Influence Of Quasi-biennial Oscillation Research Articles

Investigation of the solar activity and QBO effects on the near-space environment

AbstractBased on the wavelet method, we investigate the solar activity and quasi-biennial oscillation (QBO) signals in the near-space environment by using the modern-era retrospective analysis for research and applications, version-2 data and sounding of the atmosphere using broadband emission radiometry (SABER) onboard the thermosphere, ionosphere, mesosphere energetics, and dynamics satellite (TIMED/SABER). The results indicate that the annual variation mainly dominates from stratosphere to mesosphere at middle latitudes, which is more significant and stable in the southern hemisphere. Moreover, the temperature of top near space is affected by the solar activity, and it can vary over 5 K inner one solar cycle. The QBO signal is obviously seen in the stratospheric zonal wind and temperature, which is obviously existed only at a narrow latitude band between 10°S and 10°N, and its altitude range above the equator region is 20–40 km, while it turns to be 20–30 km over 10°S and 10°N, but seemly not existed in the mesosphere. Generally, our results would help to understand the influence of solar activity and QBO.

Spatial and Temporal Characterization of Near Space Temperature and Humidity and Their Driving Influences

Near space refers to the atmospheric region 20–100 km above Earth’s surface, encompassing the stratosphere, mesosphere, and part of the thermosphere. This region is susceptible to surface and upper atmospheric disturbances, and the atmospheric temperature and humidity profiles can finely characterize its complex environment. To analyze the relationship between changes in temperature and humidity profiles and natural activities, this study utilizes 18 years of temperature and water vapor data from the TIMED/SABER and AURA/MLS instruments to investigate the variations in temperature and humidity with altitude, time, and spatial distribution. In addition, multiple linear regression analysis is used to examine the impact mechanisms of solar activity, the El Niño–Southern Oscillation (ENSO), and the Quasi-Biennial Oscillation (QBO) on temperature and humidity. The results show that in the mid- and low-latitude regions, temperature and water vapor reach their maxima at an altitude of 50 km, with values of 265 K and 8–9 × 10⁻⁶ ppmv, respectively; the variation characteristics differ across latitudes and altitudes, with a clear annual cycle; the feedback effects of solar activity and the ENSO index on temperature and humidity in the 20–40 km atmospheric layer are significantly different. Among these factors, solar activity is the most significant influence on temperature and water vapor, with response coefficients of −0.2 to −0.16 K/sfu and 0.8 to 4 × 10⁻⁶ ppmv/sfu, respectively. Secondly, in the low-latitude stratospheric region, the temperature response to ENSO is approximately −1.5 K/MEI, while in the high-latitude region, a positive response of 3 K/MEI is observed. The response of water vapor to ENSO varies between −1 × 10⁻⁷ and −4 × 10−⁷ ppmv/sfu. In the low-latitude stratospheric region, the temperature and humidity responses to the QBO index exhibit significant differences, ranging from −1.8 to −0.6 K/10 m/s. Additionally, there are substantial differences in responses between the polar regions and the low-latitude equatorial region. Finally, a three-dimensional model coefficient was constructed to illustrate the influence of solar activity, ENSO, and QBO on temperature and humidity in the near space. The findings of this study contribute to a deeper understanding of the temperature and humidity variation characteristics in near space and provide valuable data and model references for predicting three-dimensional parameters of temperature and humidity in this region.

Quasi-Biennial Oscillation influence on Australian summer rainfall

The variability and distribution of Australia’s summer rainfall are influenced by modes of climate variability on multi-week to multi-year time scales. Here, we investigate the role of the stratospheric quasi-biennial oscillation (QBO) and demonstrate that the QBO influences rainfall variations and extremes’ responses across large regions of Australia. We find the QBO modulates convective heating to the east of the Maritime Continent and over the central South Atlantic Ocean in the austral summer. The baroclinic response and barotropic structure of the extra-tropical Rossby wave train induces anomalous circulation that affects the distribution and amount of rainfall over Australia. Our analysis and findings of QBO teleconnections with the dynamics that drive Australia’s rainfall variability and extremes represents a pathway to improve our understanding of rainfall potential predictability and scope to extend Australia’s rainfall prediction lead times.

Understanding the Mechanisms for Tropical Surface Impacts of the Quasi‐Biennial Oscillation (QBO)

AbstractThe impact of the quasi‐biennial oscillation (QBO) on tropical convection and precipitation is investigated through nudging experiments using the UK Met Office Hadley Center Unified Model. The model control simulations show robust links between the internally generated QBO and tropical precipitation and circulation. The model zonal wind in the tropical stratosphere was nudged above 90 hPa in atmosphere‐only and coupled ocean‐atmosphere configurations. The convection and precipitation in the atmosphere‐only simulations do not differ between the experiments with and without nudging, which may indicate that SST‐convection coupling is needed for any QBO influence on the tropical lower troposphere and surface. In the coupled experiments, the precipitation and sea‐surface temperature relationships with the QBO phase disappear when nudging is applied. Imposing a realistic QBO‐driven static stability anomaly in the upper‐troposphere lower‐stratosphere is not sufficient to simulate tropical surface impacts. The nudging reduced the influence of the lower troposphere on the upper branch of the Walker circulation, irrespective of the QBO, indicating that the upper tropospheric zonal circulation has been decoupled from the surface by the nudging. These results suggest that grid‐point nudging mutes relevant feedback processes occurring at the tropopause level, including high cloud radiative effects and wave mean flow interactions, which may play a key role in stratospheric‐tropospheric coupling.

Global monsoon systems and their modulation by the equatorial Quasi-Biennial Oscillation

Monthly-mean data of ERA-Interim reanalysis, precipitation, outgoing longwave radiation (OLR) and sea surface temperature(SST) are investigated for 40 years (1979-2018) to reveal the modulation of the global monsoon systems by the equatorial quasi-biennial oscillation (QBO), focusing only on the neutral El Niño-Southern Oscillation (ENSO) periods (in total 374 months). First, the climatology of the global monsoon systems is viewed with longitude-latitude plots of the precipitation, its proxies and lower tropospheric circulations for the annual mean and two solstice seasons, together with the composite differences between the two seasons. In addition to seasonal variations of Intertropical Convergence Zones (ITCZs), several regional monsoon systems are well identified with an anti-phase of the annual cycle between the two hemispheres. Precipitation-related quantities (OLR and specific humidity), surface conditions [i.e., mean sea level pressure (MSLP) and SST] and circulation fields related to moist convection systems show fundamental features of the global monsoon systems. After introducing eight QBO phases based on the leading two principal components of the zonal-mean zonal wind variations in the equatorial lower-stratosphere, the statistical significance of the composite difference in the precipitation and tropospheric circulation is evaluated for the opposite QBO phases. The composite differences show significant modulations in some regional monsoon systems, dominated by zonally asymmetric components, with the largest magnitudes for specific QBO-phases corresponding to traditional indices of the equatorial zonal-mean zonal wind at 20 and 50 hPa. Along the equator, significant QBO influence is characterized by the modulation of the Walker circulation over the western Pacific. In middle latitudes during boreal summer, for a specific QBO-phase, statistically significant modulation of low-pressure cyclonic perturbation is obtained over the Northern-Hemisphere western Pacific Ocean associated with statistically significant features of heavier precipitation over the eastern side of the cyclonic perturbation and the opposite lighter precipitation over the western side. During boreal winter, similar significant low-pressure cyclonic perturbations were found over the Northern-Hemisphere eastern Pacific and Atlantic Oceans for specific phases.

Phase Unlocking and the Modulation of Tropopause-Level Trace Gas Advection by the Quasibiennial Oscillation.

Open questions about the modulation of near-surface trace gas variability by stratosphere-troposphere tracer transport complicate efforts to identify anthropogenic sources of gases such as CFC-11 and N2O and disentangle them from dynamical influences. In this study, we explore one model's modulation of lower stratospheric tracer advection by the quasi-biennial oscillation (QBO) of stratospheric equatorial zonal-mean zonal winds at 50hPa. We assess instances of coherent modulation versus disruption through phase unlocking with the seasonal cycle in the model and in observations. We quantify modeled advective contributions to the temporal rate of change of stratospheric CFC-11 and N2O at extratropical and high-latitudes by calculating a transformed Eulerian mean (TEM) budget across isentropic surfaces from a 10-member WACCM4 ensemble simulation. We find that positive interannual variability in seasonal tracer advection generally occurs in the easterly QBO phase, as in previous work, and briefly discuss physical mechanisms. Individual simulations of the 10-member ensemble display phase-unlocking disruptions from this general pattern due to seasonally varying synchronizations between the model's repeating 28-month QBO cycle and the 12-month seasonal cycle. We find that phase locking and unlocking patterns of tracer advection calculations inferred from observations fall within the envelope of the ensemble member results. Our study bolsters evidence for variability in the interannual stratospheric dynamical influence of CFC-11 near-surface concentrations by assessing the QBO modulation of lower stratospheric advection via synchronization with the annual cycle. It identifies a likely cause of variations in the QBO influence on tropospheric abundances.

Zonally asymmetric influences of the quasi-biennial oscillation on stratospheric ozone

Abstract. The quasi-biennial oscillation (QBO), as the dominant mode in the equatorial stratosphere, modulates the dynamical circulation and the distribution of trace gases in the stratosphere. While the zonal mean QBO signals in stratospheric ozone have been relatively well documented, the zonal (longitudinal) differences in the QBO ozone signals have been less studied. Using satellite-based total column ozone (TCO) data from 1979 to 2020, zonal mean ozone data from 1984 to 2020, three-dimensional (3-D) ozone data from 2002 to 2020, and ERA5 reanalysis and model simulations from 1979 to 2020, we demonstrate that the influences of the QBO (using a QBO index at 20 hPa) on stratospheric ozone are zonally asymmetric. The global distribution of stratospheric ozone varies significantly during different QBO phases. During QBO westerly (QBOW) phases, the TCO and stratospheric ozone are anomalously high in the tropics, while in the subtropics they are anomalously low over most of the areas, especially during the winter–spring of the respective hemisphere. This confirms the results from previous studies. In the polar region, the TCO and stratospheric ozone (50–10 hPa) anomalies are seasonally dependent and zonally asymmetric. During boreal winter (December–February, DJF), positive anomalies of the TCO and stratospheric ozone are evident during QBOW over the regions from North America to the North Atlantic (120∘ W–30∘ E), while significant negative anomalies exist over other longitudes in the Arctic. In boreal autumn (September–November, SON), the TCO and stratospheric ozone are anomalously high from Greenland to Eurasia (60∘ W–120∘ E) but anomalously low in other regions over the Arctic. Weak positive TCO and stratospheric ozone anomalies exist over the South America sector (90∘ W–30∘ E) of the Antarctic, while negative anomalies of the TCO and stratospheric ozone are seen in other longitudes. The consistent features of TCO and stratospheric ozone anomalies indicate that the QBO signals in TCO are mainly determined by the stratospheric ozone variations. Analysis of meteorological conditions indicates that the QBO ozone perturbations are mainly caused by dynamical transport and also influenced by chemical reactions associated with the corresponding temperature changes. QBO affects the geopotential height and the polar vortex and subsequently the transport of ozone-rich air from lower latitudes to the polar region, which therefore influences the ozone concentrations over the polar region. The geopotential height anomalies associated with QBO (QBOW–QBOE) are zonally asymmetric with clear wave number 1 features, which indicates that QBO influences the polar vortex and stratospheric ozone mainly by modifying the wave number 1 activities.

The influence of quasi-biennial oscillation on West African Rainfall

The influence of quasi-biennial oscillation on West African Rainfall

Origins of multi-decadal variability in sudden stratospheric warmings

Abstract. Sudden stratospheric warmings (SSWs) are major disruptions of the Northern Hemisphere (NH) stratospheric polar vortex and occur on average approximately six times per decade in observation-based records. However, within these records, intervals of significantly higher and lower SSW rates are observed, suggesting the possibility of low-frequency variations in event occurrence. A better understanding of factors that influence this decadal variability may help to improve predictability of NH midlatitude surface climate, through stratosphere–troposphere coupling. In this work, multi-decadal variability of SSW events is examined in a 1000-year pre-industrial simulation of a coupled global climate model. Using a wavelet spectral decomposition method, we show that hiatus events (intervals of a decade or more with no SSWs) and consecutive SSW events (extended intervals with at least one SSW in each year) vary on multi-decadal timescales of periods between 60 and 90 years. Signals on these timescales are present for approximately 450 years of the simulation. We investigate the possible source of these long-term signals and find that the direct impact of variability in tropical sea surface temperatures, as well as the associated Aleutian Low, can account for only a small portion of the SSW variability. Instead, the major influence on long-term SSW variability is associated with long-term variability in amplitude of the stratospheric quasi-biennial oscillation (QBO). The QBO influence is consistent with the well-known Holton–Tan relationship, with SSW hiatus intervals associated with extended periods of particularly strong, deep QBO westerly phases. The results support recent studies that have highlighted the role of vertical coherence in the QBO when considering coupling between the QBO, the polar vortex and tropospheric circulation.

The Impact of the Stratosphere on the MJO in a Forecast Model

AbstractThis study examines the relationship between the Madden‐Julian oscillation (MJO) and stratospheric quasi‐biennial oscillation (QBO) in a state‐of‐the‐art global numerical weather forecast model. A set of 61‐day model integrations, with 15 ensemble members, is performed across 84 start dates during December–February of 1989–2016. For 28 of those dates—every 1 January—the stratosphere is initialized from observation, and the model simulates stronger MJO events during observed easterly QBO phases (QBOE) than westerly QBO phases (QBOW). However, in these “control experiments,” the QBO's impact on the MJO is already present in the initial conditions, and the direct influence of the model stratosphere during the simulation is unclear. To explore this more directly, the model was rerun with an artificially imposed QBOE and QBOW state, replacing the existing stratospheric initial condition above 150 hPa while leaving the troposphere unaltered. Though the imposed QBO states weaken faster in the model than in observations, their persistence is comparable to the control simulations. The MJO is stronger during imposed‐QBOE experiments than imposed‐QBOW, and differences are statistically significant by several metrics, though magnitude of the differences is smaller than in observations. Analysis suggests that the strength of the MJO response to the QBO increases for simulations with stronger upper‐tropospheric temperature differences and for simulations in which the MJO at the initialization time is strong and active over the Maritime Continent. However, tropospheric conditions still appear to have a dominant effect in explaining the apparent QBO influence in this model.

The influence of the stratospheric Quasi-Biennial Oscillation on trace gas levels at the Earth’s surface

The Quasi-Biennial Oscillation (QBO) of tropical zonal wind is one of the most important modes of interannual variability in the stratosphere. It is well established that the QBO influences the distribution of trace gases throughout the global stratosphere. What has not been clearly shown thus far is whether the stratospheric QBO has a consistent and significant impact on tropospheric trace gases. Here we clearly demonstrate that the effects of QBO variability in stratospheric transport and trace gas distributions regularly and persistently extend into the troposphere, which influences the interannual variability of long-lived trace gas mole fractions at the Earth’s surface. We show that the variability in the surface mole fractions on one- to five-year timescales is primarily driven by the QBO. The QBO influence on tropospheric constituent mole fractions arises from the modulation of the stratosphere to troposphere mass flux and is apparent in surface measurements, as well as throughout the stratosphere and troposphere in chemistry–climate model simulations of chlorofluorocarbon-11, chlorofluorocarbon-12 and nitrous oxide. The global total emissions estimated from measured changes in the global mean surface mole fractions of these ozone-depleting species, as well as other long-lived trace gases, will be improved by accurately accounting for the QBO-driven variability. Interannual variations of long-lived trace gas mole fractions at the Earth’s surface are primarily driven by the stratospheric Quasi-Biennial Oscillation, according to surface measurements and model simulations. Ignoring this variability may induce errors in estimating emissions of trace gases.

Magnitudes of Gravity Wave Pseudomomentum Flux Derived by Combining COSMIC Radio Occultation and ERA-Interim Reanalysis Data

In the present work, dry temperature profiles provided by the Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) radio occultation (RO) mission and the horizontal wind field provided by the European Centre for Medium-Range Weather Forecasts (ECMWF) reanalysis are combined for the first time to retrieve the magnitudes of gravity wave (GW) pseudomomentum flux (PMF). The vertical wave parameters, including Brunt–Väisälä frequencies, potential energy (Ep), and vertical wavelengths, are retrieved from RO temperature profiles. The intrinsic frequencies, which are retrieved from the horizontal wind field of ERA-Interim, are combined with the vertical wave parameters to derive the horizontal wavelengths and magnitudes of the PMF of GWs. The feasibility of this new strategy is validated first by comparing the distributions of GW parameters during June, July, and August (JJA) 2006 derived this way with those derived by previous studies. Then the seasonal and interannual variations of the distributions of GW PMF for three altitude ranges, 20–25 km, 25–30 km, and 30–35 km, over the globe during the seven years from June 2006 to May 2013 are presented. It is shown that the three altitude intervals share similar seasonal and interannual distribution patterns of GW PMF, while the magnitudes of GW PMF decrease with increased height and the hot spots of GW activity are the most discernable at the lowest altitude interval of 20–25 km. The maximums of PMF usually occur at latitudes around 60° in the winter hemispheres, where eastward winds prevail, and the second maximums exist over the subtropics of the summer hemispheres, where deep convection occurs. In addition, the influence of quasi-biennial oscillation (QBO) on both GW PMF and zonal winds is discernible over subtropical regions. The present work complements the GW PMF interannual variation patterns derived based on satellite observations by previous studies in terms of the altitude range, latitude coverage, and time period analyzed.

The ENSO and QBO Impact on Ozone Variability and Stratosphere‐Troposphere Exchange Relative to the Subtropical Jets

AbstractThe El Niño–Southern Oscillation (ENSO) and the Quasi‐Biennial Oscillation (QBO) are natural sources of ozone variability in the tropical and midlatitude upper troposphere and lower stratosphere. Regional tropospheric ozone variations have previously been attributed to ENSO‐ and QBO‐induced changes in stratosphere‐troposphere exchange around the subtropical jets (STJs). We use Goddard Earth Observing System Data Assimilation System analyses of Ozone Monitoring Instrument and Microwave Limb Sounder ozone data mapped to a STJ coordinate to examine the influence of ENSO and QBO on global and regional variability of upper troposphere and lower stratosphere ozone, and tropopause folding stratosphere‐troposphere exchange. These coordinates eliminate the apparent variability due to the changing location of the STJ, which is itself influenced by ENSO. The ENSO influence on ozone variability near the STJ and in tropopause folds is much greater than the QBO influence, although both are small in a hemispheric mean. On regional scales, the ozone sensitivity to ENSO associated with tropopause folding is strongly positive in some regions and strongly negative in others, which largely cancel in the hemispheric mean. Poleward of the STJs, the sensitivities of stratospheric ozone to ENSO and QBO are large and similar in magnitude but are opposite in phase. Thus, the ozone responses to ENSO and QBO can largely cancel or reinforce, depending on their relative phases. These results can be used to evaluate and compare modeled QBO and ENSO influences on ozone variability that removes the first‐order impact of differences in the location of the STJs.

Global analysis for periodic variations in gravity wave squared amplitudes and momentum fluxes in the middle atmosphere

Abstract. Atmospheric gravity waves (GWs) are an important coupling mechanism in the middle atmosphere. For instance, they provide a large part of the driving of long-period atmospheric oscillations such as the Quasi-Biennial Oscillation (QBO) and the semiannual oscillation (SAO) and are in turn modulated. They also induce the wind reversal in the mesosphere–lower thermosphere region (MLT) and the residual mean circulation at these altitudes. In this study, the variations in monthly zonal mean gravity wave square temperature amplitudes (GWSTAs) and, for the first time, absolute gravity wave momentum flux (GWMF) on different timescales such as the annual, semiannual, terannual and quasi-biennial variations are investigated by spectrally analyzing SABER observations from 2002 to 2015. Latitude–altitude cross sections of spectral amplitudes and phases of GWSTA and absolute GWMF in the stratosphere and mesosphere are presented and physically interpreted. It is shown that the time series of GWSTA and GWMF at a certain altitude and latitude results from the complex interplay of GW sources, propagation through and filtering in lower altitudes, oblique propagation superposing GWs from different source locations, and, finally, the modulation of the GW spectrum by the winds at a considered altitude and latitude. The strongest component is the annual variation, dominated in the summer hemisphere by subtropical convective sources and in the winter hemisphere by polar vortex dynamics. At heights of the wind reversal, a 180∘ phase shift also occurs, which is at different altitudes for GWSTA and GWMF. In the intermediate latitudes a semiannual variation (SAV) is found. Dedicated GW modeling is used to investigate the nature of this SAV, which is a different phenomenon from the tropical SAO also seen in the data. In the tropics a stratospheric and a mesospheric QBO are found, which are, as expected, in antiphase. Indication for a QBO influence is also found at higher latitudes. In previous studies a terannual variation (TAV) was identified. In the current study we explain its origin. In particular the observed patterns for the shorter periods, SAV and TAV, can only be explained by poleward propagation of GWs from the lower-stratosphere subtropics into the midlatitude and high-latitude mesosphere. In this way, critical wind filtering in the lowermost stratosphere is avoided and this oblique propagation is hence likely an important factor for MLT dynamics.

The Impact of the QBO on MJO Convection in Cloud-Resolving Simulations

Abstract This study examines the relationship between the Madden–Julian oscillation (MJO) and the stratospheric quasi-biennial oscillation (QBO) in a limited-area cloud-resolving model with parameterized large-scale dynamics. The model is used to simulate two consecutive MJO events that occurred during the late fall and early winter of 2011. To test the influence of the QBO on the simulated MJO events, various QBO states are imposed via the addition of characteristic wind and temperature anomalies. In experiments with only QBO temperature anomalies imposed (without corresponding zonal wind anomalies) the strength of convection during MJO active phases is amplified for the QBO easterly phase [an anomalously cold tropical tropopause layer (TTL)] compared to the westerly QBO phase (a warm TTL), as measured by outgoing longwave radiation, cloud fraction, and large-scale ascent. This response is qualitatively consistent with the observed MJO–QBO relationship. The response of precipitation is weaker, and is less consistent across variations in the simulation configuration. Experiments with only imposed QBO wind anomalies (without corresponding temperature anomalies) show much weaker effects altogether than those with imposed temperature anomalies, suggesting that TTL temperature anomalies are a key pathway through which the QBO can modulate the MJO. Sensitivity tests indicate that the QBO influence on MJO convection depends on both the amplitude and the height of the QBO temperature anomaly: lower-altitude and larger-amplitude temperature anomalies have more pronounced effects on MJO convection.

QBO Influence on MJO Amplitude over the Maritime Continent: Physical Mechanisms and Seasonality

AbstractThe quasi-biennial oscillation (QBO) is stratified by stratospheric zonal wind direction and height into four phase pairs [easterly midstratospheric winds (QBOEM), easterly lower-stratospheric winds, westerly midstratospheric winds (QBOWM), and westerly lower-stratospheric winds] using an empirical orthogonal function analysis of daily stratospheric (100–10 hPa) zonal wind data during 1980–2017. Madden–Julian oscillation (MJO) events in which the MJO convective envelope moved eastward across the Maritime Continent (MC) during 1980–2017 are identified using the Real-time Multivariate MJO (RMM) index and the outgoing longwave radiation (OLR) MJO index (OMI). Comparison of RMM amplitudes by the QBO phase pair over the MC (RMM phases 4 and 5) reveals that boreal winter MJO events have the strongest amplitudes during QBOEM and the weakest amplitudes during QBOWM, which is consistent with QBO-driven differences in upper-tropospheric lower-stratospheric (UTLS) static stability. Additionally, boreal winter RMM events over the MC strengthen during QBOEM and weaken during QBOWM. In the OMI, those amplitude changes generally shift eastward to the eastern MC and western Pacific Ocean, which may result from differences in RMM and OMI index methodologies. During boreal summer, as the northeastward-propagating boreal summer intraseasonal oscillation (BSISO) becomes the dominant mode of intraseasonal variability, these relationships are reversed. Zonal differences in UTLS stability anomalies are consistent with amplitude changes of eastward-propagating MJO events across the MC during boreal winter, and meridional stability differences are consistent with amplitude changes of northeastward-propagating BSISO events during boreal summer. Results remain consistent when stratifying by neutral ENSO phase.

The importance of stratospheric initial conditions for winter North Atlantic Oscillation predictability and implications for the signal‐to‐noise paradox

This study investigates the influence of atmospheric initial conditions on winter seasonal forecasts of the North Atlantic Oscillation (NAO). Hindcast (or reforecast) experiments – which differ only in their initial conditions – are performed over the period 1960–2009, using prescribed sea surface temperature (SST) and sea‐ice boundary conditions. The first experiment (“ERA‐40/Int IC”) is initialized using the ERA‐40 and ERA‐Interim reanalysis datasets, which assimilate upper‐air, satellite and surface observations; the second experiment (“ERA‐20C IC”) is initialized using the ERA‐20C reanalysis dataset, which assimilates only surface observations. The ensemble mean NAO skill is largest in ERA‐40/Int IC (r= 0.54), which is initialized with the superior reanalysis data. Moreover, ERA‐20C IC did not exhibit significantly more NAO hindcast skill (r= 0.38) than in a third experiment, which was initialized with incorrect (shuffled) initial conditions. The ERA‐40/Interim and ERA‐20C initial conditions differ substantially in the tropical stratosphere, where the quasi‐biennial oscillation (QBO) of zonal winds is not present in ERA‐20C. The QBO hindcasts are highly skilful in ERA‐40/Int IC – albeit with a somewhat weaker equatorial zonal wind amplitude in the lower stratosphere – but are incorrect in ERA‐20C IC, indicating that the QBO is responsible for the additional NAO hindcast skill; this is despite the model exhibiting a relatively weak teleconnection between the QBO and NAO. The influence of the QBO is further demonstrated by regressing out the QBO influence from each of the hindcast experiments, after which the difference in NAO hindcast skill between the experiments is negligible. Whilst ERA‐40/Int IC demonstrates a more skilful NAO hindcast, it appears to have a relatively weak predictable signal; this is the so‐called “signal‐to‐noise paradox” identified in previous studies. Diagnostically amplifying the (weak) QBO–NAO teleconnection increases the ensemble‐mean NAO signal with negligible impact on the NAO hindcast skill, after which the signal‐to‐noise problem seemingly disappears.

The Spatial and Temporal Variability of Global Stratospheric Gravity Waves and Their Activity during Sudden Stratospheric Warming Revealed by COSMIC Measurements

This study investigates the spatial and temporal variability of global stratospheric gravity waves (GWs) and the characteristics of GW activity during sudden stratospheric warming (SSW) using the GPS radio occultation measurements from the COSMIC mission during September 2006 to May 2013. Corresponding to the COSMIC RO observational window and analysis method, GW potential energy (Ep) with vertical scales no shorter than ∼2 km is resolved. It is found that the distributions of GW Ep over 20–30 km and 30–38 km show similar spatial and seasonal variations. The variations of GW Ep with altitude and latitude along the westerly wind are identified in different seasons over 60◦–80◦W. In the middle and high latitudes, seasonal cycles are distinct in the time–latitude and time–altitude distributions of GW activities, which show larger Ep in winters when westerly wind dominates and smaller Ep in summers when easterly wind dominates. The influence of quasi-biennial oscillation on GW activity is recognized in the tropics. GW Ep enhances closely following the occurrence of minor SSW events; while during major events, GW Ep may not enhance, and sometimes may even weaken, in the regions where reversals of zonal wind occur, probably caused by the filtering impact of the 0 m s−1 wind level on the GWs.

Surface impacts of the Quasi Biennial Oscillation

Abstract. Teleconnections between the Quasi Biennial Oscillation (QBO) and the Northern Hemisphere zonally averaged zonal winds, mean sea level pressure (mslp) and tropical precipitation are explored. The standard approach that defines the QBO using the equatorial zonal winds at a single pressure level is compared with the empirical orthogonal function approach that characterizes the vertical profile of the equatorial winds. Results are interpreted in terms of three potential routes of influence, referred to as the tropical, subtropical and polar routes. A novel technique is introduced to separate responses via the polar route that are associated with the stratospheric polar vortex, from the other two routes. A previously reported mslp response in January, with a pattern that resembles the positive phase of the North Atlantic Oscillation under QBO westerly conditions, is confirmed and found to be primarily associated with a QBO modulation of the stratospheric polar vortex. This mid-winter response is relatively insensitive to the exact height of the maximum QBO westerlies and a maximum positive response occurs with westerlies over a relatively deep range between 10 and 70 hPa. Two additional mslp responses are reported, in early winter (December) and late winter (February/March). In contrast to the January response the early and late winter responses show maximum sensitivity to the QBO winds at ∼ 20 and ∼ 70 hPa respectively, but are relatively insensitive to the QBO winds in between (∼ 50 hPa). The late winter response is centred over the North Pacific and is associated with QBO influence from the lowermost stratosphere at tropical/subtropical latitudes in the Pacific sector. The early winter response consists of anomalies over both the North Pacific and Europe, but the mechanism for this response is unclear. Increased precipitation occurs over the tropical western Pacific under westerly QBO conditions, particularly during boreal summer, with maximum sensitivity to the QBO winds at 70 hPa. The band of precipitation across the Pacific associated with the Inter-tropical Convergence Zone (ITCZ) shifts southward under QBO westerly conditions. The empirical orthogonal function (EOF)-based analysis suggests that this ITCZ precipitation response may be particularly sensitive to the vertical wind shear in the vicinity of 70 hPa and hence the tropical tropopause temperatures.

Influence of QBO on stratospheric Kelvin and Mixed Rossby gravity waves in high-top CMIP5 models

It is well established that quasi-biennial oscillation (QBO) has a substantial influence on Kelvin and mixed Rossby gravity (MRG) wave activity in the tropical lower stratosphere. In this study, we examined how QBO influences Kelvin and MRG wave activity in the lower stratosphere, based on nine high-top CMIP5 models. The results show that the Kelvin and MRG wave signals are stronger in the models with QBO, and relatively weaker in the models without QBO. The results are consistent with established theory, whereby upward-propagating Kelvin waves occurs more frequently during the easterly QBO phase, while upward-propagating MRG waves occurs during the westerly QBO phase. Without the QBO, the mean flow exhibits a near-zero easterly wind, which prevents the waves from propagating and penetrating into the stratosphere. Our analysis also shows that models with the QBO tend to have more robust signatures (in terms of amplitude and phase speed) of Kelvin and MRG waves.