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Journal ArticleDOI

The quasi-biennial oscillation

TL;DR: The quasi-biennial oscillation (QBO) as discussed by the authors dominates the variability of the equatorial stratosphere (∼16-50 km) and is easily seen as downward propagating easterly and westerly wind regimes, with a variable period averaging approximately 28 months.
Abstract: The quasi-biennial oscillation (QBO) dominates the variability of the equatorial stratosphere (∼16–50 km) and is easily seen as downward propagating easterly and westerly wind regimes, with a variable period averaging approximately 28 months. From a fluid dynamical perspective, the QBO is a fascinating example of a coherent, oscillating mean flow that is driven by propagating waves with periods unrelated to that of the resulting oscillation. Although the QBO is a tropical phenomenon, it affects the stratospheric flow from pole to pole by modulating the effects of extratropical waves. Indeed, study of the QBO is inseparable from the study of atmospheric wave motions that drive it and are modulated by it. The QBO affects variability in the mesosphere near 85 km by selectively filtering waves that propagate upward through the equatorial stratosphere, and may also affect the strength of Atlantic hurricanes. The effects of the QBO are not confined to atmospheric dynamics. Chemical constituents, such as ozone, water vapor, and methane, are affected by circulation changes induced by the QBO. There are also substantial QBO signals in many of the shorter-lived chemical constituents. Through modulation of extratropical wave propagation, the QBO has an effect on the breakdown of the wintertime stratospheric polar vortices and the severity of high-latitude ozone depletion. The polar vortex in the stratosphere affects surface weather patterns, providing a mechanism for the QBO to have an effect at the Earth's surface. As more data sources (e.g., wind and temperature measurements from both ground-based systems and satellites) become available, the effects of the QBO can be more precisely assessed. This review covers the current state of knowledge of the tropical QBO, its extratropical dynamical effects, chemical constituent transport, and effects of the QBO in the troposphere (∼0–16 km) and mesosphere (∼50–100 km). It is intended to provide a broad overview of the QBO and its effects to researchers outside the field, as well as a source of information and references for specialists. The history of research on the QBO is discussed only briefly, and the reader is referred to several historical review papers. The basic theory of the QBO is summarized, and tutorial references are provided.
Citations
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Journal ArticleDOI
TL;DR: It is demonstrated how phase angle statistics can be used to gain confidence in causal relation- ships and test mechanistic models of physical relationships between the time series and Monte Carlo methods are used to assess the statistical significance against red noise backgrounds.
Abstract: Many scientists have made use of the wavelet method in analyzing time series, often using popular free software. However, at present there are no similar easy to use wavelet packages for analyzing two time series together. We discuss the cross wavelet transform and wavelet coher- ence for examining relationships in time frequency space be- tween two time series. We demonstrate how phase angle statistics can be used to gain confidence in causal relation- ships and test mechanistic models of physical relationships between the time series. As an example of typical data where such analyses have proven useful, we apply the methods to the Arctic Oscillation index and the Baltic maximum sea ice extent record. Monte Carlo methods are used to assess the statistical significance against red noise backgrounds. A software package has been developed that allows users to perform the cross wavelet transform and wavelet coherence (http://www.pol.ac.uk/home/research/waveletcoherence/). As we are interested in extracting low s/n ratio signals in time series we discuss only CWT in this paper. While CWT is a common tool for analyzing localized intermittent os- cillations in a time series, it is very often desirable to ex- amine two time series together that may be expected to be linked in some way. In particular, to examine whether re- gions in time frequency space with large common power have a consistent phase relationship and therefore are sug- gestive of causality between the time series. Many geophys- ical time series are not Normally distributed and we suggest methods of applying the CWT to such time series. From two CWTs we construct the Cross Wavelet Transform (XWT) which will expose their common power and relative phase in time-frequency space. We will further define a measure of Wavelet Coherence (WTC) between two CWT, which can find significant coherence even though the common power is low, and show how confidence levels against red noise back- grounds are calculated. We will present the basic CWT theory before we move on to XWT and WTC. New developments such as quanti- fying the phase relationship and calculating the WTC sig- nificance level will be treated more fully. When using the methods on time series it is important to have solid mecha- nistic foundations on which to base any relationships found, and we caution against using the methods in a "scatter-gun" approach (particularly if the time series probability density functions are modified). To illustrate how the various meth- ods are used we apply them to two data sets from meteo- rology and glaciology. Finally, we will provide links to a MatLab software package.

4,586 citations


Cites background from "The quasi-biennial oscillation"

  • ...A 3 month lag is consistent with the mechanism of stratospheric forcing of the troposphere (Baldwin et al., 2001; Jevrejeva et al., 2003)....

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  • ...A 3 month lag is consis- tent with the mechanism of stratospheric forcing of the tropo- sphere (Baldwin et al., 2001; Jevrejeva et al., 2003)....

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Journal ArticleDOI
TL;DR: In this article, a review of gravity wave sources and characteristics, the evolution of the gravity wave spectrum with altitude and with variations of wind and stability, the character and implications of observed climatologies, and the wave interaction and instability processes that constrain wave amplitudes and spectral shape are discussed.
Abstract: [1] Atmospheric gravity waves have been a subject of intense research activity in recent years because of their myriad effects and their major contributions to atmospheric circulation, structure, and variability. Apart from occasionally strong lower-atmospheric effects, the major wave influences occur in the middle atmosphere, between ∼ 10 and 110 km altitudes because of decreasing density and increasing wave amplitudes with altitude. Theoretical, numerical, and observational studies have advanced our understanding of gravity waves on many fronts since the review by Fritts [1984a]; the present review will focus on these more recent contributions. Progress includes a better appreciation of gravity wave sources and characteristics, the evolution of the gravity wave spectrum with altitude and with variations of wind and stability, the character and implications of observed climatologies, and the wave interaction and instability processes that constrain wave amplitudes and spectral shape. Recent studies have also expanded dramatically our understanding of gravity wave influences on the large-scale circulation and the thermal and constituent structures of the middle atmosphere. These advances have led to a number of parameterizations of gravity wave effects which are enabling ever more realistic descriptions of gravity wave forcing in large-scale models. There remain, nevertheless, a number of areas in which further progress is needed in refining our understanding of and our ability to describe and predict gravity wave influences in the middle atmosphere. Our view of these unknowns and needs is also offered.

2,206 citations

Book ChapterDOI
01 Nov 2013
TL;DR: In this article, an overview of model capabilities as assessed in this chapter, including improvements, or lack thereof, relative to models assessed in the AR4, is presented, along with an assessment of recent work connecting model performance to the detection and attribution of climate change as well as to future projections.
Abstract: Climate models have continued to be developed and improved since the AR4, and many models have been extended into Earth System models by including the representation of biogeochemical cycles important to climate change. These models allow for policy-relevant calculations such as the carbon dioxide (CO2) emissions compatible with a specified climate stabilization target. In addition, the range of climate variables and processes that have been evaluated has greatly expanded, and differences between models and observations are increasingly quantified using ‘performance metrics’. In this chapter, model evaluation covers simulation of the mean climate, of historical climate change, of variability on multiple time scales and of regional modes of variability. This evaluation is based on recent internationally coordinated model experiments, including simulations of historic and paleo climate, specialized experiments designed to provide insight into key climate processes and feedbacks and regional climate downscaling. Figure 9.44 provides an overview of model capabilities as assessed in this chapter, including improvements, or lack thereof, relative to models assessed in the AR4. The chapter concludes with an assessment of recent work connecting model performance to the detection and attribution of climate change as well as to future projections.

1,686 citations

Journal ArticleDOI
19 Oct 2001-Science
TL;DR: Observations show that large variations in the strength of the stratospheric circulation, appearing first above ∼50 kilometers, descend to the lowermost stratosphere and are followed by anomalous tropospheric weather regimes, which precede shifts in the probability distributions of extreme values of the Arctic and North Atlantic Oscillations and the location of storm tracks.
Abstract: Observations show that large variations in the strength of the stratospheric circulation, appearing first above ∼50 kilometers, descend to the lowermost stratosphere and are followed by anomalous tropospheric weather regimes During the 60 days after the onset of these events, average surface pressure maps resemble closely the Arctic Oscillation pattern These stratospheric events also precede shifts in the probability distributions of extreme values of the Arctic and North Atlantic Oscillations, the location of storm tracks, and the local likelihood of mid-latitude storms Our observations suggest that these stratospheric harbingers may be used as a predictor of tropospheric weather regimes

1,639 citations

References
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Journal ArticleDOI
TL;DR: The Arctic Oscillation (AO) as mentioned in this paper is the signature of modulations in the strength of the polar vortex aloft, and it resembles the NAO in many respects; but its primary center of action covers more of the Arctic, giving it a more zonally symmetric appearance.
Abstract: The leading empirical orthogonal function of the wintertime sea-level pressure field is more strongly coupled to surface air temperature fluctuations over the Eurasian continent than the North Atlantic Oscillation (NAO). It resembles the NAO in many respects; but its primary center of action covers more of the Arctic, giving it a more zonally symmetric appearance. Coupled to strong fluctuations at the 50-hPa level on the intraseasonal, interannual, and interdecadal time scales, this "Arctic Oscillation" (AO)can be interpreted as the surface signature of modulations in the strength of the polar vortex aloft. It is proposed that the zonally asymmetric surface air temperature and mid-tropospheric circulation anomalies observed in association with the AO may be secondary baroclinic features induced by the land-sea contrasts. The same modal structure is mirrored in the pronounced trends in winter and springtime surface air temperature, sea-level pressure, and 50-hPa height over the past 30 years: parts of Eurasia have warmed by as much as several K, sea-level pressure over parts of the Arctic has fallen by 4 hPa, and the core of the lower stratospheric polar vortex has cooled by several K. These trends can be interpreted as the development of a systematic bias in one of the atmosphere's dominant, naturally occurring modes of variability.

3,800 citations


"The quasi-biennial oscillation" refers background or methods in this paper

  • ...The surface NAM pattern is also known as the Arctic Oscillation [Thompson and Wallace, 1998] and is broadly similar to the QBO signature shown in Figure 31, suggesting that the QBO may act to modulate the NAM....

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  • ...During winter the QBO appears to excite the “northern annular mode” (NAM) (also called the Arctic Oscillation) [Thompson and Wallace, 1998, 2000]....

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Journal ArticleDOI
TL;DR: In this paper, a simple spectral cloud ensemble was proposed to provide realistic values of the thermal forcing by convection under various synoptic conditions, such as tropical penetrative convection, tradewind cumuli, and extratropical organized convection.
Abstract: Observational studies indicate that a mass flux approach may provide a realistic framework for cumulus parameterization in large-scale models, but this approach, through the introduction of a spectral cloud ensemble, leads normally to rather complex schemes. In this paper the question is addressed whether much simpler schemes can already provide realistic values of the thermal forcing by convection under various synoptic conditions. This is done through verifying such a scheme first on data from field experiments for periods of tropical penetrative convection (GATE, Marshall Islands), tradewind cumuli (ATEX, BOMEX) and extratropical organized convection (SESAME-79) and then in a NWP model. The scheme considers a population of clouds where the cloud ensemble is described by a one-dimensional bulk model as earlier applied by Yanai et al. in a diagnostic study of tropical convection. Cumulus scale downdrafts are included. Various types of convection are represented, i.e., penetrative convection in c...

3,296 citations

Journal ArticleDOI
TL;DR: In this article, the authors compared the structure and seasonality of the Southern Hemisphere (SH) annular mode and the Northern Hemisphere (NH) mode, referred to as the Arctic Oscillation (AO), based on data from the National Centers for Environmental Prediction and National Center for Atmospheric Research reanalysis and supplementary datasets.
Abstract: The leading modes of variability of the extratropical circulation in both hemispheres are characterized by deep, zonally symmetric or ‘‘annular’’ structures, with geopotential height perturbations of opposing signs in the polar cap region and in the surrounding zonal ring centered near 458 latitude. The structure and dynamics of the Southern Hemisphere (SH) annular mode have been extensively documented, whereas the existence of a Northern Hemisphere (NH) mode, herein referred to as the Arctic Oscillation (AO), has only recently been recognized. Like the SH mode, the AO can be defined as the leading empirical orthogonal function of the sea level pressure field or of the zonally symmetric geopotential height or zonal wind fields. In this paper the structure and seasonality of the NH and SH modes are compared based on data from the National Centers for Environmental Prediction‐National Center for Atmospheric Research reanalysis and supplementary datasets. The structures of the NH and SH annular modes are shown to be remarkably similar, not only in the zonally averaged geopotential height and zonal wind fields, but in the mean meridional circulations as well. Both exist year-round in the troposphere, but they amplify with height upward into the stratosphere during those seasons in which the strength of the zonal flow is conducive to strong planetary wave‐mean flow interaction: midwinter in the NH and late spring in the SH. During these ‘‘active seasons,’’ the annular modes modulate the strength of the Lagrangian mean circulation in the lower stratosphere, total column ozone and tropopause height over mid- and high latitudes, and the strength of the trade winds of their respective hemispheres. The NH mode also contains an embedded planetary wave signature with expressions in surface air temperature, precipitation, total column ozone, and tropopause height. It is argued that the horizontal temperature advection by the perturbed zonal-mean zonal wind field in the lower troposphere is instrumental in forcing this pattern. A companion paper documents the striking resemblance between the structure of the annular modes and observed climate trends over the past few decades.

3,278 citations


"The quasi-biennial oscillation" refers background in this paper

  • ...The surface NAM pattern is also known as the Arctic Oscillation [Thompson and Wallace, 1998] and is broadly similar to the QBO signature shown in Figure 31, suggesting that the QBO may act to modulate the NAM....

    [...]

  • ...During winter the QBO appears to excite the “northern annular mode” (NAM) (also called the Arctic Oscillation) [Thompson and Wallace, 1998, 2000]....

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  • ...a deep, zonally symmetric or “annular” structure [Thompson and Wallace, 2000]....

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  • ...The leading mode of variability of the northern extratropical troposphere/stratosphere is characterized by 39, 2 / REVIEWS OF GEOPHYSICS Baldwin et al.: THE QUASI-BIENNIAL OSCILLATION ● 219 a deep, zonally symmetric or “annular” structure [Thompson and Wallace, 2000]....

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Journal ArticleDOI
TL;DR: In this article, a single layer of homogeneous incompressible fluid with free surface is treated, where the Coriolis parameter is assumed to be proportional to the latitude, and a strong east-west current was formed along the equator.
Abstract: Quasi-horizontal wave motions in the equatorial area are discussed. A single layer of homogeneous incompressible fluid with free surface is treated. The Coriolis parameter is assumed to be proportional to the latitude. In general, waves of two different types are obtained as solutions, one being the inertio-gravity wave and the other Rossby wave. They are distinguished from each other by the difference of frequencies and by the relationships between pressure and velocity fields. For the solutions of the lowest mode (waves confined near the equator), however, the distinction between the Rossby and the inertio-gravity waves is not clear. The wave moves westward and the frequency of this wave is compared to that of the gravity wave, if wave length is large. With the increase of the wave number the frequency decreases and approaches to that of the Rossby type wave. The pressure and wind fields of this wave show somewhat mixed character of the two types, and change continuously with the wave number. In this connection it seems impossible to \"filter out\" gravity waves from large scale motions. Another interesting feature of the equatorial disturbances is that the low frequency waves are trapped near the equator. It is shown that the both waves of inertio-gravity type and of the Rossby type have appreciable amplitude only near the equator. The characteristic north-south extent of the waves is (c/ ) 1/2, where c is the velocity of long gravity waves and is the Rossby parameter. This expression is identical with that derived by Bretherton (1964) for inertio-gravity oscillations in a meridional plane. In the later half, \"forced stationary motion\" in the equatorial region is treated. Based on the same model, mass sources and sinks are introduced periodically in the east-west direction. Then the motions and surface topography caused by them are calculated. As expected, high and low pressures appear where mass source and sink are given respectively. But these high and low cells are splitted into two parts separated by troughs or ridges located along the equator. Strong east-west current was formed along the equator. The flow directs from source to sink and it is intensified by the turning of the circular flow in the higher latitudes.

2,355 citations


"The quasi-biennial oscillation" refers background or methods in this paper

  • ...The solutions included a Rossby mode, and a mode which became known as the (mixed) Rossby-gravity mode....

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  • ...The propagation of various tropical waves is depicted by orange arrows, with the QBO driven by upward propagating gravity, inertia-gravity, Kelvin, and Rossby-gravity waves....

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  • ...Kelvin and Rossby-gravity waves....

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  • ...Long records of rawinsonde data from high-quality stations have been used to derive seasonal and QBOrelated variations of Kelvin and Rossby-gravity wave activity near the equator [Maruyama, 1991; Dunkerton, 1991b, 1993; Shiotani and Horinouchi, 1993; Sato et al., 1994; Wikle et al., 1997]....

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  • ...For example, Takahashi et al. [1997] suggested that the easterly acceleration of the model QBO was due to gravity waves as well as Rossby waves from the NH winter and Rossby-gravity waves, while westerly acceleration was due to Kelvin and gravity waves....

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Journal ArticleDOI
TL;DR: The role of wave-induced forces in the extratropical overworld is discussed in this paper, where the authors focus on the role of waves and eddies in the overworld overworld and show that the global exchange rate is determined by details of near-tropopause phenomena such as penetrative cumulus convection or small-scale mixing associated with upper level fronts and cyclones.
Abstract: In the past, studies of stratosphere-troposphere exchange of mass and chemical species have mainly emphasized the synoptic- and small-scale mechanisms of exchange This review, however, includes also the global-scale aspects of exchange, such as the transport across an isentropic surface (potential temperature about 380 K) that in the tropics lies just above the tropopause, near the 100-hPa pressure level Such a surface divides the stratosphere into an “overworld” and an extratropical “lowermost stratosphere” that for transport purposes need to be sharply distinguished This approach places stratosphere-troposphere exchange in the framework of the general circulation and helps to clarify the roles of the different mechanisms involved and the interplay between large and small scales The role of waves and eddies in the extratropical overworld is emphasized There, wave-induced forces drive a kind of global-scale extratropical “fluid-dynamical suction pump,” which withdraws air upward and poleward from the tropical lower stratosphere and pushes it poleward and downward into the extratropical troposphere The resulting global-scale circulation drives the stratosphere away from radiative equilibrium conditions Wave-induced forces may be considered to exert a nonlocal control, mainly downward in the extratropics but reaching laterally into the tropics, over the transport of mass across lower stratospheric isentropic surfaces This mass transport is for many purposes a useful measure of global-scale stratosphere-troposphere exchange, especially on seasonal or longer timescales Because the strongest wave-induced forces occur in the northern hemisphere winter season, the exchange rate is also a maximum at that season The global exchange rate is not determined by details of near-tropopause phenomena such as penetrative cumulus convection or small-scale mixing associated with upper level fronts and cyclones These smaller-scale processes must be considered, however, in order to understand the finer details of exchange Moist convection appears to play an important role in the tropics in accounting for the extreme dehydration of air entering the stratosphere Stratospheric air finds its way back into the troposphere through a vast variety of irreversible eddy exchange phenomena, including tropopause folding and the formation of so-called tropical upper tropospheric troughs and consequent irreversible exchange General circulation models are able to simulate the mean global-scale mass exchange and its seasonal cycle but are not able to properly resolve the tropical dehydration process Two-dimensional (height-latitude) models commonly used for assessment of human impact on the ozone layer include representation of stratosphere-troposphere exchange that is adequate to allow reasonable simulation of photochemical processes occurring in the overworld However, for assessing changes in the lowermost stratosphere, the strong longitudinal asymmetries in stratosphere-troposphere exchange render current two-dimensional models inadequate Either current transport parameterizations must be improved, or else, more likely, such changes can be adequately assessed only by three-dimensional models

2,342 citations


"The quasi-biennial oscillation" refers background in this paper

  • ...This, in turn, might affect the strength of the planetary wave driven “extratropical pump” [Holton et al., 1995]....

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  • ...The climatological circulation is characterized by large-scale ascent in the tropics, broad poleward transport in the stratosphere, and compensating sinking through the extratropical tropopause [Holton et al., 1995]....

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