The Experts below are selected from a list of 19854 Experts worldwide ranked by ideXlab platform

Marco A Giorgetta - One of the best experts on this subject based on the ideXlab platform.

  • Tropical Deep Convection Impact on Southern Winter Stationary Waves and Its Modulation by the Quasi-Biennial Oscillation
    Journal of Climate, 2019
    Co-Authors: Cristina Peña-ortiz, Elisa Manzini, Marco A Giorgetta
    Abstract:

    AbstractThe impact of tropical deep convection on southern winter stationary waves and its modulation by the Quasi-Biennial Oscillation (QBO) have been investigated in a long (210 year) climate mod...

  • The Quasi-Biennial Oscillation in a warmer climate: sensitivity to different gravity wave parameterizations
    Climate Dynamics, 2015
    Co-Authors: Sebastian Schirber, Thomas Krismer, Elisa Manzini, Marco A Giorgetta
    Abstract:

    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.

  • Quasi-Biennial Oscillation of the tropical stratospheric aerosol layer
    Atmospheric Chemistry and Physics Discussions, 2014
    Co-Authors: R. Hommel, Marco A Giorgetta, C. Timmreck, H. F. Graf
    Abstract:

    Abstract. This study describes how aerosol in an aerosol-coupled climate model of the middle atmosphere is influenced by the Quasi-Biennial Oscillation (QBO) during times when the stratosphere is largely unperturbed from volcanic material. In accordance with satellite observations, the tropical stratospheric aerosol load is predominately influenced by QBO induced anomalies in the vertical advection. Large impacts are seen in the size of aerosols, in particular in the region where aerosol evaporates. This turns the quasi-static balance between processes maintaining the vertical extent of the Junge layer in the tropics into a cyclic balance when considering this dominant mode of atmospheric variability. Global aerosol-interactive models without a QBO are only able to simulate the quasi-static balance state. To assess the global impact of stratospheric aerosols on climate processes, those partly non-linear relationships between the QBO and stratospheric aerosols have to be taken into account.

  • Wave Forcing of the Quasi-Biennial Oscillation in the Max Planck Institute Earth System Model
    Journal of the Atmospheric Sciences, 2014
    Co-Authors: Thomas Krismer, Marco A Giorgetta
    Abstract:

    AbstractThis study investigates the resolved wave forcing of the Quasi-Biennial Oscillation (QBO) in the Max Planck Institute Earth System Model truncated at T63 with 95 vertical levels. The model, which parameterizes unresolved gravity waves, internally generates a QBO. The resolved waves contribute up to 50% and 30% to the total wave forcing (resolved plus parameterized) of the QBO westerly and easterly jet, respectively, mostly owing to waves with zonal wavenumbers lower than 20 and frequencies lower than 0.5 cpd. At higher frequencies and wavenumbers, the model underestimates the strength of the tropospheric wave sources when compared to Tropical Rainfall Measuring Mission (TRMM) observations and applies strong horizontal diffusion, which explains the shortage of wave momentum at these scales (relative to recent studies based on high-resolution models). The study further relates the vertical structure of equatorial Kelvin waves, which contribute most to the transport and deposition of westerly wave mo...

  • Sensitivity of the Quasi-Biennial Oscillation to CO2 doubling
    Geophysical Research Letters, 2005
    Co-Authors: Marco A Giorgetta, Martin C. Doege
    Abstract:

    [1] The Quasi-Biennial Oscillation (QBO) is a major Oscillation with a characteristic period of 28 months that influences substantially the stratospheric circulation and chemistry. This exploratory study investigates its possible change by direct simulation of the QBO in a doubled CO2 climate for a range of gravity wave sources. It is found that a QBO spin-up is likely. In the most extreme case the period is nearly halved. Important accelerating factors are the decreased tropical upwelling and increased gravity wave drag, while increased filtering of eastward gravity waves in the upper tropical troposphere acts as a decelerating factor.

Kevin Hamilton - One of the best experts on this subject based on the ideXlab platform.

  • overview of experiment design and comparison of models participating in phase 1 of the sparc quasi biennial Oscillation initiative qboi
    Geoscientific Model Development, 2017
    Co-Authors: Neal Butchart, Scott Osprey, Kevin Hamilton, Yoshio Kawatani, James Anstey, C Mclandress, Andrew C Bushell, Francois Lott, J F Scinocca, Timothy N Stockdale
    Abstract:

    Abstract. The Stratosphere–troposphere Processes And their Role in Climate (SPARC) Quasi-Biennial Oscillation initiative (QBOi) aims to improve the fidelity of tropical stratospheric variability in general circulation and Earth system models by conducting coordinated numerical experiments and analysis. In the equatorial stratosphere, the QBO is the most conspicuous mode of variability. Five coordinated experiments have therefore been designed to (i) evaluate and compare the verisimilitude of modelled QBOs under present-day conditions, (ii) identify robustness (or alternatively the spread and uncertainty) in the simulated QBO response to commonly imposed changes in model climate forcings (e.g. a doubling of CO2 amounts), and (iii) examine model dependence of QBO predictability. This paper documents these experiments and the recommended output diagnostics. The rationale behind the experimental design and choice of diagnostics is presented. To facilitate scientific interpretation of the results in other planned QBOi studies, consistent descriptions of the models performing each experiment set are given, with those aspects particularly relevant for simulating the QBO tabulated for easy comparison.

  • An unexpected disruption of the atmospheric Quasi-Biennial Oscillation
    Science (New York N.Y.), 2016
    Co-Authors: Scott Osprey, James A. Anstey, Neal Butchart, Jeff Knight, Adam A. Scaife, Kevin Hamilton, Verena Schenzinger, Chunxi Zhang
    Abstract:

    One of the most repeatable phenomena seen in the atmosphere, the Quasi-Biennial Oscillation (QBO) between prevailing eastward and westward wind jets in the equatorial stratosphere (approximately 16 to 50 kilometers altitude), was unexpectedly disrupted in February 2016. An unprecedented westward jet formed within the eastward phase in the lower stratosphere and cannot be accounted for by the standard QBO paradigm based on vertical momentum transport. Instead, the primary cause was waves transporting momentum from the Northern Hemisphere. Seasonal forecasts did not predict the disruption, but analogous QBO disruptions are seen very occasionally in some climate simulations. A return to more typical QBO behavior within the next year is forecast, although the possibility of more frequent occurrences of similar disruptions is projected for a warming climate.

  • The Quasi-Biennial Oscillation in a Double CO2 Climate
    Journal of the Atmospheric Sciences, 2011
    Co-Authors: Yoshio Kawatani, Kevin Hamilton, Shingo Watanabe
    Abstract:

    Abstract The 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 t...

  • Nonlinear Representation of the Quasi-Biennial Oscillation
    Journal of the Atmospheric Sciences, 2009
    Co-Authors: Lionel Pandolfo, Kevin Hamilton
    Abstract:

    A nonlinear principal component analysis (NLPCA) is applied to monthly mean zonal wind observations from January 1956 through December 2007 taken at seven pressure levels between 10 and 70 hPa in the stratosphere near the equator to represent the well-known Quasi-Biennial Oscillation (QBO) and investigate its variability and structure. The NLPCA is conducted using a simplified two‐hidden layer feed-forward neural network that alleviates the problems of nonuniqueness of solutions and data overfitting that plague nonlinear techniques of principal component analysis. The QBO is used as a test bed for the new compact model of NLPCA. The two nonlinear principal components of the dataset of the equatorial stratospheric zonal winds, determined by the compact NLPCA, offer a clear picture of the QBO. In particular, their structure shows that the QBO phase consists of a predominant 28.3-month cycle that is modulated by an 11-yr cycle as well as by longer cycles. The differences in wind variability between westerly and easterly regimes and between Northern Hemisphere winter and summer seasons and the tendency for a seasonal synchronization of the QBO phases are well captured.

  • Longitudinal Variation of the Stratospheric Quasi-Biennial Oscillation
    Journal of the Atmospheric Sciences, 2004
    Co-Authors: Kevin Hamilton, Albert Hertzog, F. Vial, Georgiy L. Stenchikov
    Abstract:

    The longitudinal dependence of interannual variations of tropical stratospheric wind is examined in a detailed general circulation model simulation and in the limited observations available. A version of the SKYHI model is run with an imposed zonally symmetric zonal momentum source that forces the zonal-mean zonal wind evolution in the tropical stratosphere to be close to an estimate of the observed zonal wind based on radiosonde observations at Singapore during the period 1978‐99. This amounts to a kind of simple assimilation model in which only the zonal-mean wind field in the tropical stratosphere is assimilated, and other quantities are allowed to vary freely. A total of five experiments were run, one covering the full 1978‐99 period and four for 1989‐99. The results at and above about 30 hPa are fairly simple to characterize. When the zonal-mean wind near the equator at a particular level is easterly, the monthly mean wind has only very small zonal contrasts. When mean westerlies are present near the equator, significant zonal asymmetries occur at low latitudes, most notably easterly anomalies over South America and westerly anomalies in the eastern Pacific region. These anomalies generally display a continuous meridional phase propagation with the extratropical quasi-stationary eddy field in the winter hemisphere. The net result is a significantly weaker peak-to-peak amplitude of the Quasi-Biennial Oscillation (QBO) in zonal wind over the South American sector than over the rest of the equatorial band. The zonal contrast in QBO amplitude near 10 hPa exceeds 10%. In the lower stratosphere the zonal asymmetries in the prevailing wind are fairly small. Asymmetries seem to reflect the upward extension of the tropospheric Walker circulation, and are less strongly modulated by the Quasi-Biennial Oscillation in zonal-mean circulation. The model results were checked against limited station observations at Nairobi (1.38S, 36.78E), Singapore (1.48N, 103.98E), Rochambeau (4.88N, 52.48W), and Bogota (4.78N, 74.18W). Overall reasonable agreement was found between the monthly mean zonal winds in the model simulation and these station data. The low-latitude wind field in monthly mean NCEP gridded analyses was also examined. These analyses have some obviously unrealistic features in the tropical stratosphere, but some of the behavior seen in the SKYHI model simulations can be identified as well in the NCEP analyses.

J P Mccormack - One of the best experts on this subject based on the ideXlab platform.

  • The influence of the 11‐year solar cycle on the quasi‐biennial Oscillation
    Geophysical Research Letters, 2003
    Co-Authors: J P Mccormack
    Abstract:

    [1] The zonally averaged CHEM2D photochemical-dynamical middle atmosphere model is used to investigate the effect of the 11-year cycle in solar ultraviolet (UV) irradiance on the Quasi-Biennial Oscillation (QBO) in equatorial lower stratospheric zonal wind. Model calculations show the duration of the westerly (easterly) phase of the modeled QBO is ∼1 month shorter (longer) at solar maximum than at solar minimum. This effect is most apparent when the modeled QBO period is 28 months, and it is dependent on the magnitude of the imposed solar UV variations. The model results also show that a realistic simulation of the semi-annual Oscillation in equatorial zonal wind is necessary to produce solar cycle changes in QBO behavior. This is the first fully interactive modeling study to show that changes in solar UV can influence the behavior of the QBO, and lends support to the current working theory of sun-climate connections.

  • the influence of the 11 year solar cycle on the quasi biennial Oscillation
    Geophysical Research Letters, 2003
    Co-Authors: J P Mccormack
    Abstract:

    [1] The zonally averaged CHEM2D photochemical-dynamical middle atmosphere model is used to investigate the effect of the 11-year cycle in solar ultraviolet (UV) irradiance on the Quasi-Biennial Oscillation (QBO) in equatorial lower stratospheric zonal wind. Model calculations show the duration of the westerly (easterly) phase of the modeled QBO is ∼1 month shorter (longer) at solar maximum than at solar minimum. This effect is most apparent when the modeled QBO period is 28 months, and it is dependent on the magnitude of the imposed solar UV variations. The model results also show that a realistic simulation of the semi-annual Oscillation in equatorial zonal wind is necessary to produce solar cycle changes in QBO behavior. This is the first fully interactive modeling study to show that changes in solar UV can influence the behavior of the QBO, and lends support to the current working theory of sun-climate connections.

Darryn W Waugh - One of the best experts on this subject based on the ideXlab platform.

  • Modifications of the Quasi-Biennial Oscillation by a geoengineering perturbation of the stratospheric aerosol layer
    Geophysical Research Letters, 2014
    Co-Authors: Valentina Aquila, Luke D Oman, Chaim I Garfinkel, Paul Newman, Darryn W Waugh
    Abstract:

    This paper examines the impact of geoengineering via stratospheric sulfate aerosol on the Quasi-Biennial Oscillation (QBO) using the NASA Goddard Earth Observing System version 5 Chemistry Climate Model. We performed four 30 year simulations with a continuous injection of sulfur dioxide on the equator at 0° longitude. The four simulations differ by the amount of sulfur dioxide injected (5 Tg/yr and 2.5 Tg/yr) and the altitude of the injection (16 km–25 km and 22 km–25 km). We find that such an injection dramatically alters the Quasi-Biennial Oscillation, prolonging the phase of easterly shear with respect to the control simulation. This is caused by the increased aerosol heating and associated warming in the tropical lower stratosphere and higher residual vertical velocity. In the case of maximum perturbation, i.e., highest stratospheric aerosol burden, the lower tropical stratosphere is locked into a permanent westerly QBO phase.

  • Modifications of the Quasi-Biennial Oscillation by a geoengineering perturbation of the stratospheric aerosol layer
    Geophysical Research Letters, 2014
    Co-Authors: Valentina Aquila, Luke D Oman, Chaim I Garfinkel, Paul Newman, Darryn W Waugh
    Abstract:

    This paper examines the impact of geoengineering via stratospheric sulfate aerosol on the Quasi-Biennial Oscillation (QBO) using the NASA Goddard Earth Observing System version 5 Chemistry Climate Model. We performed four 30 year simulations with a continuous injection of sulfur dioxide on the equator at 0° longitude. The four simulations differ by the amount of sulfur dioxide injected (5 Tg/yr and 2.5 Tg/yr) and the altitude of the injection (16 km–25 km and 22 km–25 km). We find that such an injection dramatically alters the Quasi-Biennial Oscillation, prolonging the phase of easterly shear with respect to the control simulation. This is caused by the increased aerosol heating and associated warming in the tropical lower stratosphere and higher residual vertical velocity. In the case of maximum perturbation, i.e., highest stratospheric aerosol burden, the lower tropical stratosphere is locked into a permanent westerly QBO phase.

Elisa Manzini - One of the best experts on this subject based on the ideXlab platform.

  • Tropical Deep Convection Impact on Southern Winter Stationary Waves and Its Modulation by the Quasi-Biennial Oscillation
    Journal of Climate, 2019
    Co-Authors: Cristina Peña-ortiz, Elisa Manzini, Marco A Giorgetta
    Abstract:

    AbstractThe impact of tropical deep convection on southern winter stationary waves and its modulation by the Quasi-Biennial Oscillation (QBO) have been investigated in a long (210 year) climate mod...

  • The Quasi-Biennial Oscillation in a warmer climate: sensitivity to different gravity wave parameterizations
    Climate Dynamics, 2015
    Co-Authors: Sebastian Schirber, Thomas Krismer, Elisa Manzini, Marco A Giorgetta
    Abstract:

    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.

  • predictability of the quasi biennial Oscillation and its northern winter teleconnection on seasonal to decadal timescales
    Geophysical Research Letters, 2014
    Co-Authors: Adam A. Scaife, Mark P. Baldwin, Elisa Manzini, Martin B. Andrews, Maria Athanassiadou, Alberto Arribas, Nick Dunstone, Jeff R Knight, Craig Maclachlan, Wolfgang A Muller
    Abstract:

    The predictability of the Quasi-Biennial Oscillation (QBO) is examined in initialized climate forecasts extending out to lead times of years. We use initialized retrospective predictions made with coupled ocean-atmosphere climate models that have an internally generated QBO. We demonstrate predictability of the QBO extending more than 3 years into the future, well beyond timescales normally associated with internal atmospheric processes. Correlation scores with observational analyses exceed 0.7 at a lead time of 12 months. We also examine the variation of predictability with season and QBO phase and find that skill is lowest in winter. An assessment of perfect predictability suggests that higher skill may be achievable through improved initialization and climate modeling of the QBO, although this may depend on the realism of gravity wave source parameterizations in the models. Finally, we show that skilful prediction of the QBO itself does not guarantee predictability of the extratropical winter teleconnection that is important for surface winter climate prediction.

  • Forcing of the Quasi-Biennial Oscillation from a broad spectrum of atmospheric waves
    Geophysical Research Letters, 2002
    Co-Authors: Marco A Giorgetta, Elisa Manzini, Erich Roeckner
    Abstract:

    [1] The circulation of the stratosphere, and its influence on the trace constituent distribution, is an important component of the climate system, which must be included in simulations of global climate change. However, the ability to simulate a dominant stratospheric phenomenon, the Quasi-Biennial Oscillation (QBO) in equatorial zonal wind, is an outstanding challenge in climate modeling. Although confined to the tropics, the QBO affects the circulation and the interannual variability of the entire stratosphere, parts of the mesosphere and possibly also of the troposphere. Here we show that the QBO is successfully simulated in a general circulation model (GCM) of the newest generation. Key factors are a sufficient spatial resolution, a realistic simulation of tropical convection, and the consideration of the effects of gravity waves. From this simulation it is inferred that a broad spectrum of atmospheric waves is necessary to generate the QBO in the model.