Quasi-biennial Oscillation Signal Research Articles

QBO and Pinatubo signals in the mass flux at 100hPa during the period from 1986 to 1995

The residual mean meridional circulation is calculated by the transformed Eulerian-mean (TEM) momentum and continuity equations for 10 years from December 1985 to November 1995. The tropical upward mass flux at 100 hPa is estimated from the extratropical downward mass fluxes in the southern and northern hemispheres. The multiple regression statistical model is used to obtain quantitatively the signal of the quasi-biennial oscillation (QBO) and the trend. The results show that the tropical upward mass flux is enhanced during QBO easterly phase and reduced during QBO westerly phase. The signal of Mount Pinatubo volcanic eruption in June 1991 is detected. An abrupt increase of the mass flux is shown in the latter half of 1991, and the increase ceases at the end of 1993, about 2.5 years after the eruption.

Mechanisms for the Extratropical QBO in Circulation and Ozone

Abstract A two-and-a-half-dimensional interactive stratospheric model (i.e., a zonally averaged dynamical-chemical model combined with a truncated spectral dynamical model), whose equatorial zonal wind was relaxed toward the observed Singapore zonal wind, was able to reproduce much of the observed quasi-biennial oscillation (QBO) variability in the column ozone, in its vertical distribution in the low and middle latitudes, and also in the high southern polar latitudes. To reveal the mechanisms responsible for producing the modeled QBO signal over the globe, several control runs were also performed. The authors find that the ozone variability in the lower stratosphere—and hence also in the column—is determined mainly by two dynamical mechanisms. In the low to midlatitudes it is created by a “direct QBO circulation.” Unlike the classic picture of a nonseasonal two-cell QBO circulation symmetric about the equator, a more correct picture is a direct QBO circulation that is strongly seasonal, driven by the sea...

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...

Potential role of the quasi‐biennial oscillation in the stratosphere‐troposphere exchange as found in water vapor in general circulation model experiments

The tropical tropopause is considered to be the main region of upward transport of tropospheric air carrying water vapor and other tracers to the tropical stratosphere. The lower tropical stratosphere is also the region where the quasi‐biennial oscillation (QBO) in the zonal wind is observed. The QBO is positioned in the region where the upward transport of tropospheric tracers to the overworld takes place. Hence the QBO can in principle modulate these transports by its secondary meridional circulation. This modulation is investigated in this study by an analysis of general circulation model (GCM) experiments with an assimilated QBO. The experiments show, first, that the temperature signal of the QBO modifies the specific humidity in the air transported upward and, second, that the secondary meridional circulation modulates the velocity of the upward transport. Thus during the eastward phase of the QBO the upward moving air is moister and the upward velocity is less than during the westward phase of the QBO. It was further found that the QBO period is too short to allow an equilibration of the moisture in the QBO region. This causes a QBO signal of the moisture which is considerably smaller than what could be obtained in the limiting case of indefinitely long QBO phases. This also allows a high sensitivity of the mean moisture over a QBO cycle to the El Niño‐Southern Oscillation (ENSO) phenomena or major tropical volcanic eruptions. The interplay of sporadic volcanic eruptions, ENSO, and QBO can produce low‐frequency variability in the water vapor content of the tropical stratosphere, which renders the isolation of the QBO signal in observational data of water vapor in the equatorial lower stratosphere difficult.

Interannual Variability of Stratospheric Zonal Wind Forced by the Northern Lower-Stratospheric Large-Scale Waves

An interactive stratospheric model was run for 13 years while being forced with the observed daily varying Northern Hemisphere (NH) waves (numbers 1 to 3) on the 368-K isentropic surface (which lies near 150 mb) from 1980 to 1993. It reproduced much of the observed interannual variability (IAV) in the high-latitude monthly mean stratospheric zonal winds during NH winter, especially in December. In the model, wavenumber 1 played the major role in producing the high-latitude IAV. In addition, observations (from 1974 to 1995) revealed some strong anticorrelations between the large-scale 150-mb stationary wave-1 amplitude and the 10-mb high-latitude zonal wind during the NH early winter. It therefore seems that planetary wave variability in the lower stratosphere is the direct cause of much of the IAV in the monthly mean stratospheric zonal wind during the NH winter (especially in December). During late winter agreement between model and observations is improved by increasing the damping of planetary waves in the Tropics. Since the variability in the NH high-latitude winter zonal wind is so well modeled it does not seem likely that the high-latitude stratosphere is strongly chaotic, at least in early winter and on the timescale of a month. It also implies that if the stratospheric equatorial quasi-biennial oscillation (QBO) affects the extratropics in December (as implied by observations) then it must do so by somehow influencing the planetary waves at 150 mb. It is shown that this influence is disrupted by strong tropospheric activity, as in the early winter of 1987. The model also produces a QBO signal in the Southern Hemisphere Tropics during NH winter, due to the variability in the wave-induced cross-equatorial flow.

Effects of an Imposed Quasi-Biennial Oscillation in a Comprehensive Troposphere–Stratosphere–Mesosphere General Circulation Model

A 48-yr integration was performed using the Geophysical Fluid Dynamics Laboratory SKYHI troposphere‐ stratosphere‐mesosphere GCM with an imposed zonal momentum forcing designed to produce a quasi-biennial oscillation (QBO) in the tropical stratosphere. In response to this forcing, the model generates a QBO in the tropical circulation that includes some very realistic features, notably the asymmetry between the strength of the descending easterly and westerly shear zones, and the tendency for the initial westerly accelerations to appear quite narrowly confined to the equator. The extratropical circulation in the Northern Hemisphere (NH) winter stratosphere is affected by the tropical QBO in a manner similar to that observed. In particular, the polar vortex is generally weaker in winters in which there are easterlies in the tropical middle stratosphere. Roughly twothirds of the largest midwinter polar warmings occur when the equatorial 30-mb winds are easterly, again in rough agreement with observations. Despite this effect, however, the total interannual variance in the zonalmean extratropical circulation in the model apparently is slightly decreased by the inclusion of the tropical QBO. The observed QBO dependence of the winter-mean stratospheric extratropical stationary wave patterns is also quite well reproduced in the model. The QBO was also found to have a profound influence on stratospheric stationary waves at low latitudes. Near and above 10 mb the NH stationary waves were found to penetrate across the equator during the westerly QBO phase, but to be restricted to latitudes poleward of ;108N during the easterly phase. This means that the equatorial QBO in prevailing wind near and above 10 mb has a significant zonally asymmetric component. If this is also true in the real atmosphere, there are important implications for the adequacy of the current observational rawindsonde network near the equator. Analysis of the zonal-mean zonal momentum budget in the tropical stratosphere reveals that the resolved waves in the model are strong enough to force the realized accelerations through much of the QBO cycle. The exception appears to be the easterly acceleration phase below about 20 mb. The implications of this for the generation of a self-consistent QBO by GCMs will be considered. The effects of the imposed QBO on the troposphere were found to be very modest. There does appear to be a statistically significant weakening (by ; 1ms 21) of the high-latitude winter vortex in the middle and upper troposphere. Given the very high predictability of the stratospheric QBO itself, this effect could possibly be used to enhance the skill of seasonal weather forecasts. No significant QBO influence was found in the model precipitation field.

QBO and Pinatubo signals in the mass flux at 100 hPa and stratospheric circulation

The residual mean meridional circulation is calculated by the transformed Eulerian‐mean (TEM) momentum and continuity equations for the period from December 1988 to November 1995. Our method does not rely on radiative calculations. The tropical upward mass flux at 100 hPa is estimated from the extratropical downward mass fluxes in the southern and northern hemispheres. The multiple regression statistical model is used to obtain quantitatively the signal of the quasi‐biennial oscillation (QBO) and the trend. The results show that the tropical upward mass flux is enhanced during QBO easterly phase and reduced during QBO westerly phase. As a residual, the signal of Mount Pinatubo volcanic eruption in June 1991 is detected. This reflects the aerosol effects to the residual circulation in the lower stratosphere after the eruption. An abrupt increase of the mass flux is shown in the latter half of 1991, and the increase ceases at the end of 1993, about 2.5 years after the eruption.

An analysis of tropical transport: Influence of the quasi‐biennial oscillation

An analysis of over 4 years of Upper Atmosphere Research Satellite (UARS) measurements of CH4, HF, O3, and zonal wind are used to study the influence of the quasi‐biennial oscillation (QBO) on constituent transport in the tropics. At the equator, spectral analysis of the Halogen Occultation Experiment (HALOE) and Microwave Limb Sounder (MLS) observations reveals QBO signals in constituent and temperature fields at altitudes between 20 and 45 km. Between these altitudes, the location of the maximum QBO amplitude roughly corresponds with the location of the largest vertical gradient in the constituent field. Thus, at 40 km where CH4 and HF have strong vertical gradients, QBO signals are correspondingly large, while at lower altitudes where the vertical gradients are weak, so are the QBO variations. Similarly, ozone, which is largely under dynamical control below 30 km in the tropics, has a strong QBO signal in the region of sharp vertical gradients (∼28 km) below the ozone peak. Above 35 km, annual and semi‐annual variations are also found to be important components of the variability of long‐lived tracers. Therefore, above 30 km, the variability in CH4 and HF at the equator is represented by a combination of semiannual, annual, and QBO timescales. A one‐dimensional vertical transport model is used to further investigate the influence of annual and QBO variations on tropical constituent fields. QBO‐induced vertical motions are calculated from observed high resolution Doppler imager (HRDI) zonal winds at the equator, while the mean annually varying tropical ascent rate is obtained from the Goddard two‐dimensional model. Model simulations of tropical CH4 confirm the importance of both the annual cycle and the QBO in describing the HALOE CH4 observations above 30 km. Estimates of the tropical ascent rate and the variation due to the annual cycle and QBO are also discussed.

Isolation of the Ozone QBO in SAGE II Data by Singular-Value Decomposition

Abstract Detailed structure of the global quasi-biennial oscillation (QBO) in ozone is analyzed using Stratospheric Aerosol and Gas Experiment II ozone and nitrogen dioxide data. Emphasis is placed on the midlatitude QBO, in particular its vertical structure and seasonal synchronization. The global QBO signal is isolated using a combination of singular-value decomposition and regression analyses, which combine to act as an accurate QBO digital filter. Results show that the midlatitude ozone QBO has a two-cell structure in the vertical (similar to that at the equator), with in-phase maxima in the lower and middle stratosphere. Both upper- and lower-level anomalies contribute important fractions to the midlatitude column amounts. The lower-level maxima have a broad latitudinal structure (˜15°–60°), and collocation with the strongest background gradients suggests that these anomalies result from mean vertical transport. The, middle stratosphere signal maximizes in the subtropics (10°–40°) and is likely due t...

Residual Circulation in the Stratosphere and Lower Mesosphere as Diagnosed from Microwave Limb Sounder Data

Results for the residual circulation in the stratosphere and lower mesosphere between September 1991 and August 1994 are reported. This circulation is diagnosed by applying an accurate radiative transfer code to temperature, ozone, and water vapor measurements acquired by the Microwave Limb Sounder (MLS) onboard the Upper Atmosphere Research Satellite (UARS), augmented by climatological distributions of methane, nitrous oxide, nitrogen dioxide, surface albedo, and cloud cover. The sensitivity of the computed heating rates to the presence of Mt. Pinatubo aerosols is explored by utilizing aerosol properties derived from the measurements obtained by the Improved Stratospheric and Mesospheric Sounder instrument, also onboard UARS. The computed vertical velocities exhibit a Semiannual oscillation (SAO) around the tropical stratopause, with the region of downward velocities reaching maximum spatial extent in February and August. This behavior reflects the semiannual oscillation in temperature and ozone and mimics that seen in past studies of the October 1978–May 1979 period based on data from the Limb Infrared Monitor of the Stratosphere onboard the Nimbus 7 satellite. The SAO vertical velocities are stronger during the northern winter phase, as expected if planetary waves from the winter hemisphere are involved in driving the SAO. A possible quasi-biennial oscillation (QBO) signal extending from the middle into the upper stratosphere is also hinted at, with the equatorial vertical velocities in the region 10–1 hPa significantly smaller (or even negative) in 1993/94 than in 1992/93. Despite the short data record, the authors believe that this pattern reflects a QBO signal rather than a coincidental interannual variability, since the time–height section of vertical velocity at the equator resembles that of the zonal wind. Wintertime high-latitude descent rates are usually greater in the Northern Hemisphere, but they also exhibit significant variability there. In the three northern winters analyzed in this study, strong downward velocities are diagnosed in the lower stratosphere during stratospheric warmings and are associated with enhanced wave forcing (computed as the momentum residual) in the mid- and upper stratosphere. The implications of the computed circulation for the distribution of tracers are illustrated by the example of the “double-peaked” structure in the water vapor distribution measured by MLS.

The Solar Signal in the North Polar VORTEX in January-February. A Brief Review for an Aspect of STEP Working Group V.

The Quasi-Biennial Oscillation (QBO) in the equatorial stratosphere affects the extratropical cyclonic vortex in the stratosphere by disposing it to act in either of two regimes: weak and warm in the east years of the QBO versus cold and intense in the west years. This influence of the QBO is, however, thwarted at the peaks of the 11-year sunspot cycle: at low solar activity the extratropical vortex reacts as expected, but at high solar activity the QBO signal is reversed. The high interannual variability in the arctic stratosphere is responsible for the lack of ozone depletion on the large scale observed in Antarctica.

Phase–Space Characteristics of the Tropical Stratospheric Quasi-Biennial Oscillation

Abstract Extended empirical orthogonal functions (EOFs) are used to define a phase space for the analysis of tropical stratospheric wind data, extending our previous study of the quasi-biennial oscillation (QBO) in several manners. First, the sensitivity of the analysis to the length of the window (w) is discussed in some detail. As w increases, the leading pair of EOFs become more concentrated on the period near 28 months; simultaneously, the signals contained in higher-order EOFs become more significant, with more clearly defined periodicities; however, for large w more EOFs are required to represent the same variance. There appear to be two stable regimes: when w is less than 20 months the first two EOFs describe a QBO with some irregularities in the onset of easterly wind regimes, whereas when w exceeds 30 months such irregularities are represented by the third and fourth EOFS. Second, the first pair of EOFs with w = 40 are regarded as representing a pure QBO signal, subject to variations in cycle len...

The stratospheric quasi-biennial oscillation observed in the semidiurnal ground pressure data

Abstract. Ground pressure observations made at Macao (22°N, 113°E) from 1953 to 1991 are analyzed and compared with the stratospheric quasi-biennial oscillation (QBO) data obtained during the same interval. The periods of the two phenomena and their time evolution are found to be close to each other. Furthermore, the time series of the stratospheric winds and the S2(p) QBO signature are highly correlated, thus confirming earlier analysis. On this basis, pressure measurements obtained at Batavia (now Djakarta: 6°S, 107°E) from 1870 to 1944 are used to trace back the QBO phenomenon before the advent of systematic stratospheric balloon measurements. The inferred period, which varies between 25 and 32 months, suggests that the QBO has been present in the atmosphere at least since 1870.

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.

The stratospheric quasi-biennial oscillation observed in the semidiurnal ground pressure data

Ground pressure observations made at Macao (22°N, 113°E) from 1953 to 1991 are analyzed and compared with the stratospheric quasi-biennial oscillation (QBO) data obtained during the same interval. The periods of the two phenomena and their time evolution are found to be close to each other. Furthermore, the time series of the stratospheric winds and the S2(p) QBO signature are highly correlated, thus confirming earlier analysis. On this basis, pressure measurements obtained at Batavia (now Djakarta: 6°S, 107°E) from 1870 to 1944 are used to trace back the QBO phenomenon before the advent of systematic stratospheric balloon measurements. The inferred period, which varies between 25 and 32 months, suggests that the QBO has been present in the atmosphere at least since 1870.

Global QBO in Circulation and Ozone. Part I: Reexamination of Observational Evidence

Observational evidence for a global quasi-biennial oscillation (QBO) pattern is reviewed. In particular, the presence of an extratropical, as well as an equatorial, component of the QBO signal in column ozone is established. It is found that the ozone interannual variability is such that as one moves away from the Tropics, the frequency spectrum of the anomaly changes from one that is dominated by the equatorial QBO frequency of 1/30 mo to a two-peak spectrum around the two frequencies: 1/30 mo and 1/20 mo. Instead of treating the 1/20 mo frequency as a separate phenomenon to be filtered away in extracting the QBO in the extratropics, as was previously done, the authors argue that both peaks are integral parts of the extratropical QBO phenomenon. The 1/20 mo frequency happens to be the difference combination of the QBO frequency 1/30 mo and the annual frequency 1/12 mo. Therefore, it can represent the result of the QBO modulating an annual cycle. The authors suggest that previous methods of extracting the extratropical QBO signal severely underestimated the contribution of the QBO to the interannual variability of ozone when data are filtered to pass only the component with the period of equatorial QBO. Further, it is argued that the transport of equatorial QBO ozone anomaly by a non-QBO circulation can at most account for 6-8 Dobson units (DU) of the observed interannual variability of column ozone in the extratropics. The remaining variability (up to 20 DU) probably cannot be produced without an anomaly in the transporting circulation in the extratropics.

SOI and QBO signals in the African region

Climatic determinants of summer (Nov-Mar) rainfall over southern Africa are investigated through analysis of sea surface temperatures (SST), outgoing longwage radiation (OLR) and tropospheric wind with respect to the Southern Oscillation Index (SOI) and the stratospheric quasi-biennial oscillation (QBO). Index-to-field correlation maps are presented at various lags for the austral spring and summer seasons to establish the spatial dependence and evolution of coherent, statistically significant features. The SOI signal is reflected in upper-level zonal wind anomalies over the equatorial Atlantic Ocean during spring. SSTs in the central Indian Ocean are significantly negatively correlated with the SOI in summer. On the other hand, OLR correlations are weak over southern Africa in the summer, implying that the SOI signal may not dominate interannual convective variability. QBO correlations with SST are relatively weak, but with 200 hPa zonal winds over the western equatorial Ocean, positive correlations are noted. A standing wave pattern is described in the sub-tropics. The OLR correlation pattern represents a dipole with increased convection over eastern and southern Africa in contrast to reduced convection over Madagascar when the QBO is in west phase. Contingency analyses indicate that the global indices are unreliable predictors in isolation. However the characteristics and domain of influence of SOI and QBO signals are identified and may offer useful inputs to objective multivariate models for different modes of southern African rainfall variability.

A Quasi-Biennial Oscillation Signal in General Circulation Model Simulations

The quasi-biennial oscillation (QBO) is a free atmospheric mode that affects the equatorial lower stratosphere. With a quasi-regular frequency, the mean equatorial zonal wind alternates from easterly to westerly regimes. This oscillation is zonally symmetric about the equator, has its largest amplitude in the latitudinal band from 20 degrees S to 20 degrees N, and has a mean period of about 27 months. The QBO appears to originate in the momentum deposition produced by the damping in the stratosphere of equatorial waves excited by diabatic thermal processes in the troposphere. The results of three 10-year simulations obtained with three general circulation models are reported, all of which show the development in the stratosphere of a QBO signal with a period and a spatial propagating structure that are in good agreement with observations without any ad hoc parameterization of equatorial wave forcing. Although the amplitude of the oscillation in the simulations is still less than the observed value, the result is promising for the development of global climate models.

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

Variability in the upward propagating semidiurnal tide due to effects of QBO in the lower atmosphere

We report on a series of numerical experiments designed to investigate the effects of the quasibiennial oscillation (QBO) in the stratosphere on upward propagating semidiurnal tides. Our results indicate that significant variability (5–20 m/s) in semidiurnal winds in the mesosphere is attributable to the QBO of stratospheric mean winds and temperatures. While QBO variability in the background atmosphere is largely confined to low latitudes in the lower atmosphere, the QBO signature in upward propagating semidiurnal tides during January is largely evident in the neutral wind field at high latitudes in the mesosphere. Subsequent Hough mode decompositions of our tidal calculations indicate that the QBO primarily affects the (2,2) and (2,3) modes. The absolute magnitudes of the effect on these modes are comparable but the largest percent change is evident in the (2,3) mode. Our results also show that the QBO of stratospheric ozone does not affect semidiurnal tidal forcing.