Easterly Quasi-biennial Oscillation Research Articles

Sensitivity of Easterly QBO’s Boreal Winter Teleconnections and Surface Impacts to SSWs

Abstract The quasi-biennial oscillation (QBO) is thought to influence boreal winter surface conditions over Asia and around the North Atlantic. Confirming if these responses are robust is complicated by the QBO having multiple pathways to influence surface conditions as well as internal variability. The reanalysis record suggests that sudden stratospheric warmings (SSWs), breakdowns of the polar vortex that can elicit persistent surface impacts, are more frequent during easterly QBO (EQBO). Hence, this modulated frequency of SSWs may account for some of the EQBO surface responses. However, many climate models do not reproduce this QBO–SSW relationship, perhaps because it is noise or because the model QBOs are deficient. We circumvent these issues by using an ensemble of fixed boundary condition branched simulations in which a realistic EQBO is prescribed in control simulations previously devoid of a QBO, allowing us to isolate the transient atmospheric response to EQBO. Imposing EQBO accelerates the tropical upper-tropospheric wind, shifts the subtropical jet poleward, and attenuates the polar vortex. Interestingly, the latter is not entirely dependent on the statistically significant increase in SSW frequency due to EQBO. Corroborating observations, EQBO is associated with warmer surface temperatures over Asia and negative North Atlantic Oscillation (NAO) conditions. We then subsample the branched/control simulations based on which EQBO members have SSWs. The negative NAO response is primarily associated with more frequent SSWs, while the Asia warming develops irrespective of SSWs. These results have implications for wintertime predictability and clarify the pairing of particular QBO teleconnections with certain surface impacts. Significance Statement The QBO is one of the few parts of the Earth system that is predictable months in advance and that also elicits global effects on surface temperature, circulation, and precipitation. Unfortunately, climate models and operational forecast systems do not simulate the QBO well and it is not always clear how robust the global impacts of the QBO are. Here, we impose the QBO in idealized model simulations, which modulates wintertime surface temperature and precipitation over Asia, the North Atlantic, Europe, and Africa in a manner consistent with observations. This work substantiates the importance of climate and forecast models properly simulating the QBO.

Influence of Natural Tropical Oscillations on Ozone Content and Meridional Circulation in the Boreal Winter Stratosphere

The dependence of ozone content in the polar stratosphere upon different phases of the quasi-biennial oscillation (QBO) of the zonal wind and the El Niño–Southern Oscillation (ENSO) during winter was studied. The monthly (from November to January) mean residual meridional circulation (RMC) was calculated for four different combinations of the main phases of ENSO and QBO using MERRA2 reanalysis data. It has been demonstrated that the QBO phase manifests itself in different vertical distributions of ozone in the equatorial stratosphere, as well as in strengthening/weakening of the secondary meridional circulation in the tropics. The enhancement of the RMC from the tropical to the polar stratosphere is stronger at altitudes where ozone is higher in the tropics under El Niño conditions. The RMC modification and intensification are observed from ozone-depleted areas under La Niña conditions. A “cumulative” effect is observed by February under La Niña conditions and the easterly QBO, which is expressed in the lowest ozone content in the polar stratosphere. The numerical experiments carried out using the Middle and Upper Atmosphere Model (MUAM) confirmed tendencies in changes in the meridional transport detected from the reanalysis data for different combinations of QBO and ENSO.

Contributions of Parameterized Gravity Waves and Resolved Equatorial Waves to the QBO Period in a Future Climate of CESM2

AbstractContributions of the resolved waves and parameterized gravity waves to changes in the quasi‐biennial oscillation (QBO) in a future simulation (2015–2100) under the SSP370 scenario are investigated using the Community Earth System Model 2 (CESM2) with enhanced vertical resolution and are compared with those from four CESM2 historical simulations (1979–2014). The maximum QBO amplitude of the future simulation is 26.0 m s−1, which is slightly less than that of the historical simulations (27.4–29.3 m s−1). However, the QBO period in the future simulation is much shorter: 21.6 months in the early‐future (2015–2050) and 12 months in the late‐future (2065–2100) period, than in the historical simulations (23.5–30.9 months). The shortened QBO period in the future is primarily due to increases in both resolved wave forcing and parameterized gravity wave drag (GWD) in the stratosphere, with a more significant contribution by the GWD. As convective activity becomes stronger in the future simulation, the momentum flux of parameterized convective gravity waves at the cloud top increases, resulting in stronger GWD in the stratosphere. The increases in the magnitude of westward GWD dominate those of eastward GWD in the stratosphere. This is due to a significant increase in westward momentum flux in the troposphere, especially during the descending easterly QBO, and enhanced westerlies in the lowermost stratosphere, which introduces a westward anomaly. For the resolved waves, Kelvin wave forcing is a key contributor to increased eastward forcing in the future simulation, with relatively minor contributions by other equatorial planetary waves.

QBO modulation of MJO teleconnections in the North Pacific: impact of preceding MJO phases

This study examines the influence of the Quasi-Biennial Oscillation (QBO) on the Madden-Julian Oscillation (MJO) teleconnections in the North Pacific using ERA5 data. It is found that the Rossby wave trains induced by MJO phase 6–7 exhibit greater strength and robustness during the westerly QBO winter (WQBO) than during the easterly QBO winter (EQBO), although the MJO itself is weaker during the former. This counter-intuitive dependency of MJO teleconnections on the QBO is attributed to the preexisting MJO teleconnections prior to the MJO phase 6–7. The MJO phase 6–7 is more frequently preceded by stronger MJO phase 3–4 during the EQBO than during the WQBO. The preceding MJO phase 3–4 teleconnections, which have opposed signs to the MJO phase 6–7 teleconnections, result in a considerable attenuation of the MJO phase 6–7 teleconnections by destructive interference. This result is supported by linear model experiments. The subseasonal-to-seasonal prediction models also indicate improved prediction skills of MJO phase 6–7 teleconnections during the WQBO compared to the EQBO. These results suggest that enhanced MJO activities during the EQBO do not necessarily result in stronger and more robust MJO teleconnections in the North Pacific.

Arctic Sea Ice Loss, Long‐Term Trends in Extratropical Wave Forcing, and the Observed Strengthening of the QBO‐MJO Connection

AbstractA modulation has been identified of the tropical Madden‐Julian oscillation (MJO) by the stratospheric quasi‐biennial oscillation (QBO) such that the MJO in boreal winter is ∼40% stronger and persists ∼10 days longer during the easterly QBO phase (QBOE) than during the westerly phase. A proposed mechanism is reductions of tropical lower stratospheric static stability during QBOE caused by (a) the QBO induced meridional circulation; and (b) QBO influences on extratropical wave forcing of the stratospheric residual meridional circulation during early winter. Here, long‐term variability of the QBO‐MJO connection and associated variability of near‐tropopause tropical static stability and extratropical wave forcing are investigated using European Center reanalysis data for the 1959–2021 period. During the most reliable (post‐satellite) part of the record beginning in 1979, a strengthening of the QBO‐MJO modulation has occurred during a time when tropical static stability in the lowermost stratosphere and uppermost troposphere has been decreasing and extratropical wave forcing in early winter has been increasing. A high inverse correlation (R = −0.87) is obtained during this period between early winter wave forcing anomalies and wintertime tropical lower stratospheric static stability. Regression relationships are used to show that positive trends in early winter wave forcing during this period have likely contributed to decreases in tropical static stability, favoring a stronger QBO‐MJO connection. As shown in previous work, increased sea level pressure anomalies over northern Eurasia produced by Arctic sea ice loss may have been a significant source of the observed positive trends in early winter wave forcing.

Modulation of the intraseasonal variability in early summer precipitation in eastern China by the Quasi-Biennial Oscillation and the Madden–Julian Oscillation

Abstract. Using the reanalysis and multiple observations, the possible impact of the Madden–Julian Oscillation (MJO) on early summer (June–July) rainfall in eastern China and its modulation by the Quasi-Biennial Oscillation (QBO) are examined. The composite results show that the suppressed (enhanced) convection anomalies for MJO phases 8–1 (4–5) are more concentrated over the maritime continent and the western Pacific during easterly QBO (EQBO). As a consequence, more significant wet (dry) anomalies develop in South (eastern) China during MJO phases 8–1 (4–5) configured with easterly (westerly) QBO. The enhancement and expansion of the anomalous tropical convection band do not necessarily correspond to enhancement of the extratropical circulation response to MJO phases 8–1 (4–5) configured with westerly (easterly) QBO. The anomalous high (low) over the maritime continent and western Pacific associated with MJO phases 8–1 (4–5) is intensified (deepened) during easterly (westerly) QBO, leading to large southwesterly (northeasterly) anomalies in South China and the coasts, carrying abundant (sparse) moisture. Two anomalous meridional circulation cells are observed for MJO phases 8–1 in the East Asia sector, with downwelling anomalies around 5–20∘ N, upwelling anomalies around 20–30∘ N, and another downwelling branch northward of 30∘ N, which are enhanced during easterly QBO. The anomalous meridional circulation cells are reversed for MJO phases 4–5, which are stronger during westerly QBO with the anomalous downwelling and dry anomalies covering eastern China. The combined impact of MJO phases 8–1 and easterly QBO on the early summer rainfall is noticeable in 1996, 2016, and 2020. The enormous rainfall amount appeared along the Yangtze River in 1996, 2016, and 2020 due to the extended period of MJO phases 8–1 under the background of the easterly QBO.

Projected Strengthening Impact of the Quasi-Biennial Oscillation on the Southern Hemisphere by CMIP5/6 Models

Abstract Using 20 quasi-biennial oscillation (QBO)-resolving models from phases 5/6 of the Coupled Model Intercomparison Project (CMIP5/6), this study examines the projected Southern Hemisphere (SH) extratropical response to the QBO. Nine of the 22 models simulate decelerated circumpolar westerlies during easterly QBO (EQBO) in the historical climate as is observed, though only ∼30% of the observed change is reproduced in the multimodel ensemble mean (MME) of these high-skill models. These high-skill models project an enhanced stratospheric QBO teleconnection for both emissions scenarios. Further, the stratospheric wind anomaly in high latitudes is projected to move to midlatitudes in future scenarios. The climatological subtropical jet is projected to strengthen, while tropical easterlies are projected to weaken. As a consequence, upward wave activity in the future appears to become more sensitive to the QBO phase. Enhanced upward propagation of waves in mid- to high latitudes during EQBO are much stronger in future scenarios than in historical simulations. The anomalous downwelling over the Antarctic, as part of the Brewer–Dobson deep branch response to EQBO, is also projected to strengthen, corresponding to increased warm anomalies. In future scenarios, the areal extent of the deep convective response to EQBO over the Maritime Continent widens and includes a sharper reduction in outgoing longwave radiation, albeit with southward expansion from Indonesia to Australia. The enhancement and spatial shift in the stratospheric polar vortex pathway and tropical convection pathway subsequently lead to changes in the tropospheric QBO signal. An annular mode–like response forms in the troposphere and near-surface in the present climate, whereas this pattern shifts farther equatorward in future projections with circulation anomalies in the tropical Indian Ocean amplifying. Significance Statement The influence of the equatorial stratospheric quasi-biennial oscillation (QBO) has been reported to intensify in the Northern Hemisphere under climate change, even as the QBO wind magnitude weakens. It is still unknown whether the influence of the QBO on the Southern Hemisphere intensifies or weakens in a warm climate. Using simulations from multiple models, this study finds that the influence of the QBO on the Southern Hemisphere is projected to strengthen in most models even as the QBO weakens. Two QBO pathways are explored, including the stratospheric polar vortex route and the tropical convection pathway, both of which are projected to amplify in both the moderate- and high-emissions scenarios. In addition to the amplification of the QBO signals in the Southern Hemisphere extratropics, an equatorward shift of the maximum anomalies is also projected. That is, the maximum easterly anomalies in the Antarctic stratosphere during easterly QBO and the annular mode–like pattern in the troposphere or the near-surface shifts farther equatorward.

A Strengthened Teleconnection of the Quasi‐Biennial Oscillation and Tropical Easterly Jet in the Past Decades in E3SMv1

AbstractThe Energy Exascale Earth System Model version 1 (E3SMv1) is a new global model that can generate a weakened tropical easterly jet (TEJ) during the easterly quasi‐biennial oscillation (EQBO) and a strengthened TEJ during the westerly QBO (WQBO), which is referred to as the QBO‐TEJ teleconnection. Although the longitudinal structure of the QBO‐TEJ teleconnection is different between reanalysis and simulations, we hypothesize that simulations can still provide insights into the observed long‐term changes in the QBO‐TEJ teleconnection. The Atmospheric Model Intercomparison Project historical simulations (AMIP‐historical) generate a strengthened trend in the correlation between the QBO and TEJ indices in the period of 1950–2014 as in reanalysis. The comparison of the AMIP‐historical and the Coupled Model Intercomparison Project coupled historical simulations (coupled‐historical) shows that the warming of the Indian Ocean has played a vital role in the strengthened QBO‐TEJ teleconnection over the past six decades.

QBO deepens MJO convection

The underlying mechanism that couples the Quasi-Biennial Oscillation (QBO) and the Madden-Julian oscillation (MJO) has remained elusive, challenging our understanding of both phenomena. A popular hypothesis about the QBO-MJO connection is that the vertical extent of MJO convection is strongly modulated by the QBO. However, this hypothesis has not been verified observationally. Here we show that the cloud-top pressure and brightness temperature of deep convection and anvil clouds are systematically lower in the easterly QBO (EQBO) winters than in the westerly QBO (WQBO) winters, indicating that the vertical growth of deep convective systems within MJO envelopes is facilitated by the EQBO mean state. Moreover, the deeper clouds during EQBO winters are more effective at reducing longwave radiation escaping to space and thereby enhancing longwave cloud-radiative feedback within MJO envelopes. Our results provide robust observational evidence of the enhanced MJO activity during EQBO winters by mean state changes induced by the QBO.

Evaluation of Processes Related to Stratosphere–Troposphere Coupling in GEFSv12 Subseasonal Hindcasts

Abstract The representation of the stratosphere and stratosphere–troposphere coupling processes is evaluated in the subseasonal Global Ensemble Forecast System, version 12 (GEFSv12), hindcasts. The GEFSv12 hindcasts develop systematic stratospheric biases with increasing lead time, including a too strong boreal wintertime stratospheric polar vortex. In the tropical stratosphere, the GEFSv12 winds and temperatures associated with the quasi-biennial oscillation (QBO) tend to decay with lead time such that they underestimate the observed amplitudes; consistently, the QBO-associated mean meridional circulation is too weak. The hindcasts predict extreme polar vortex events (including sudden stratospheric warmings and vortex intensifications) about 13–14 days in advance, and extreme lower-stratospheric eddy heat flux events about 6–10 days in advance. However, GEFSv12’s ability to predict these events is likely affected by its zonal-mean circulation biases, which increases the rates of false alarms and missed detections. Nevertheless, GEFSv12 shows stratosphere–troposphere coupling relationships that agree well with reanalysis and other subseasonal forecast systems. For instance, GEFSv12 reproduces reanalysis relationships between polar vortex strength and the Northern Annular Mode in the troposphere. It also exhibits enhanced weeks 3–5 prediction skill of the North Atlantic Oscillation index when initialized during strong and weak polar vortex states compared to neutral states. Furthermore, GEFSv12 shows significant differences in Madden–Julian oscillation (MJO) amplitudes and enhanced MJO predictive skill in week 4 during easterly versus westerly QBO phases, though these results are sensitive to the level used to define the QBO. Our results provide a baseline from which future GEFS updates may be measured.

On the magnitude and sensitivity of the quasi-biennial oscillation response to a tropical volcanic eruption

Abstract. Volcanic eruptions that inject sulfur dioxide into the stratosphere have the potential to alter large-scale circulation patterns, such as the quasi-biennial oscillation (QBO), which can affect weather and transport of chemical species. Here, we conduct simulations of tropical volcanic eruptions using the UM-UKCA aerosol-climate model with an explicit representation of the QBO. Eruptions emitting 60 Tg of SO2 (i.e. the magnitude of the 1815 Mt. Tambora eruption) and 15 Tg of SO2 (i.e. the magnitude of the 1991 Mt. Pinatubo eruption) were initiated at the Equator during two different QBO states. We show that tropical eruptions delay the progression of the QBO phases, with the magnitude of the delay dependent on the initial wind shear in the lower stratosphere and a much longer delay when the shear is easterly than when it is westerly. The QBO response in our model is driven by vertical advection of momentum by the stronger tropical upwelling caused by heating due to the increased volcanic sulfate aerosol loading. Direct aerosol-induced warming with subsequent thermal wind adjustment, as proposed by previous studies, is found to only play a secondary role. This interpretation of the response is supported by comparison with a simple dynamical model. The dependence of the magnitude of the response on the initial QBO state results from differences in the QBO secondary circulation. In the easterly shear zone of the QBO, the vertical component of the secondary circulation is upward and reinforces the anomalous upwelling driven by volcanic aerosol heating, whereas in the westerly shear zone the vertical component is downward and opposes the aerosol-induced upwelling. We also find a change in the latitudinal structure of the QBO, with the westerly phase of the QBO strengthening in the hemisphere with the lowest sulfate aerosol burden. Overall, our study suggests that tropical eruptions of Pinatubo magnitude or larger could force changes to the progression of the QBO, with particularly disruptive outcomes for the QBO if the eruption occurs during the easterly QBO shear.

Quasi-biennial modulation of total ozone distribution from equator to polar regions based on observations

The modulation of total ozone (TOZ) by the quasi-biennial oscillation (QBO) not only at the equator, but also at subtropics, middle and high latitudes has long been known. As well, a large number of studies have shown seasonal synchronization and interhemispheric asymmetry of QBO effects in the TOZ in extratropical regions. However, the proposed mechanisms of seasonality and asymmetry as a consequence of the interaction of the quasi-biennial and annual cycles are not well understood. A great problem in comparing model simulations with observations and, hence, in verifying theoretical assumptions about the mechanism of extratropical ozone QBO, is the continually changing phase relationship between the annual and quasi-biennial cycles. This issue is exacerbated by the fact that, as is commonly believed, the wind QBO period is not constant, but varies irregularly from one cycle to another. In this study the influence of the QBO of equatorial stratospheric wind on the global distribution of the TOZ has been analyzed using the SBUV Merged Ozone Data Set (MOD). The investigation of the QBO effects in the TOZ in the subtropics, middle and high latitudes of both hemispheres is based on accounting for the seasonal regularities of the wind QBO, which leads to a limited number of only certain combinations of the quasi-biennial and annual cycles that can be realized. The smaller variability serves to clarify the nature of changes in extratropical ozone. A detailed analysis of the TOZ data for 1970–2021 showed that the interannual TOZ variability at subtropics, middle and high latitudes depends on the alternation of wind QBO cycles of different scenarios. The key moments of the TOZ variations are clearly associated with certain points in time during the downward propagation of the easterly and westerly QBO wind regimes and the seasonally dependent QBO circulation produced by them. These results contradict the general view that it is difficult to isolate the effect of QBO on the TOZ in extratropics because of the irregularly varying QBO period, and can be used in comparing simulated and observed extratropical ozone QBO, as well as in long-term forecast of variations in the TOZ distribution.

Captured QBO‐MJO Connection in a Subseasonal Prediction System

AbstractThe quasi‐biennial oscillation (QBO) impacts the Madden‐Julian Oscillation (MJO) activity with a stronger MJO in QBO easterly (QBOE) than QBO westerly (QBOW) winters. However, this relationship is poorly represented in the current generation climate models. For the first time, this paper applies a stratospheric zonal‐mean nudging in a subseasonal prediction system to capture it. Two strong MJO cases in a QBO‐neutral winter are investigated. The QBO temperature and zonal wind anomalies are added separately as well as together to the stratosphere using nudging in MJO case hindcast. Only by nudging the QBO temperature anomalies while leaving the zonal wind free, can the prediction system capture the observed QBO‐MJO connection. The tropopause instability is found positively correlated to the MJO amplitude, but it cannot fully explain the captured connection. The free‐evolving zonal wind anomalies in the stratosphere due to the nudged QBO temperature are crucial for the captured connection.

Southern Hemisphere Response to the Quasi-Biennial Oscillation in the CMIP5/6 Models

Abstract Using 25 quasi-biennial oscillation (QBO)-resolving models from phases 5 and 6 of the Coupled Model Intercomparison Project (CMIP5/6), this study systematically explores the Southern Hemisphere (SH) extratropical response to the QBO. The observed maximum stratospheric polar vortex–weakening response to the 20-hPa easterly QBO appears in the austral spring, which is reproduced qualitatively by seven models. The Eliassen–Palm (EP) flux convergence associated with enhanced upward propagation of waves from the SH midlatitude troposphere is also reasonably simulated following easterly QBO in these seven models. The Brewer–Dobson (BD) circulation response, especially its deep branch, has a large bias among models. The anomalous upwelling of the deep branch of the BD circulation and the cold anomalies are confined to midlatitudes above 20 hPa in reanalyses, while this deep branch is simulated to expand to the Antarctic stratosphere with unrealistically cold anomalies for some models. Models with a decent stratospheric pathway tend to simulate the observed negative Southern Annular Mode (SAM)-like response in the troposphere. Further, the largest enhanced convection center is observed over the tropical Indo-Pacific Ocean and simulated by 12 models, most of which also simulate a more reasonable height anomaly pattern in the troposphere. The near-surface response to the QBO also resembles the SAM pattern with warm anomalies over eastern Antarctic, which are simulated by only five models. Seven models simulate the observed wet signals in the midlatitude southern Indo-Pacific Ocean, where an anomalous low develops. Overall, models with a better reproduction of the SH stratospheric polar vortex response exhibit a more reasonable near-surface response, though for all models the response is too weak. Significance Statement The equatorial stratospheric quasi-biennial oscillation (QBO) has been widely reported to influence the Northern Hemisphere. This paper focuses on the possible impact of the QBO on the Southern Hemisphere (SH) using 25 models. The maximum QBO signal in the Antarctic stratosphere forms in austral spring when the climatological vortex intensity has weakened relative to austral winter. The timing and magnitude of the QBO signal is simulated with different degrees of success by the 25 models, and those with a reasonable polar vortex response tend to better simulate the near-surface response. The QBO has another pathway for the extratropical circulation change via modifying tropical convection, and models also simulate a large spread in the tropical convection change associated with the QBO.

Development of a Statistical Subseasonal Forecast Tool to Predict California Atmospheric Rivers and Precipitation Based on MJO and QBO Activity

AbstractThis paper examines the empirical relationship between the Madden–Julian oscillation (MJO), the quasi‐biennial oscillation (QBO), and atmospheric river (AR) activity and precipitation in California on subseasonal time scales. We introduce an experimental forecast tool that uses observed anomaly patterns during a 38 yr period to predict the probability of above‐ and below‐normal AR activity and precipitation at lead times of 1–6 weeks based on the phase and amplitude of the MJO and QBO. The hindcast prediction skill of probabilistic AR activity and precipitation forecasts is evaluated for Northern, Central, and Southern California, as well as two sets of smaller geographical domains. These smaller domains are more relevant for water resource management and allow us to investigate the sensitivity of prediction skill to domain size. Consistent with previous studies, our results demonstrate that subseasonal AR activity and precipitation in California are strongly modulated by the MJO and QBO. The anomaly patterns of AR activity and precipitation vary considerably throughout the cool season, with a tendency toward below‐normal AR activity and precipitation during easterly QBO and above‐normal AR activity and precipitation during westerly QBO in JFM. The opposite patterns are generally observed in OND, but the anomaly signals are weaker and less coherent for AR activity. Certain combinations of MJO phase, QBO phase, lag time, and season yield notably higher skill scores, reinforcing the notion of “windows of opportunity” for skillful subseasonal‐to‐seasonal predictions. In California, these forecasts of opportunity are predominantly associated with easterly QBO in JFM and FMA.

Modulation by the QBO of the Relationship between the NAO and Northeast China Temperature in Late Winter

Abstract The North Atlantic Oscillation (NAO) generally has an in-phase relationship with surface air temperature (SAT) anomalies over Northeast China in late winter. The present study shows that such an NAO–SAT relationship becomes stronger during easterly phases of the quasi-biennial oscillation (QBO), but is relatively weak during westerly phases. Observational evidence reveals that the modulation effect of the QBO on the NAO–SAT relationship over Northeast China is attributable to QBO-induced changes in the spatial structure of the NAO and associated stratosphere–troposphere coupling. During easterly QBO phases, the NAO has a strong connection with the Northern Hemisphere stratospheric polar vortex, facilitating a hemisphere-wide structure of the NAO and thus a downstream extension of NAO signal from the Euro–Atlantic sector toward Northeast Asia. During westerly QBO phases, however, the NAO has a limited connection with the stratospheric polar vortex. In this case, the NAO features a classically regional mode, with the signal in the SAT field mainly confined to the Euro-Atlantic sector. By examining historical simulations from six climate models participating in CMIP6 and including stratospheric processes, it is found that none of these models captures a significant difference in the spatial structure of the NAO and its connection with the stratospheric polar vortex between the easterly and westerly QBO phases in late winter. A key reason may be related to the poor performance of the models in simulating the Holton–Tan effect, which is critical for linking the QBO and NAO. Significance Statement The QBO is the dominant mode of equatorial stratospheric variability and important for seasonal forecasting. This study aims to better understand the surface influence of the QBO by examining the relationship between the NAO and temperature anomalies over China according to the phases of the QBO during boreal winter. Our results highlight the importance of the QBO in modulating the spatial structure and thus climate impact of the NAO via stratosphere–troposphere coupling in observations, while state-of-the-art climate models perform poorly in simulating the QBO-related stratosphere–troposphere coupling and thus the QBO–NAO connection. These findings have important implications for seasonal prediction and model development in the future.

Numerical simulation of stratospheric QBO impact on the planetary waves up to the thermosphere

With the help of numerical simulation, a detailed analysis of the dynamical effect of the stratospheric quasi-biennial oscillation (QBO) of the equatorial zonal wind on the planetary waves (PWs) up to thermospheric heights is carried out for the first time. The 3-dimensional nonlinear mechanistic model of middle and upper atmosphere (MUAM) is used, which is capable of simulating the general atmospheric circulation from the surface up to 300–400 km altitude. The amplitudes of stationary and westward travelling PWs with periods from 4 to 10 days are calculated based on ensembles of model simulations for conditions corresponding to the easterly and westerly QBO phases. Fluxes of wave activity and refractive indices of the atmosphere are calculated to analyze the detailed behavior of the PWs. The important result to emerge is that the stratospheric QBO causes statistically significant changes in the amplitudes of individual wave components up to 25% in the mesosphere-lower thermosphere and 10% changes above 200 km. This change in wave structures should be especially noticeable in the atmosphere during periods of low solar activity, when the direct contribution of solar activity fluctuations is minimized. Propagating from the troposphere to the upper atmosphere, PWs contribute to the propagation of the QBO signal not only from the equatorial region to extratropical latitudes, but also from the stratosphere to the thermosphere. The need for a detailed analysis of large-scale wave disturbances in the upper atmosphere and their relationship with the underlying layers is due, in particular, to their significant impact on satellite navigation and communication systems, which is caused by amplitude and phase fluctuations of the radio signal.

Impacts of the stratospheric quasi-biennial oscillation on the tropospheric circulation and climate in the Northeast Asia–North Pacific region in early summer

Previous studies have indicated that the stratospheric quasi-biennial oscillation (QBO) has a global impact on winter weather, but relatively less attention has been paid to its effect in summer. Using ERA5 data, this study reports that the QBO has a significant impact on the tropospheric circulation and surface air temperature (SAT) in the extratropics in Northeast Asia and the North Pacific in early summer. Specifically, a QBO-induced mean meridional circulation prevails from Northeast Asia to the North Pacific in the westerly QBO years, exhibiting westerly anomalies in 20°–35°N and easterly anomalies in 35°–65°N from the lower stratosphere to troposphere. This meridional pattern of zonal wind anomalies can excite positive vorticity and thus lead to anomalous low pressure and cyclonic circulation from Northeast Asia to the North Pacific, which in turn cause northerly wind anomalies and decreased SAT in Northeast Asia in June. Conversely, in the easterly QBO years, the QBO-related circulation and SAT anomalies are generally in an opposite polarity to those in the westerly QBO years. These findings provide new evidence of the impact of the QBO on the extratropical climate, and may benefit the prediction of SAT in Northeast Asia in early summer.摘要本文研究了平流层准两年振荡 (QBO) 对东北亚-北太平洋地区初夏对流层环流和地表气温的影响. 在QBO西风位相年, 东北亚至北太平洋地区存在一支由QBO引发的平均经向环流异常, 该经向环流异常可在东北亚至北太平洋地区激发正涡度, 并形成异常气旋式环流. 气旋左侧出现的异常偏北风导致6月东北亚地表气温下降. QBO东风位相年的结果与西风位相年大致相反. 这些结果为QBO对热带外地区天气,气候的影响提供了新的证据, 并为东北亚初夏地表气温的预测提供了新的线索.

Non‐Linear Response of the Extratropics to Tropical Climate Variability

AbstractThere are established El Niño‐Southern Oscillation (ENSO) and Quasi‐biennial Oscillation (QBO) teleconnections to the extratropics that alter the strength of the polar vortex. Here, we demonstrate non‐linearity in the stratospheric polar vortex response to ENSO and QBO combined, in climate model simulations and observational data. We show that the combined effect of El Niño and QBO easterly phase (QBO‐E) on the polar vortex is greater than the linear combination of the individual responses to El Niño and QBO‐E separately. Observations support the model results, despite a relatively small sample of observed cases. We propose that non‐linearity in the teleconnection to ENSO and QBO is caused by the combined effects of an increase in wave driving from El Niño and more poleward propagation of wave activity during QBO‐E. Our results have implications for extreme winters when both El Niño and QBO‐E are active.

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.