerosols. Five major volcanic eruptions after 1960 (i.e., Agung, St. Helens, El Chichón, Nevado del Ruiz and Pinatubo) have been considered in a numerical study conducted with a composition-climate coupled model including an aerosol microphysics code for aerosol formation and growth. Model results are compared between an ensemble of numerical simulations including volcanic aerosols and their radiative effects (VE) and a reference simulations ensemble (REF) with no radiative impact of the volcanic aerosols. Differences of VE-REF show enhanced diabatic heating rates; increased stratospheric temperatures and mean zonal westerly winds; increased planetary wave amplitude; and tropical upwelling. The impact on stratospheric upwelling is found to be larger when the volcanically perturbed stratospheric aerosol is confined to the tropics, as tends to be the case for eruptions which were followed by several months with easterly shear of the quasi-biennial oscillation (QBO), e.g., the Pinatubo case. Compared to an eruption followed by a period of westerly QBO, such easterly QBO eruptions are quite different, with meridional transport to mid- and high-latitudes occurring later, and at higher altitude, with a consequent decrease in cross-tropopause removal from the stratosphere, and therefore longer decay timescale. Comparing the model-calculated e-folding time of the volcanic aerosol mass during the first year after the eruptions, an increase is found from 8.1 and 10.3 months for El Chichón and Agung (QBO westerly shear), to 14.6 and 30.7 months for Pinatubo and Ruiz (QBO easterly shear). The corresponding e-folding time of the global-mean radiative flux changes goes from 9.1 and 8.0 months for El Chichón and Agung, to 28.7 and 24.5 months for Pinatubo and Ruiz.

Synchronisation of the equatorial QBO by the annual cycle in tropical upwelling in a warming climate

The response of the period of the quasi‐biennial oscillation (QBO) to increases in tropical upwelling are considered using a one‐dimensional model. We find that the imposition of the annual cycle in tropical upwelling creates substantial variability in the period of the QBO. The annual cycle creates synchronisation regions in the wave forcing space, within which the QBO period locks onto an integer multiple of the annual forcing period. Outside of these regions, the QBO period undergoes discrete jumps as it attempts to find a stable relationship with the oscillator forcing. The resulting set of QBO periods can be either discrete or broad‐banded, depending on the intrinsic period of the QBO.We use the same model to study the evolution of the QBO period as the strength of tropical upwelling increases, as would be expected in a warmer climate. The QBO period lengthens and migrates closer towards 36‐ and 48‐month locking regions as upwelling increases. The QBO period does not vary continuously with increased upwelling, however, but instead transitions through a series of two‐ and three‐cycles before becoming locked to the annual cycle. Finally, some observational evidence for the cyclical behaviour of the QBO periods in the real atmosphere is presented.

Attribution of variations in the quasi-biennial oscillation period from the duration of easterly and westerly phases

This study reports the main features of quasi-biennial oscillation (QBO) period variability at stratospheric levels from 70 to 10 hPa and its attribution from the duration variability of westerly and easterly phases using monthly mean zonal wind data from August 1956 to July 2013, archived by Free University of Berlin. A total of 24 QBO events have been distinguished based on the zonal wind field and wavelet analysis for it. The QBO period varies in phase at various stratospheric levels and shows no significant long-term trend but decadal to multi-decadal variability. The noted case-to-case variations in QBO period are due to variations in durations of the westerly and easterly phases at the same level. The highly coupled variability of the easterly duration in the upper levels above 30 hPa and westerly durations in the lower levels below, which manifests the stalling or accelerating of the descent rate of easterly wind regimes around 30 hPa, is found to be the dominant variability of the easterly and westerly durations at various stratospheric levels. Accordingly, the period of QBO in the lower levels below 40 hPa/upper levels above 20 hPa is determined by the westerly/easterly durations there in about 75 % of the 24 QBO events; and at 30 hPa, variations in the durations of both easterly and westerly phases contribute to the QBO period variability. On the contrary, in only 4 out of 24 QBO events, the variations of the westerly/easterly durations in the upper/lower levels are greater than the variations of the easterly/westerly durations in the upper/lower levels, making deterministic contributions to the QBO period variability.

Correlation of the quasi-biennial oscillations in galactic cosmic rays and in the solar activity indices

Quasi-biennial oscillation (QBO) is a well-known variation in solar activity, interplanetary parameters, geomagnetic disturbances and cosmic rays. Solar QBO is translated to the space via open magnetic flux and modulates intensity of cosmic rays. The highest negative correlation exists in the QBO of cosmic rays with QBO in the heliospheric magnetic field strength B as well as with QBO in the scalar product BV, where V is the solar wind velocity, cosmic ray being delayed by ≈ 1 month. During ≈ 50 years of cosmic ray monitoring the QBO periods demonstrated some intermittency. It is argued that the Gnevyshev Gap effect and the step-like changes in the cosmic ray intensity appeared to be a part of QBO in cosmic rays.

The quasi-biennial oscillation in a warmer climate: sensitivity to different gravity wave parameterizations

In order to simulate the quasi-biennial oscillation (QBO) with a realistic period and amplitude, general circulation models commonly include parameterizations of small scale gravity waves (GW). In this work, we explore how different GW parameterization setups determine the response of QBO properties to a warmer climate. Atmosphere-only experiments in both present day and warmer climate serve as testbed to analyze the effect of four different GW parameterization setups, active in the tropics. Having tuned the GW parameterizations to produce a realistic QBO in present day climate, we analyze changes of QBO properties in the warmer climate. The QBO period decreases in two parameterization setups by ~30 %, while the QBO period remains unchanged in the remaining two parameterization setups. In all parameterization setups, the QBO amplitude in the warmer climate weakens below 10 hPa but shows different behaviour above 10 hPa. We show that changes in QBO amplitude and changes in QBO period are inconsistent among experiments. In the chosen experimental design, the inconsistent future change in QBO properties among the suite of experiments depends solely on the choice of the GW parameterization setup.

The QBO in two GISS global climate models: 1. Generation of the QBO

AbstractThe adjustment of parameterized gravity waves associated with model convection and finer vertical resolution has made possible the generation of the quasi‐biennial oscillation (QBO) in two Goddard Institute for Space Studies (GISS) models, GISS Middle Atmosphere Global Climate Model III and a climate/middle atmosphere version of Model E2. Both extend from the surface to 0.002 hPa, with 2° × 2.5° resolution and 102 layers. Many realistic features of the QBO are simulated, including magnitude and variability of its period and amplitude. The period itself is affected by the magnitude of parameterized convective gravity wave momentum fluxes and interactive ozone (which also affects the QBO amplitude and variability), among other forcings. Although varying sea surface temperatures affect the parameterized momentum fluxes, neither aspect is responsible for the modeled variation in QBO period. Both the parameterized and resolved waves act to produce the respective easterly and westerly wind descent, although their effect is offset in altitude at each level. The modeled and observed QBO influences on tracers in the stratosphere, such as ozone, methane, and water vapor are also discussed. Due to the link between the gravity wave parameterization and the models' convection, and the dependence on the ozone field, the models may also be used to investigate how the QBO may vary with climate change.

Examining the stratospheric response to the solar cycle in a coupled WACCM simulation with an internally generated QBO

Abstract. The response of the stratosphere to the combined interaction of the quasi-biennial oscillation (QBO) and the solar cycle in ultraviolet (UV) radiation, and the influence of the solar cycle on the QBO, are investigated using the Whole Atmosphere Community Climate Model (WACCM). Transient simulations were performed beginning in 1850 that included fully interactive ocean and chemistry model components, observed greenhouse gas concentrations, volcanic eruptions, and an internally generated QBO. Over the full length of the simulations we do not find a solar cycle modulation of either the QBO period or amplitude. We also do not find a persistent wintertime UV response in polar stratospheric geopotential heights when stratifying by the QBO phase. Over individual ~40 year periods of the simulation, a statistically significant correlation is sometimes found between the northern polar geopotential heights in February and UV irradiance during the QBO's westerly phase. However, the sign of the correlation varies over the simulation, and is never significant during the QBO's easterly phase. Complementing this is the analysis of four simulations using a QBO prescribed to match observations over the period 1953–2005. Again, no consistent correlation is evident. In contrast, over the same period, meteorological reanalysis shows a strong positive correlation during the QBO westerly phase, although it weakens as the period is extended. The results raise the possibility that the observed polar solar–QBO correlation may have occurred because of the relatively short data record and the presence of additional external forcings rather than a direct solar–QBO interaction.

ENSO influence on QBO modulations of the tropical tropopause

AbstractWe examine the El Niño–Southern Oscillation (ENSO) influence on the quasi‐biennial oscillation (QBO) modulation of the cold‐point tropopause (CPT) temperatures. An analysis of approximately five decades (in most cases 1950s to near‐present) of radiosonde data from 10 near‐equatorial stations, distributed along the Equator, shows that the ENSO influence on the QBO is quite zonally symmetric. At all stations analyzed, the QBO has larger amplitude and longer period during La Niña conditions than during El Niño over this total period. We also show that as a consequence of the ENSO influences on QBO periods and amplitudes, the differences between the warmer CPT temperatures during QBO westerly shear conditions and colder temperatures during QBO easterly shear conditions are larger during La Niña than during El Niño for all stations for the entire period considered here. This strengthens earlier findings that the greatest dehydration of air entering the stratosphere from the troposphere occurs during the winter under La Niña and easterly QBO conditions. In addition, stratosphere/troposphere wind and temperature profiles are derived to establish the degree of QBO downward penetration necessary to influence zonal winds and temperatures in the upper troposphere.

The Quasi-Biennial Oscillation in a Double CO2 Climate

AbstractThe effects of anticipated twenty-first-century global climate change on the stratospheric quasi-biennial oscillation (QBO) have been studied using a high-resolution version of the Model for Interdisciplinary Research on Climate (MIROC) atmospheric GCM. This version of the model is notable for being able to simulate a fairly realistic QBO for present-day conditions including only explicitly resolved nonstationary waves. A long control integration of the model was run with observed climatological sea surface temperatures (SSTs) appropriate for the late twentieth century, followed by another integration with increased atmospheric CO2 concentration and SSTs incremented by the projected twenty-first-century warming in a multimodel ensemble of coupled ocean–atmosphere runs that were forced by the Special Report on Emissions Scenarios (SRES) A1B scenario of future atmospheric composition. In the experiment for late twenty-first-century conditions the QBO period becomes longer and QBO amplitude weaker than in the late twentieth-century simulation. The downward penetration of the QBO into the lowermost stratosphere is also curtailed in the late twenty-first-century run. These changes are driven by a significant (30%–40%) increase of the mean upwelling in the equatorial stratosphere, and the effect of this enhanced mean circulation overwhelms counteracting influences from strengthened wave fluxes in the warmer climate. The momentum fluxes associated with waves propagating upward into the equatorial stratosphere do strengthen overall by ∼(10%–15%) in the warm simulation, but the increases are almost entirely in zonal phase speed ranges that have little effect on the stratospheric QBO but that would be expected to have important influences in the mesosphere and lower thermosphere.

Sensitivity of GCM tropical middle atmosphere variability and climate to ozone and parameterized gravity wave changes

This paper describes the impact of changing the current imposed ozone climatology upon the tropical Quasi‐Biennial Oscillation (QBO) in a high top climate configuration of the Met Office U.K. general circulation model. The aim is to help distinguish between QBO changes in chemistry climate models that result from temperature‐ozone feedbacks and those that might be forced by differences in climatology between previously fixed and newly interactive ozone distributions. Different representations of zonal mean ozone climatology under present‐day conditions are taken to represent the level of change expected between acceptable model realizations of the global ozone distribution and thus indicate whether more detailed investigation of such climatology issues might be required when assessing ozone feedbacks. Tropical stratospheric ozone concentrations are enhanced relative to the control climatology between 20–30 km, reduced from 30–40 km and enhanced above, impacting the model profile of clear‐sky radiative heating, in particular warming the tropical stratosphere between 15–35 km. The outcome is consistent with a localized equilibrium response in the tropical stratosphere that generates increased upwelling between 100 and 4 hPa, sufficient to account for a 12 month increase of modeled mean QBO period. This response has implications for analysis of the tropical circulation in models with interactive ozone chemistry because it highlights the possibility that plausible changes in the ozone climatology could have a sizable impact upon the tropical upwelling and QBO period that ought to be distinguished from other dynamical responses such as ozone‐temperature feedbacks.

Modulation of the Period of the Quasi-Biennial Oscillation by the Solar Cycle

Abstract The authors examine the mechanism of solar cycle modulation of the Quasi-Biennial Oscillation (QBO) period using the Two-and-a-Half-Dimensional Interactive Isentropic Research (THINAIR) model. Previous model results (using 2D and 3D models of varying complexity) have not convincingly established the proposed link of longer QBO periods during solar minima. Observational evidence for such a modulation is also controversial because it is only found during the period from the 1960s to the early 1990s, which is contaminated by volcanic aerosols. In the model, 200- and 400-yr runs without volcano influence can be obtained, long enough to establish some statistical robustness. Both in model and observed data, there is a strong synchronization of the QBO period with integer multiples of the semiannual oscillation (SAO) in the upper stratosphere. Under the current level of wave forcing, the period of the QBO jumps from one multiple of SAO to another and back so that it averages to 28 months, never settling down to a constant period. The “decadal” variability in the QBO period takes the form of “quantum” jumps; these, however, do not appear to follow the level of the solar flux in either the observation or the model using realistic quasi-periodic solar cycle (SC) forcing. To understand the solar modulation of the QBO period, the authors perform model runs with a range of perpetual solar forcing, either lower or higher than the current level. At the current level of solar forcing, the model QBO period consists of a distribution of four and five SAO periods, similar to the observed distribution. This distribution changes as solar forcing changes. For lower (higher) solar forcing, the distribution shifts to more (less) four SAO periods than five SAO periods. The record-averaged QBO period increases with the solar forcing. However, because this effect is rather weak and is detectable only with exaggerated forcing, the authors suggest that the previous result of the anticorrelation of the QBO period with the SC seen in short observational records reflects only a chance behavior of the QBO period, which naturally jumps in a nonstationary manner even if the solar forcing is held constant, and the correlation can change as the record gets longer.

Seasonal and quasi‐biennial variations in the migrating diurnal tide observed by Thermosphere, Ionosphere, Mesosphere, Energetics and Dynamics (TIMED)

We present periodic variations of the migrating diurnal tide from Thermosphere, Ionosphere, Mesosphere, Energetics and Dynamics (TIMED) temperature and wind data from 2002 to 2007 and meteor radar data at Maui (20.75°N, 156.43°W). There are strong quasi‐biennial oscillation (QBO) signatures in the amplitude of the diurnal tidal temperature in the tropical region and in the wind near ±20°. The magnitude of the QBO in the diurnal tidal amplitude reaches about 3 K in temperature and about 7 m/s (Northern Hemisphere) and 9 m/s (Southern Hemisphere) in meridional wind. The period of the diurnal tide QBO is around 24–25 months in the mesosphere but is quite variable with altitude in the stratosphere. Throughout the mesosphere, the amplitude of the diurnal tide reaches maximum during March/April of years when the QBO in lower stratospheric wind is in the eastward phase. Because the tide shows amplification only during a limited time of the year, there are not enough data yet to determine whether the tidal variation is truly biennial (24‐month period) or is quasi‐biennial. The semiannual (SAO) and annual oscillations (AO) in the diurnal tide support previous findings: tidal amplitude is largest around equinoxes (SAO signal) and is larger during the vernal equinox (AO signal). TIMED Sounding of the Atmosphere using Broadband Emission Radiometry (TIMED/SABER) temperature and atmospheric pressure data are used to calculate the balance wind and the tides in horizontal wind. The comparison between the calculations and the wind observed by TIMED Doppler Interferometer (TIDI) and meteor radar indicates qualitative agreement, but there are some differences as well.

Nonstationary Synchronization of Equatorial QBO with SAO in Observations and a Model

Abstract It has often been suggested that the period of the quasi-biennial oscillation (QBO) has a tendency to synchronize with the semiannual oscillation (SAO). Apparently the synchronization is better the higher up the observation extends. Using 45 yr of the 40-yr European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-40) data of the equatorial stratosphere up to the stratopause, the authors confirm that this synchronization is not just a tendency but a robust phenomenon in the upper stratosphere. A QBO period starts when a westerly SAO (w-SAO) descends from the stratopause to 7 hPa and initiates the westerly phase of the QBO (w-QBO) below. It ends when another w-SAO, a few SAO periods later, descends again to 7 hPa to initiate the next w-QBO. The fact that it is the westerly but not the easterly SAO (e-SAO) that initiates the QBO is also explained by the general easterly bias of the angular momentum in the equatorial stratosphere so that the e-SAO does not create a zero-wind line, unlike the w-SAO. The currently observed average QBO period of 28 months, which is not an integer multiple of SAO periods, is a result of intermittent jumps of the QBO period from four SAO to five SAO periods. The same behavior is also found in the Two and a Half Dimensional Interactive Isentropic Research (THINAIR) model. It is found that the nonstationary behavior in both the observation and model is caused not by the 11-yr solar-cycle forcing but by the incompatibility of the QBO’s natural period (determined by its wave forcing) and the “quantized” period determined by the SAO. The wave forcing parameter for the QBO period in the current climate probably lies between four SAO and five SAO periods. If the wave forcing for the QBO is tuned so that its natural period is compatible with the SAO period above (e.g., at 24 or 30 months), nonstationary behavior disappears.

A reexamination of the QBO period modulation by the solar cycle

Using the updated Singapore wind from 1953 to 2007 for the lower stratosphere 70–10 hPa, courtesy of Barbara Naujokat of Free University of Berlin, we examine the variation of the period of the Quasi‐Biennial Oscillation (QBO) as a function of height and its modulation in time by the 11‐year solar cycle. The analysis is supplemented by the ERA‐40 reanalysis up to 1 hPa. Previously, it was reported that the descent of the easterly shear zone tends to stall near 30 hPa during solar minimum, leading to a lengthened QBO westerly duration near 44–50 hPa and the reported anticorrelation of the westerly duration and the solar cycle. Using an objective method, continuous wavelet transform (CWT), for the determination of local QBO period, we find that the whole QBO period is almost invariant with respect to height, so that the stalling mechanism affects only the partition of the whole period between easterly and westerly durations. Using this longest data set available for equatorial stratospheric wind, which spans five and half solar cycles (six solar minima), we find that in three solar minima, the QBO period is lengthened, while in the remaining almost three solar cycles, the QBO period is lengthened instead at solar maxima. We suggest that the decadal variation of the QBO period originates in the upper stratosphere, where the solar‐ozone radiative influence is strong. The solar modulation of the QBO period is found to be nonstationary; the averaged effect cannot be determined unless the data record is much longer. In shorter records, the correlation can change sign, as we have found in segments of the longest record available, with or without lag.

Radar observations of long‐term variability of mesosphere and lower thermosphere winds over Tirunelveli (8.7°N, 77.8°E)

The horizontal wind data acquired by MF radar at Tirunelveli (8.7°N, 77.8°E) for the period January 1993 to July 2006 are used to study long‐term variability of equatorial mesosphere and lower thermosphere (MLT) winds. The monthly mean zonal wind exhibits dominant semiannual variability with westward winds during equinox and eastward winds during solstice. The first westward phase, which occurs during spring equinox, undergoes interannual variability with larger westward winds during the years 1993, 1995 and 1997. This interannual variability has been interpreted as quasi‐biennial oscillation (QBO) in the MLT winds. However, this QBO is absent during the year 1999, as the next large westward wind phase occurs during the year 2000. Thus the period of QBO is extended from nearly 2 to 3 years (a). During these years, the period of stratospheric QBO winds is also extended to 3 a. Besides SAO and QBO, as the zonal winds undergo intraseasonal oscillations with periodicities resembling those of Madden‐Julian Oscillation (MJO) in the troposphere, the relation between the two is examined on the basis of the existing hypothesis. The present study reveals long‐term variation in the amplitude of the annual oscillation noticed in monthly mean meridional winds. A decreasing trend is observed in the annual mean northward winds during the years 1993–2006 in contrast to decreasing trend in the annual mean southward winds reported from midlatitudes. These observations are discussed in the context of our current understanding of long‐term variability of MLT winds.

Influence of the Quasi‐Biennial Oscillation on the ECMWF model short‐range‐forecast errors in the tropical stratosphere

AbstractThis paper addresses the impact of the Quasi‐Biennial Oscillation (QBO) on the background‐error covariances in the tropical atmosphere of the ECMWF model. The tropical short‐range‐forecast‐error covariances are represented in terms of equatorial waves coupled to convection.By comparing the forecast‐error proxy data from two different phases of the QBO, it is shown that the phase of the QBO has an effect on the distribution of tropical forecast‐error variances between various equatorial waves. The influence of the QBO is limited to the stratospheric levels between 50 hPa and 5 hPa. In the easterly QBO phase, the percentage of error variance in Kelvin waves is significantly greater than in the westerly phase. In the westerly phase, westward‐propagating inertio‐gravity waves become more important, at the expense of Kelvin modes, eastward‐propagating mixed Rossby‐gravity waves and inertio‐gravity modes. Comparison of datasets from two easterly phases shows that the maxima of stratospheric error variance in various equatorial modes follow the theory of the interaction of waves with descending shear zones of the horizontal wind.Single‐observation experiments illustrate an impact of the phase of the QBO on stratospheric analysis increments, which is mostly seen in the balanced geopotential field. Idealized 3D‐Var assimilation experiments suggest that background‐error statistics from the easterly QBO period are on average more useful for the multivariate variational assimilation, as a consequence of a stronger mass‐wind coupling due to increased impact of Kelvin waves in the easterly phase.By comparing the tropical forecast errors in two operational versions of the model a few years apart, it is shown here that recent model improvements, primarily in the model physics, have substantially reduced the errors in both wind and geopotential throughout the tropical atmosphere. In particular, increased wind‐field errors associated with the intertropical convergence zone have been removed. Consequently, the ability of the applied background‐error model to represent the error fields has improved. Copyright © 2007 Royal Meteorological Society