Showing results for tags 'ssw'. - 33andrain Jump to content

Search the Community

Showing results for tags 'ssw'.



More search options

  • Search By Tags

    Type tags separated by commas.
  • Search By Author

Content Type


Forums

  • 33andrain Community
    • 33andrain Spotlight
    • Thread Archive
    • Off Topic
  • 33andrain University
    • Research Portal

Product Groups

  • Premium Services
  • Winter Essentials

Find results in...

Find results that contain...


Date Created

  • Start

    End


Last Updated

  • Start

    End


Filter by number of...

Joined

  • Start

    End


Group


Location

Found 72 results

  1. On the Generation and Maintenance of the 2012/13 Sudden Stratospheric Warming Authors: Fen Xu, X. San Liang Published: July 2017 Abstract: Using a newly developed analysis tool, multiscale window transform (MWT), and the MWT-based localized multiscale energetics analysis, the 2012/13 sudden stratospheric warming (SSW) is diagnosed for an understanding of the underlying dynamics. The fields are first reconstructed onto three scale windows: that is, mean window, sudden warming window or SSW window, and synoptic window. According to the reconstructions, the major warming period may be divided into three stages: namely, the stages of rapid warming, maintenance, and decay, each with different mechanisms. It is found that the explosive growth of temperature in the rapid warming stage (28 December–10 January) results from the collaboration of a strong poleward heat flux and canonical transfers through baroclinic instabilities in the polar region, which extract available potential energy (APE) from the mean-scale reservoir. In the course, a portion of the acquired APE is converted to and stored in the SSW-scale kinetic energy (KE), leading to a reversal of the polar night jet. In the stage of maintenance (11–25 January), the mechanism is completely different: First the previously converted energy stored in the SSW-scale KE is converted back, and, most importantly, in this time a strong barotropic instability happens over Alaska–Canada, which extracts the mean-scale KE to maintain the high temperature, while the mean-scale KE is mostly from the lower atmosphere, in conformity with the classical paradigm of mean flow–wave interaction with the upward-propagating planetary waves. This study provides an example that a warming may be generated in different stages through distinctly different mechanisms. Link to full paper: https://journals.ametsoc.org/doi/10.1175/JAS-D-17-0002.1
  2. Enhanced Stratosphere/Troposphere Coupling During Extreme Warm Stratospheric Events with Strong Polar-Night Jet Oscillation Author: Dieter H.W. Peter, Andrea Schneidereit and Alexey Y. Karpechko Published: November 29th, 2018 Abstract: Extreme warm stratospheric events during polar winters from ERA-Interim reanalysis and CMIP5-ESM-LR runs were separated by duration and strength of the polar-night jet oscillation (PJO) using a high statistical confidence level of three standard deviations (strong-PJO events). With a composite analysis, we demonstrate that strong-PJO events show a significantly stronger downward propagating signal in both, northern annular mode (NAM) and zonal mean zonal wind anomaly in the stratosphere in comparison with non-PJO events. The lower stratospheric EP-flux-divergence difference in ERA-Interim was stronger in comparison to long-term CMIP5-ESM-LR runs (by a factor of four). This suggests that stratosphere–troposphere coupling is stronger in ERA-Interim than in CMIP5-ESM-LR. During the 60 days following the central date (CD), the Arctic oscillation signal was more intense during strong-PJO events than during non-PJO events in ERA-Interim data in comparison to CMIP5-ESM-LR runs. During the 15-day phase after CD, strong PJO events had a significant increase in stratospheric ozone, upper tropospheric zonally asymmetric impact, and a regional surface impact in ERA-Interim. Finally, we conclude that the applied high statistical threshold gives a clearer separation of extreme warm stratospheric events into strong-PJO events and non-PJO events including their different downward propagating NAM signal and tropospheric impacts. Link to full paper: https://www.mdpi.com/2073-4433/9/12/467/htm Credit goes to @sebastiaan1973 for finding this paper
  3. Eurasian snow cover variability and links to winter climate in the CMIP5 models Author: Jason C. Furtado, Judah L. Cohen, Amy H. Butler, Emily E. Riddle and Arun Kumar Published: 31st January, 2015 Abstract: Observational studies and modeling experiments illustrate that variability in October Eurasian snow cover extent impacts boreal wintertime conditions over the Northern Hemisphere (NH) through a dynamical pathway involving the stratosphere and changes in the surface-based Arctic Oscillation (AO). In this paper, we conduct a comprehensive study of the Eurasian snow–AO relationship in twenty coupled climate models run under pre-industrial conditions from the Coupled Model Intercomparison Project Phase 5 (CMIP5). Our analyses indicate that the coupled climate models, individually and collectively, do not capture well the observed snow–AO relationship. The models lack a robust lagged response between October Eurasian snow cover and several NH wintertime variables (e.g., vertically propagating waves and geopotential heights). Additionally, the CMIP5 models do not simulate the observed spatial distribution and statistics of boreal fall snow cover across the NH including Eurasia. However, when analyzing individual 40-year time slices of the models, there are periods of time in select models when the observed snow–AO relationship emerges. This finding suggests that internal variability may play a significant role in the observed relationship. Further analysis demonstrates that the models poorly capture the downward propagation of stratospheric anomalies into the troposphere, a key facet of NH wintertime climate variability irrespective of the influence of Eurasian snow cover. A weak downward propagation signal may be related to several factors including too few stratospheric vortex disruptions and weaker-than-observed tropospheric wave driving. The analyses presented can be used as a roadmap for model evaluations in future studies involving NH wintertime climate variability, including those considering future climate change. Link to full paper: http://web.mit.edu/jlcohen/www/papers/Furtado_etal_CD15.pdf Credit goes to Tom @Isotherm for recommending this excellent paper.
  4. Role of Finite-Amplitude Eddies and Mixing in the Life Cycle of Stratospheric Sudden Warmings Authors: Sandro W. Lubis, Clare S. Y. Huang, Noboru Nakamura, Nour-Eddine Omrani and Martin Jucker Published: 26th October, 2018 Abstract: Despite the advances in theories and data availability since the first observation of stratospheric sudden warmings (SSWs) in the 1950s, some dynamical aspects of SSWs remain elusive, including the roles of wave transience at finite amplitude and irreversible wave dissipation due to mixing. This is likely due to a limitation of the traditional theory for SSWs that is tailored to small-amplitude waves and is unsuitable for large-scale wave events. To circumvent these difficulties, the authors utilized a novel approach based on finite-amplitude wave activity theory to quantify the roles of finite-amplitude wave transience and mixing in the life cycle of SSWs. In this framework, a departure from the exact nonacceleration relation can be directly attributed to irreversible mixing and diabatic forcings. The results show that prior to the warming event, an increase in pseudomomentum/wave activity largely compensates for the anomalous Eliassen–Palm flux convergence, while the total wave dissipation due to mixing (enstrophy dissipation) and radiative forcing only plays a secondary role. After the vortex breaks down, enhanced mixing increases irreversible wave dissipation and in turn slows down vortex recovery. It is shown that (i) a rapid recovery of the polar vortex is characterized by weak wave transience that follows a nonacceleration relation reversibly and (ii) a delayed recovery is attributed to stronger and more persistent irreversible wave dissipation due to mixing, a deviation from the classical nonacceleration relation. The results highlight the importance of mixing in the asymmetry between breakdown and recovery of the polar vortex during SSWs. Link to full paper: This is behind an AMS paywall but there is another link to the Researchgate site with the early online release with the full "preliminary" version from September 2018 (just scroll down that page) and there is also a personal pdf download version available from there. https://www.researchgate.net/publication/327854069_Role_of_Finite-Amplitude_Eddies_and_Mixing_in_the_Life_Cycle_of_Stratospheric_Sudden_Warmings
  5. Sudden Stratospheric Warmings – developing a new classification based on vertical depth, applying theory to a SSW in 2018, and assessing predictability of a cold air outbreak following this SSW Author: L. van Galen Published: August 2018 Abstract: n this study a new classification has been developed to classify Sudden Stratospheric Warmings (SSWs) based on their vertical depth. This new classification was developed because the depth of a SSW has been found to be important for the magnitude of tropospheric impact (Gerber et al., 2009; Palmeiro et al., 2015), and the official SSW classification does not tell anything about the vertical extent of a SSW. The new classification adapted from a previously developed classification of Kramer (2016; hereafter referred to as K16)). It prescribes that the zonal mean zonal wind between 60N and 70N should reverse over a depth of at least 80 hPa between 10 and 100 hPa for at least two days in a 5-day period. This classification was termed a ‘Deep Stratospheric Warming’ (DSW). In the stratosphere, the new DSW classification was compared to both the SSW-classification and the K16-classification. However, the K16-classification included events that were too weak to be considered important for tropospheric impact. Coupled to the fact that K-16-events did not contain any information about the vertical depth of SSWs, this classification was not taken into account in subsequent analyses. Compared to the official SSW classification, it appeared that DSWs were less frequent; whereas SSWs occurred about 6 times per decade, DSWs were found to occur only 4 times per decade. Furthermore, the SSW and DSW classifications were connected: most DSWs occurred a few days to weeks after the SSW date. This was explained by the behavior of SSWs and DSWs: first a warming at 10 hPa took place (the SSW date). After that, the wind reversal extended downward in an irregular fashion, during which after some SSWs at some point the wind reversal extended sufficiently far to the lower stratosphere to cause a DSW to be classified. Connected to the differences in classification date, the DSWs were rarely located close to the moment of rapid warming in the stratosphere. Thus, unlike SSWs, the DSWs did not show the ‘sudden warming’ behavior. Rather, (most of) the DSWs seemed to be a result warm anomalies in the mid-stratosphere that developed sometime during or after the SSW and subsequently downwelled on a timescale of about a week. In terms of upper tropospheric impact, the DSW classification resulted in a similar response compared to the SSW classification: anomalously high temperatures near the pole and a southward displaced jet stream. Furthermore, the jet stream was found to become slightly more meridionally oriented in the midlatitudes and slightly more zonally oriented in the subtropics, but this signal was fairly weak. The main difference between the DSW and SSW was that the DSW effects were mostly stronger in magnitude than the SSW effects. Thus, a DSW results in stronger upper tropospheric impacts compared to a SSW. A similar story was found in terms of surface impacts. Both the SSW and DSW classifications resulted in positive pressure anomalies over the pole and generally negative pressure anomalies in the midlatitudes (indicative of a negative Arctic Oscillation); but the anomalies were more pronounced after a DSW. The surface temperature response after both a SSW and DSW was chaotic, though, showing that the DSW classification did not result in a stronger or more structured surface temperature response. Finally, a thermodynamic perspective was presented to explain why the DSW classification resulted in a stronger tropospheric response compared to the SSW classification method (only in terms of a -AO and a weaker jet stream in the midlatitudes). The cause was speculated to be that the warming accompanied by a DSW extended further to the upper troposphere. This is important for two reasons, being: 1) A deeper warming causes the pressure surfaces to be less tilted from the midlatitudes to the pole, resulting in a more pronounced weakening of midlatitude westerlies, including the polar jet stream. 2) A warming that extends more towards the troposphere exerts more pressure on the underlying air, because the density of air closer to the troposphere is higher. This results in a higher surface pressure. ` In short, the new DSW classification enables to better assess the zonal mean timing and intensity of tropospheric impact compared to the SSW classification. However, no specific conclusions could be drawn about the zonally asymmetric effects of DSWs. Link to full paper: http://bibliotheek.knmi.nl/knmipubIR/IR2018-05.pdf Credit goes to @sebastiaan1973 for finding this presentation - thank you.
  6. The Downward Influence of Sudden Stratospheric Warmings: Association with Tropospheric Precursors Author: Ian White Published: Oct 2018 Abstract: Tropospheric features preceding Sudden Stratospheric Warming events (SSWs) are identified using a large compendium of events obtained from a chemistry-climate model. In agreement with recent observational studies, it is found that approximately one third of SSWs are preceded by extreme episodes of wave activity in the lower troposphere. The relationship becomes stronger in the lower stratosphere, where ∼60% of SSWs are preceded by extreme wave activity at 100 hPa. Additional analysis characterises events that do or do not appear to subsequently impact the troposphere, referred to as downward and non-downward propagating SSWs, respectively. On average, tropospheric wave activity is larger preceding downward-propagating SSWs compared to non-downward propagating events, and associated in particular with a doubly-strengthened Siberian High. Of the SSWs that were preceded by extreme lower-tropospheric wave activity, ∼2/3 propagated down to the troposphere, and hence the presence of extreme lower-tropospheric wave activity can only be used probablistically to predict a slight increase or decrease at the onset, of the likelihood of tropospheric impacts to follow. However, a large number of downward and non-downward propagating SSWs must be considered (> 35), before the difference becomes statistically significant. The precursors are also robust upon comparison with composites consisting of randomly-selected tropospheric NAM events. The downward influence and precursors to split and displacement events are also examined. It is found that anomalous upward wave-1 fluxes precede both cases. Splits exhibit a near instantaneous, barotropic response in the stratosphere and troposphere, while displacements have a stronger long-term influence. Link to full paper: https://cims.nyu.edu/~gerber/pages/documents/white_etal-JC-submitted.pdf
  7. Solar and QBO Influences on the Timing of Stratospheric Sudden Warmings Author: Lesley J. Gray Published: Dec 2004 Abstract: The interaction of the 11-yr solar cycle (SC) and the quasi-biennial oscillation (QBO) and their influence on the Northern Hemisphere (NH) polar vortex are studied using idealized model experiments and ECMWF Re-Analysis (ERA-40). In the model experiments, the sensitivity of the NH polar vortex to imposed easterlies at equatorial/subtropical latitudes over various height ranges is tested to explore the possible influence from zonal wind anomalies associated with the QBO and the 11-yr SC in those regions. The experiments show that the timing of the modeled stratospheric sudden warmings (SSWs) is sensitive to the imposed easterlies at the equator/subtropics. When easterlies are imposed in the equatorial or subtropical upper stratosphere, the onset of the SSWs is earlier. A mechanism is proposed in which zonal wind anomalies in the equatorial/subtropical upper stratosphere associated with the QBO and 11-yr SC either reinforce each other or cancel each other out. When they reinforce, as in Smin–QBO-east (Smin/E) and Smax–QBO-west (Smax/W), it is suggested that the resulting anomaly is large enough to influence the development of the Aleutian high and hence the time of onset of the SSWs. Although highly speculative, this mechanism may help to understand the puzzling observations that major warmings often occur in Smax/W years even though there is no strong waveguide provided by the QBO winds in the lower equatorial stratosphere. The ERA-40 data are used to investigate the QBO and solar signals and to determine whether the observations support the proposed mechanism. Composites of ERA-40 zonally averaged zonal winds based on the QBO (E/W), the SC (min/max), and both (Smin/E, Smin/W, Smax/E, Smax/W) are examined, with emphasis on the Northern Hemisphere winter vortex evolution. The major findings are that QBO/E years are more disturbed than QBO/W years, primarily during early winter. Sudden warmings in Smax years tend to occur later than in Smin years. Midwinter warmings are more likely during Smin/E and Smax/W years, although the latter result is only barely statistically significant at the 75% level. The data show some support for the proposed mechanism, but many more years are required before it can be fully tested. Link to full paper: https://journals.ametsoc.org/doi/10.1175/JAS-3297.1
  8. More Frequent Sudden Stratospheric Warming Events due to Enhanced MJO Forcing Expected in a Warmer Climate Authors: Kang and Tziperman Published: July 2017 Abstract: Sudden stratospheric warming (SSW) events influence the Arctic Oscillation and midlatitude extreme weather. Observations show SSW events to be correlated with certain phases of the Madden–Julian oscil- lation (MJO), but the effect of the MJO on SSW frequency is unknown, and the teleconnection mechanism, its planetary wave propagation path, and time scale are still not completely understood. The Arctic stratosphere response to increased MJO forcing expected in a warmer climate using two models is studied: the compre- hensive Whole Atmosphere Community Climate Model and an idealized dry dynamical core with and without MJO-like forcing. It is shown that the frequency of SSW events increases significantly in response to stronger MJO forcing, also affecting the averaged polar cap temperature. Two teleconnection mechanisms are identified: a direct propagation of MJO-forced transient waves to the Arctic stratosphere and a nonlinear enhancement of stationary waves by the MJO-forced transient waves. The MJO-forced waves propagate poleward in the lower stratosphere and upper troposphere and then upward. The cleaner results of the idealized model allow identifying the propagating signal and suggest a horizontal propagation time scale of 10–20 days, followed by additional time for upward propagation within the Arctic stratosphere, although there are significant uncertainties involved. Given that the MJO is predicted to be stronger in a warmer climate, these results suggest that SSW events may become more frequent, with possible implications on tropospheric high-latitude weather. However, the effect of an actual warming scenario on SSW frequency involves additional effects besides a strengthening of the MJO, requiring further investigation. Link to full paper: https://www.seas.harvard.edu/climate/eli/reprints/Kang-Tziperman-2017.pdf
  9. No robust evidence of future changes in major stratospheric sudden warmings: a multi-model assessment from CCMI Authors: Blanca Ayarzagüena, et al. Published: 13 August 2018 Abstract: Major mid-winter stratospheric sudden warmings (SSWs) are the largest instance of wintertime variability in the Arctic stratosphere. Because SSWs are able to cause significant surface weather anomalies on intra-seasonal timescales, several previous studies have focused on their potential future change, as might be induced by anthropogenic forcings. However, a wide range of results have been reported, from a future increase in the frequency of SSWs to an actual decrease. Several factors might explain these contradictory results, notably the use of different metrics for the identification of SSWs and the impact of large climatological biases in single-model studies. To bring some clarity, we here revisit the question of future SSW changes, using an identical set of metrics applied consistently across 12 different models participating in the Chemistry–Climate Model Initiative. Our analysis reveals that no statistically significant change in the frequency of SSWs will occur over the 21st century, irrespective of the metric used for the identification of the event. Changes in other SSW characteristics – such as their duration, deceleration of the polar night jet, and the tropospheric forcing – are also assessed: again, we find no evidence of future changes over the 21st century. Link to full paper: https://www.atmos-chem-phys.net/18/11277/2018/acp-18-11277-2018.pdf
  10. Northern Hemisphere Stratospheric Pathway of Different El Niño Flavors in Stratosphere-Resolving CMIP5 Models Authors: N. Calvo, M. Iza, M. M. Hurwitz, E. Manzini, C. Peña-Ortiz, A. H. Butler, C. Cagnazzo, S. Ineson and C. I. Garfinkel Published: 10th May, 2017 Abstract: The Northern Hemisphere (NH) stratospheric signals of eastern Pacific (EP) and central Pacific (CP) El Niño events are investigated in stratosphere-resolving historical simulations from phase 5 of the Coupled Model Intercomparison Project (CMIP5), together with the role of the stratosphere in driving tropospheric El Niño teleconnections in NH climate. The large number of events in each composite addresses some of the previously reported concerns related to the short observational record. The results shown here highlight the importance of the seasonal evolution of the NH stratospheric signals for understanding the EP and CP surface impacts. CMIP5 models show a significantly warmer and weaker polar vortex during EP El Niño. No significant polar stratospheric response is found during CP El Niño. This is a result of differences in the timing of the intensification of the climatological wavenumber 1 through constructive interference, which occurs earlier in EP than CP events, related to the anomalous enhancement and earlier development of the Pacific–North American pattern in EP events. The northward extension of the Aleutian low and the stronger and eastward location of the high over eastern Canada during EP events are key in explaining the differences in upward wave propagation between the two types of El Niño. The influence of the polar stratosphere in driving tropospheric anomalies in the North Atlantic European region is clearly shown during EP El Niño events, facilitated by the occurrence of stratospheric summer warmings, the frequency of which is significantly higher in this case. In contrast, CMIP5 results do not support a stratospheric pathway for a remote influence of CP events on NH teleconnections. Link to full paper: https://journals.ametsoc.org/doi/pdf/10.1175/JCLI-D-16-0132.1
  11. Impact of the Stratosphere on the Winter Tropospheric Teleconnections between ENSO and the North Atlantic and European Region Authors: Chiara Cagnazzor and Elisa Manzini Published: 21st August, 2008 Abstract: The possible role of stratospheric variability on the tropospheric teleconnection between El Niño–Southern Oscillation (ENSO) and the North Atlantic and European (NAE) region is addressed by comparing results from two ensembles of simulations performed with an atmosphere general circulation model fully resolving the stratosphere (with the top at 0.01 hPa) and its low-top version (with the top at 10 hPa). Both ensembles of simulations consist of nine members, covering the 1980–99 period and are forced with prescribed observed sea surface temperatures. It is found that both models capture the sensitivity of the averaged polar winter lower stratosphere to ENSO in the Northern Hemisphere, although with a reduced amplitude for the low-top model. In late winter and spring, the ENSO response at the surface is instead different in the two models. A large-scale coherent pattern in sea level pressure, with high pressures over the Arctic and low pressures over western and central Europe and the North Pacific, is found in the February–March mean of the high-top model. In the low-top model, the Arctic high pressure and the western and central Europe low pressure are very much reduced. The high-top minus low-top model difference in the ENSO temperature and precipitation anomalies is that North Europe is colder and the Northern Atlantic storm track is shifted southward in the high-top model. In addition, it has been found that major sudden stratospheric warming events are virtually lacking in the low-top model, while their frequency of occurrence is broadly realistic in the high-top model. Given that this is a major difference in the dynamical behavior of the stratosphere of the two models and that these events are favored by ENSO, it is concluded that the occurrence of sudden stratospheric warming events affects the reported differences in the tropospheric ENSO–NAE teleconnection. Given that the essence of the high-top minus low-top model difference is a more annular (or zonal) pattern of the anomaly in sea level pressure, relatively larger over the Arctic and the NAE regions, this interpretation is consistent with the observational evidence that sudden stratospheric warmings play a role in giving rise to persistent Arctic Oscillation anomalies at the surface. Link to full paper: https://journals.ametsoc.org/doi/pdf/10.1175/2008JCLI2549.1
  12. Effect of Madden–Julian Oscillation Occurrence Frequency on the Interannual Variability of Northern Hemisphere Stratospheric Wave Activity in Winter Authors: Feiyang Wang, Wenshou Tian, Fei Xie, Jiankai Zhang and Yuanyuan Han Published: 12th March, 2018 (on line: 1st June, 2018) Abstract: This study uses reanalysis datasets and numerical experiments to investigate the influence of the occurrence frequency of the individual phases of the Madden–Julian oscillation (MJO) on the interannual variability of stratospheric wave activity in the middle and high latitudes of the Northern Hemisphere during boreal winter [November–February (NDJF)]. Our analysis reveals that the occurrence frequency of MJO phase 4 in winter is significantly positively correlated with the interannual variability of the Eliassen–Palm (E–P) flux divergence anomalies in the northern extratropical stratosphere; that is, higher (lower) occurrence frequency of MJO phase 4 corresponds to weaker (stronger) upward wave fluxes and increased (decreased) E–P flux divergence anomalies in the middle and upper stratosphere at mid-to-high latitudes, which implies depressed (enhanced) wave activity accompanied by a stronger (weaker) polar vortex in that region. The convection anomalies over the Maritime Continent related to MJO phase 4 excite a Rossby wave train that propagates poleward to middle and high latitudes, and is in antiphase with the climatological stationary waves of wavenumber 1 at middle and high latitudes. As the spatial distribution of the convection anomalies during MJO phase 7 has an almost opposite, but weaker, pattern to that during MJO phase 4, the occurrence frequency of MJO phase 7 has an opposite and weaker effect on the northern extratropical stratosphere to MJO phase 4. However, the other MJO phases of 1, 2, 3, 5, 6 and 8, cannot significantly influence the northern extratropical stratosphere because the wave responses in these phases are neither totally in nor out of phase with the background stationary wavenumber 1. Link to full paper: This excellent very recent paper was behind an AMS paywall but I found a copy which I was able to download onto a personal pdf file and then convert to a word document and I copy the full paper below (with charts and diagrams added separately): ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------ Effect of Madden–Julian Oscillation Occurrence Frequency on the Interannual Variability of Northern Hemisphere Stratospheric Wave Activity in Winter FEIYANG WANG AND WENSHOU TIAN Key Laboratory for Semi-Arid Climate Change of the Ministry of Education, College of Atmospheric Science, Lanzhou University, Lanzhou, China FEI XIE College of Global Change and Earth System Science, Beijing Normal University, Beijing, China JIANKAI ZHANG AND YUANYUAN HAN Key Laboratory for Semi-Arid Climate Change of the Ministry of Education, College of Atmospheric Science, Lanzhou University, Lanzhou, China (Manuscript received 17 July 2017, in final form 12 March 2018) ABSTRACT 1. Introduction Planetary wave activity in the extratropical strato- sphere plays an important role in the dynamical coupling between the troposphere and stratosphere (e.g., Kuroda and Kodera 1999; Perlwitz and Graf 2001; Kushner and Polvani 2004; Perlwitz and Harnik 2004). Previous studies have found that stratospheric wave activity has increased in the early winter but decreased significantly in the late winter during past decades (e.g., Newman and Nash 2000; Randel et al. 2002; Hu and Tung 2003). As wave activity in the extratropical stratosphere is a precursor to strato- spheric events, the stratospheric polar vortex and Brewer–Dobson circulation (BDC) evolve with the wave activity (Polvani and Waugh 2004; Butchart et al. 2006; Garcia and Randel 2008; Garfinkel et al. 2015). The changing Arctic polar vortex and BDC can, in turn, in- fluence the tropospheric weather and climate across a wide range of time scales (Thompson and Wallace 2001; Karpechko and Manzini 2012; Xie et al. 2016; Zhang et al. 2016). It is, therefore, important to understand the factors that control the variability of the wave activity in the northern extratropical stratosphere. The stratospheric planetary wave activity shows pro- nounced interannual variability in winter in the Northern Hemisphere. It is well known that the interannual vari- ability of the extratropical stratosphere is related to tropical variability, such as the quasi-biennial oscillation (QBO; Holton and Tan 1980, 1982; Garfinkel et al. 2012a; Lu et al. 2014) and El Niño–Southern Oscillation (ENSO; Calvo Fernández et al. 2004, 2009; Manzini et al. 2006; Camp and Tung 2007; Garfinkel and Hartmann 2008; Cagnazzo and Manzini 2009; Cagnazzo et al. 2009; Ren et al. 2012; Xie et al. 2012; Zhang et al. 2015a,b). Strato- spheric planetary waves originate predominately in the troposphere, and their variations are caused by variability in two main factors: wave propagation from the tropo- sphere into the stratosphere, and tropospheric wave ac- tivity intensity. Wave propagation from the troposphere into the stratosphere can be affected by the QBO. The QBO affects extratropical wave propagation in two ways: the first is the stratospheric waveguide change due to the modulation by the QBO of the latitudinal location of the zero-wind line (i.e., the critical line for stationary waves) in the subtropics (Holton and Tan 1980, 1982), and the other is changes to planetary wave propagation and breaking caused by the effect of the QBO-induced meridional circulation on the refractive index (e.g., Garfinkel et al. 2012a; Lu et al. 2014). Variations in the intensity of tropospheric wave activity are mainly driven by tropical processes, for example, by ENSO. Warm ENSO events induce a deepening of the winter Aleutian low via the Pacific–North American (PNA) pattern, leading to an increase in wavenumber-1 eddies and a weakened vortex. However, the extratropical atmo- spheric circulation is not only influenced by ENSO and QBO, but also the Madden–Julian oscillation (MJO). The MJO is the dominant mode of intraseasonal variability in the tropical atmosphere (Madden and Julian 1971, 1972, 1994). A typical MJO event begins with a convective disturbance over the far equatorial western Indian Ocean and then intensifies and propa- gates eastward slowly (;5m s21) to the equatorial cen- tral Pacific Ocean. An MJO can be divided into eight phases as the convection center propagates. Previous studies have shown that these intraseasonal anomalies of moist deep convection in the tropics influence the teleconnection patterns over the middle and high lati- tudes, such as the PNA pattern (e.g., Matthews et al. 2004; Mori and Watanabe 2008; Johnson and Feldstein 2010), the North Atlantic Oscillation (NAO; e.g., Cassou 2008; Lin et al. 2009), and the Arctic Oscillation (AO; e.g., Zhou and Miller 2005; L’Heureux and Higgins 2008). Moreover, Lin et al. (2015) have also demonstrated that the seasonal mean convective activity related to MJO phases 3–5 is a possible driver of the seasonal mean NAO variability in boreal winter. Ac- cording to the theoretical studies of Matsuno (1966) and Gill (1980), the coherence between tropical and extra- tropical responses triggered by MJO-related convection is a consequence of Rossby wave trains that extend eastward and poleward across the middle and high lat- itudes. A model-based study by Seo and Son (2012) suggested that the anomalous tropical heating related to MJO phase 3 results in a Rossby wave train traveling north from the tropics into the northern Pacific and North America, and then turning south toward the equatorial African continent. The spatial structure of such a Rossby wave train is similar to that of the PNA pattern. Furthermore, Yoo et al. (2011, 2012) showed that the surface air temperature in the Arctic is also linked to the tropical MJO through the poleward propagation of wave trains. In addition, the ozone transport between the upper troposphere and lower stratosphere over the northern extratropics and Arctic is also affected by the MJO-related teleconnection (Li et al. 2013). The above studies illustrate that the MJO is able to influence wave activity in the northern extra- tropical troposphere. However, the connection between the MJO and wave activity in the northern extratropical stratosphere has received relatively little attention. Newman and Sardeshmukh (2008) have shown a link between tropical diabatic heating on intraseasonal time scales and the polar vortex. Garfinkel et al. (2012b) found a clear correlation between wave activity in the extratropical stratosphere and the MJO. They suggested that Northern Hemisphere sudden stratospheric warm- ing (SSW) events tend to follow certain MJO phases with a delay of a few days. Liu et al. (2014) investigated the connection between the equatorial MJO and dif- ferent types of the Northern Hemisphere midwinter major SSWs. Subsequently, Garfinkel et al. (2014) pointed out more clearly that MJO phase 7, in which convective anomalies propagate into the tropical central Pacific, leads to a North Pacific low, more heat flux in the troposphere, and a weakened vortex, whereas MJO phase 3 leads to the opposite effects. More recently, Schwartz and Garfinkel (2017) found that slightly more than half of SSW events follow MJO phases 6 and 7. However, these studies focused solely on the relation- ship between these two processes over intraseasonal time scales. The question that arises here is the follow- ing: Can the MJO influence the interannual variability of Northern Hemisphere stratospheric wave activity? Even though the MJO operates over intraseasonal time scales, the occurrence frequency of the individual phases of the MJO actually shows year-to-year variability. FIG. 1. Composite OLR (Wm22) anomalies during the eight MJO phases in boreal winter (NDJF). Daily OLR data for the period 1979–2013 were obtained from the CDC (NOAA) and the MJO phases were defined using the real-time multivariate MJO (RMM) index. Only days with MJO amplitude greater than 1.0 were used. OLR anomalies were calculated by removing the daily seasonal cycle and then applying a 100-day high-pass digital filter to the daily time series. The purpose of this paper is to investigate whether the in- terannual variability of the occurrence frequency of the individual phases of the MJO can significantly affect Northern Hemisphere stratospheric wave activity. The remainder of this paper is organized as follows. Section 2 introduces the datasets, methods, and model. Section 3 demonstrates the statistical relationship be- tween the interannual variability of the occurrence fre- quency of the individual phases of the MJO and wave activity in the northern extratropical stratosphere, and the associated mechanism is analyzed in section 4. Fi- nally, we present our conclusions in section 5. 2. Data, methods, and model Interpolated (2.58 longitude 3 2.58 latitude) daily mean outgoing longwave radiation (OLR) data from 1979 to 2013 were obtained from the Climate Diagnostic Center (CDC) of the National Oceanic and Atmospheric Ad- ministration (NOAA). Note that the analysis in this study is limited to the period 1979–2013, corresponding to the availability of the OLR data. The OLR can serve as a proxy for deep convection in the tropics, with lower OLR values corresponding to enhanced convective activity. To identify MJO events, the daily multivariate MJO index (Wheeler and Hendon 2004), which characterizes the state of the MJO in terms of its amplitude and phase, was obtained from the Australian Bureau of Meteorology (online at http://www.bom.gov.au/climate/mjo/). This MJO index consists of the principal components of the leading combined empirical orthogonal functions (EOFs) of the 200- and 850-hPa zonal wind and OLR averaged over the latitude band between 158S and 158N. Based on the index, an MJO cycle (typically ;40– 60 days) is divided into eight phases. The MJO is con- sidered as being active when the amplitude of the MJO index exceeds 1.0. Figure 1 shows the composited OLR anomalies during the eight MJO phases. The original OLR MJO index (OOMI), which is obtained online from the NOAA/Earth System Research Laboratory (online at https://www.esrl.noaa.gov/psd/mjo/mjoindex/oomi.1x.txt), was also used to verify the results in this study. The meteorological fields analyzed in this study were obtained from the National Centers for Environmental Prediction (NCEP)–National Center for Atmospheric Research(NCAR) reanalysis dataset (Kalnay et al. 1996). The dataset contains daily averages of variables on a 2.58 3 2.58 grid at 17 vertical pressure levels ex- tending from 1000 to 10 hPa, with 6 levels in the strato- sphere (100, 70, 50, 30, 20, and 10 hPa). The reanalysis data from the European Centre for Medium-Range Weather Forecasts (ECMWF) interim reanalysis (ERA- Interim; Dee et al. 2011) were also used to verify the results in this study, and were obtained as daily mean fields at 37 discrete pressure levels, on a 18 3 18 horizontal grid. We used wave activity analysis to investigate the en- ergy propagation of stationary Rossby waves. The wave activity flux is parallel to the group velocity of stationary Rossby waves, making it a useful indicator for identify- ing the propagation direction and source of stationary atmospheric Rossby waves. To assess the influence of MJO-related processes on wave activity in the extra- tropical stratosphere, the quasigeostrophic version of the E–P flux divergence was calculated using the NCEP– NCAR daily fields based on the original definition as follows (Edmon et al. 1980) Here p is the pressure; a is the radius of Earth; u is the latitude; f is the Coriolis parameter; u is the potential temperature; and u and y are the zonal and meridional components of the wind, respectively. Eddy flux terms are computed from the zonal anomalies for each day. The E–P fluxes include the wave momentum flux and wave heat flux. The E–P flux divergence reflects the eddy forcing on the zonal mean flow, which can serve as a measure of the wave activity, and a negative (pos- itive) E–P flux divergence represents easterly (westerly) eddy forcing of the mean flow (Andrews et al. 1987). Rossby wave ray tracing was used to further analyze the trajectory of the stationary Rossby wave train and characterize the impact of the background flow on the propagation of wave energy. This theory-based tech- nique uses a curve that is locally tangential to the group velocity vector, and has been widely used to trace the Rossby wave responses to tropical heating anomalies (Hoskins and Karoly 1981; Hoskins and Ambrizzi 1993), and also in research into atmospheric teleconnection mechanisms (Xu et al. 2013; Sun et al. 2015, 2017; Wu et al. 2016; Zheng et al. 2016). The trajectory of a wave ray can be calculated numerically from the angle of the wave front propagation, which is determined from the ratio of the zonal and meridional group velocities. As the Rossby wave propagation trajectories are closely dependent on the basic state, the Rossby wave rays were calculated here for the seasonal climatological flow using the equations in Li and Li (2012), Li et al. (2015), and Zhao et al. (2015) to delineate the propagation behavior of wave energy associated with the MJO. We used the NCAR Community Earth System Model (CESM), version 1.0.6, which is a global climate model (Hurrell et al. 2013). In particular, our model experiments were carried out using version 4 of the Whole Atmosphere Community Climate Model (WACCM4). WACCM4 also incorporates the Community Atmospheric Model, version 4 (CAM4), and as such includes all of its physical parameterizations (Neale et al. 2013). This improved version of WACCM uses a coupled system of four components: atmosphere, ocean, land, and sea ice (Holland et al. 2012). WACCM4 has a finite volume dynamical core, with 66 vertical levels extending from the ground to 4.5 3 1026 hPa (;145-km geometric altitude), and a vertical resolution of 1.1–1.4 km in the tropical tropopause layer and the lower stratosphere (below a height of 30 km). The simulations presented in this paper are performed at a horizontal resolution of 1.98 3 2.58, and include interactive chemistry (Garcia et al. 2007). More details about WACCM4 are available in Marsh et al. (2013). In this study, all relevant daily data of 34-yr boreal winter [November–February (NDJF)] from 1979 to 2013 were analyzed. For all the fields except for the MJO index, the seasonal cycle was removed, and then a 100- day high-pass digital filter was performed on the daily time series. Then, the seasonal means are constructed by averaging variables over NDJF, resulting in 34 winter fields. Note that an area average over the region 408–908N and 10–100 hPa was applied to the filtered E–P flux di- vergence for calculating the time series of wave activity in the extratropical stratosphere. The high-pass filter was chosen to retain atmospheric variations in intra- seasonal time scale and exclude other factors (e.g., QBO or ENSO) that may contaminate the connection be- tween MJO and wave activity in the extratropical stratosphere. After applying the high-pass filter to the E–P flux divergence time series, the standard deviation of the filtered seasonal mean time series was reduced, but the reduction was no more than half of the standard deviation of the unfiltered time series. 3. The correlation between MJO occurrence frequency and wave activity in the northern extratropical stratosphere Figure 2 shows the time series of the occurrence fre- quency of the eight MJO phases, and the E–P flux divergence anomalies in the northern extratropical stratosphere, during winter (NDJF) from NCEP–NCAR reanalysis data. The occurrence frequency of the in- dividual MJO phases was calculated by summing the occurrence days of each phase during winter in each year. Only the days with MJO amplitude greater than FIG. 2. Time series of the occurrence frequency of the eight MJO phases (day; red lines) and northern extratropical stratospheric E–P flux divergence anomalies (m s21 day21; blue lines) during winter (NDJF) from 1979 to 2013 based on NCEP–NCAR reanalysis data. The occurrence frequency of the individual MJO phases was calculated by summing the occurrence days of each phase during winter in each year. Only days with MJO amplitude greater than 1.0 were included. The time series of E–P flux divergence anomalies was obtained by applying a spatial average over the region 408–908N and 10–100 hPa and a time average in winter, after removing the seasonal cycle and then applying a 100-day high-pass filter to the time series. The correlation coefficient between these two linearly detrended time series is shown in the top-right corner of each panel. One (two) asterisk(s) indicate that the correlation coefficient is significant at the 90% (95%) confidence level based on the Student’s t test. 1.0 were included. A 100-day high-pass filter was ap- plied to the E–P flux divergence anomalies before performing the spatial and time average. After the 100- day high-pass filtering, the variations in the averaged E–P flux divergence anomalies highlight the in- terannual variations in the high-frequency northern extratropical stratospheric wave activity in winter. We found that strong in-phase variability exists between the occurrence frequency of MJO phase 4 and the northern extratropical stratospheric E–P flux di- vergence anomalies, with a correlation coefficient of 0.55 that is significant at the 95% confidence level (Fig. 2g). Note that the occurrence frequency of MJO phase 7 has a relatively large anticorrelation with the E–P flux divergence anomalies (R 5 20.31, significant at the 90% confidence level, Fig. 2f). These results suggest that an increase (decrease) in the occurrence frequency of MJO phase 4 during the boreal winter corresponds to weaker (stronger) wave activity in the northern extratropical stratosphere, and vice versa for MJO phase 7. Note that we also used different MJO amplitude threshold values (1.25 and 1.5) and different periods of high-pass filtering (120 and 80 days) to test the robustness of the link between the occurrence TABLE 1. The left column is the correlation coefficients between the frequency of occurrence of the individual MJO phases and E–P flux divergence anomalies in the extratropical stratosphere for an MJO amplitude threshold of 1.25 and 1.5 and the right column for filtering period of 120 and 80 days. When calculating the correlation coefficients for the MJO amplitude threshold of 1.25 and 1.5, the 100-day high-pass filtering is used. When calculating the correlation coefficients for filtering period of 120 and 80 days, the MJO amplitude threshold value 1.0 is used. The E–P flux divergence is calculated from the NCEP–NCAR reanalysis data. One and two asterisks indicate that the correlation coefficient is significant at the 90% and 95% confidence levels, respectively, based on the Student’s t test. frequency of MJO phases and the northern strato- spheric wave activity found in our study. Table 1 lists the correlation coefficients between the frequency of occurrence of the individual MJO phase and E–P flux divergence anomalies under the choice of different MJO amplitude threshold values and different periods of high-pass filtering. It is apparent that the results are not sensitive to the selections of the MJO amplitude threshold value and the reasonable change in the pe- riod of the filtering. The corresponding results from the ERA-Interim data are well consistent with the corre- lations between the occurrence frequencies of MJO phases 4 and 7 and the northern extratropical strato- spheric wave activity anomalies (Table 2). In addition, previous studies have suggested that some factors, such as QBO, ENSO, North Pacific SST, sub- polar snow cover, or sea ice, can affect northern extra- tropical stratospheric wave activity (e.g., Cohen and Jones 2011; Garfinkel et al. 2012a; Hurwitz et al. 2012; Kim et al. 2014; Chen et al. 2016). The link between the occurrence frequency of MJO phases and high- frequency variability of stratospheric wave activity may be affected by these factors. However, the 100-day high- pass filter performed on the time series in this study eliminate the effects of the signals with time scales lon- ger than 100 days. Meanwhile, we found that this link between the occurrence frequency of MJO phases and variability of wave activity in the northern stratosphere is stable when a multiple linear regression was further applied to remove the effects of the abovementioned TABLE 2. As in Table 1, but for E–P flux divergence from ERA-Interim data. factors on the filtered wave activity of the extratropical stratosphere (not shown). Figure 1 shows that the tropical convection is strength- ened over the Maritime Continent but suppressed over the central Pacific during MJO phase 4. In contrast, the pattern of convection anomalies during MJO phase 7 is approximately opposite to that associated with MJO phase 4. This explains why the occurrence frequencies of MJO phases 4 and 7 are oppositely correlated with the northern extratropical stratospheric wave activity anomalies (Figs. 2g,f). Using the data plotted in Fig. 1, the OLR anomalies spatially averaged over the Maritime Continent (158S–58N, 908–1508E) and equatorial central Pacific (158S–58N, 1608E–1508W) during the eight MJO phases are shown in Fig. 3; the convection anomalies at the center of the OLR anomalies during MJO phase 4 are more intense than those that develop during MJO phase 7. Under these circumstances, we can expect that the correlation coefficient between the occurrence frequency of MJO phase 4 and the E–P flux divergence anomalies (Fig. 2g) is more significant than that between MJO phase 7 and the E–P flux divergence anomalies (Fig. 2f). Note that the intensities of convection anomalies at the center of the OLR anomalies FIG. 3. Composite OLR (Wm22) anomalies spatially averaged over the Maritime Continent (158S–58N, 908–1508E) and equatorial central Pacific (158S–58N, 1608E–1508W) during the eight MJO phases in winter (NDJF). Only days when the MJO amplitude was greater than 1.0 were used. OLR anomalies were calculated by removing the seasonal cycle and then applying a 100-day high-pass digital filter to the daily time series. during MJO phases 5 and 8 are also as large as those during MJO phases 4 and 7, respectively. The difference between the influences of the other MJO phases and these two special phases on wave activity will be discussed in the next section. Garfinkel et al. (2014) showed that the enhanced convection in the tropical central Pacific associated with MJO phase 7 leads to a weakened Arctic polar vortex. Our study shows a similar result, with the increased occurrence frequency of MJO phase 7 corresponding to stronger wave activity in the northern extratropical stratosphere. They also showed that the weakened wave activity corresponds to suppressed convection in the central Pacific related to MJO phase 3; however, we have shown that the increased occurrence frequency of MJO phase 4 is related to weaker wave activity in the extratropical stratosphere. These differences may be caused by the different time scales considered by these two studies. Garfinkel et al. (2014) focused on the effect of one MJO phase on extratropical circulation with an intraseasonal time scale, whereas this study investigates the variability on interannual time scales of the link between the occurrence frequency of MJO phase and the northern stratospheric wave activity. The correlation coefficients between the E–P flux divergence anomalies and the occurrence frequency of MJO phases 1, 2, 3, 5, 6, and 8 are small and not significant (Figs. 2a–e,h), suggesting that the connection at interannual time scales between the occurrence frequencies of these MJO phases and wave activity is weak. A transient experiment (E1) is performed with WACCM4 to further confirm the above correlation, using natural and anthropogenic external forcings, including spectrally resolved solar variability (Lean et al. 2005), time varying greenhouse gases (GHGs) (from scenario A1B of IPCC 2001), volcanic aerosols [from the Stratospheric Processes and their Role in Climate (SPARC) Chemistry Climate Model Validation (CCMVal) REF-B2 scenario recommendations], and a nudged QBO (the time series in CESM is determined from the observed climatology over the period 1955– 2005). E1 is a historical simulation integrated over the period 1955–2005. All the forcing data used in this study are available from the CESM model input data repository. Note that previous studies have pointed out that the simulated MJO strength in WACCM is underestimated (Inness et al. 2003; Zhang et al. 2006; Subramanian et al. 2011; Liu et al. 2015; Yang et al. 2017; Kang and Tziperman 2017). However, these studies also pointed out that the CAM-based WACCM, like most atmospheric general circulation models, can reproduce the eastward propagating intraseasonal zonal winds and OLR in the tropical Indian and Pacific Oceans and the responses to MJO in the troposphere and stratosphere. These previous studies indicated that WACCM has the ability to simulate MJO activity, but the simulated intensity of MJO activity is relative weaker than the observation. The results show that the correlation between the observed occurrence frequencies of the eight MJO phases and the northern extratropical stratospheric wave activity anomalies (Fig. 2) is well simulated by the WACCM4 model (Fig. 4) for the period 1960–2005 (the first 5 years are for the spinup period). That is, the northern extratropical stratospheric E–P flux divergence anomalies have the strongest correlation with the occurrence frequency of MJO phase 4 (Fig. 4g) and are anticorrelated with MJO phase 7 (Fig. 4f). To obtain more evidence of the potential influence of MJO phases 4 and 7 on the northern winter extra tropical stratosphere, we defined MJO phases 4 and 7 occurrence frequency indices (Fig. 5). These indices were calculated by removing the mean from the occurrence frequency time series of the MJO phases 4 and 7 in winter (the red lines in Figs. 2g,f). A positive value of the index indicates a high frequency year for MJO phase 4 or 7, whereas a negative value indicates a low frequency year. Figure 5 shows the interannual variability of the indices of MJO phases 4 and 7. It is interesting that MJO phase 4 has a low occurrence frequency in the 1980s but a high occurrence frequency in the 1990s, while the variation of MJO phase 7 is generally opposite to that of MJO phase 4. These two time series show a negative correlation (R 5 20.41, significant at the 95% confidence level). This interesting phenomenon deserves future investigation. The composite anomalies of the E–P flux, E–P flux divergence, and zonal mean temperature from the NCEP–NCAR reanalysis data for high and low occurrence frequency years of MJO phases 4 and 7 during winter for the period 1979–2013 in the northern extratropical stratosphere are shown in Fig. 6. In winters with a high occurrence frequency of MJO phase 4 (Fig. 6a), the weakened upward wave fluxes and stronger E–P flux divergence anomalies in the middle and upper stratosphere at middle and high latitudes imply depressed wave activity, and this is accompanied by negative temperature anomalies in the same region. Conversely, in winters with a low occurrence frequency of MJO phase 4 (Fig. 6b), the enhanced upward wave fluxes and stronger E–P flux convergence imply enhanced wave activity and positive temperature anomalies. As expected, the changes in upward wave fluxes, E–P flux divergence, and zonal-mean temperature in the middle and upper stratosphere at middle and high latitudes associated with MJO phase 7 are just the opposite FIG. 4. As in Fig. 2, but from the WACCM4 experiment (E1). of those associated with MJO phase 4 (Figs. 6c,d). Note that the modulation of the E–P flux during MJO phase 4 is more noticeable than that during MJO phase 7, which is consistent with the correlation analysis (Figs. 2g,f). 4. Mechanism by which the MJO affects the northern extratropical stratosphere It has been demonstrated that tropical forcing can influence the stratospheric polar vortex by modulating the PNA teleconnection pattern in the Northern Hemisphere (Garfinkel and Hartmann 2008; Xie et al. 2012). Subsequently, the wave trains in the upper troposphere can enhance planetary wave propagation into the subpolar stratosphere, which weakens the stratospheric polar vortex. Further investigation is required as to whether the anomalies in the stratospheric circulation and temperature associated with high and low occurrence-frequency years of MJO phases 4 and 7 are also tied to teleconnection pattern and corresponding wave activity in the upper troposphere. To illuminate the connection between tropical MJO phases 4 and 7 and northern extratropical stratospheric wave activity, Fig. 7 shows the geopotential height anomalies at 200 hPa during the high and low occurrence frequency years of MJO phases 4 and 7. The geopotential height anomalies were again calculated by removing the seasonal cycle and then applying a 100 day high pass digital filter to the daily data. As suggested by Seo and Son (2012), anomalous tropical heating related to the MJO results in the Rossby wave train traveling north from the forcing region to the northern Pacific and North America, then turning south toward the equatorial African continent. Figures 7a and 7b show opposite pattern of geopotential height anomalies in the high and low occurrence frequency years of MJO phase 4. This pattern resembles a Rossby wave train traveling north toward the northern Pacific and North America, and FIG. 5. MJO phases (a) 4 and (b) 7 occurrence-frequency indices. The indices were calculated by removing the mean from the occurrence-frequency time series of the MJO phases 4 and 7 in winter (please refer to Fig. 2). then turning south toward the African continent. The spatial structure of the Rossby wave train is similar to that in the PNA pattern. Figures 7c and 7d are the same as in Figs. 7a and 7b, but for MJO phase 7. The pattern of geopotential height anomalies in years with high and low occurrence frequency of MJO phase 7 is generally opposite to that of MJO phase 4. As the intensity of the convection anomalies at the center of the OLR anomalies during MJO phase 4 is larger than during MJO phase 7 (Fig. 3), the geopotential height anomalies in MJO phase 7 (Figs. 7c,d) are smaller than in MJO phase 4 (Figs. 7a,b). To examine the horizontal structures of planetary wave anomalies, Fig. 7 also shows the climatological stationary waves of wavenumber 1 accompanied by the geopotential height anomalies. There is a positive (negative) anomaly superimposed on the Aleutian low over the northern Pacific during the high (low) occurrence frequency years of MJO phase 4. This would lead to a weakened (strengthened) tropospheric wave forcing of wavenumber 1. The teleconnections and poleward traveling of Rossby wave in the upper troposphere can, in turn, alter planetary wave propagation into the subpolar stratosphere, where the waves dissipate, decelerating the stratospheric polar vortex (Garfinkel and Hartmann 2008). Combining Figs. 6 and 7, the mechanism by which the MJO affects the northern extratropical stratospheric planetary wave can be summarized as follows: the propagation of tropical Rossby waves to middle and high latitudes is triggered in both the high and low occurrence-frequency years of MJO phase 4. FIG. 6. Latitude–height cross sections of composite E–P flux (vectors; horizontal term: 107m3 s22 and vertical term: 105 Pam2 s22), E–P flux divergence (shaded; ms21 day21), and zonal-mean temperature (contours; K) anomalies during (a),(c) high- and (b),(d) lowoccurrence- frequency years ofMJO phases (a),(b) 4 and (c),(d) 7. Solid contours are positive, dashed contours are negative, and zero contours are thickened. Contour interval for the zonal-mean temperature anomalies is 0.02 K. FIG. 7. Geopotential height (contours; gpm) anomalies during the (a),(c) high- and (b),(d) low-occurrencefrequency years of MJO phases (a),(b) 4 and (c),(d) 7 at 200 hPa associated with winter-averaged stationary waves of wavenumber 1 (shaded) from the NCEP–NCAR reanalysis data. The geopotential height anomalies were also calculated by removing the daily seasonal cycle and then applying a 100-day high-pass digital filter to the daily data. Solid contours are positive, and dashed contours are negative. Contour interval for the geopotential height anomalies is 0.6 gpm. However, the anomalous waves during the high (low) occurrence frequency years of MJO phase 4 (Figs. 7a,b, the geopotential height anomalies) are out of phase (in phase) with the climatological stationary wavenumber 1 in the Northern Hemisphere. This wave interference leads to weakened (strengthened) planetary waves along the polar wave guide during the high (low) frequency periods of MJO phase 4. Under this condition, it is expected that fewer (more) planetary waves propagate vertically into the stratosphere when the occurrence frequency of the MJO phase 4 is high (low). This explains why there is less (more) E–P flux and positive (negative) E–P flux divergence anomalies during the years with high (low) occurrence frequency of MJO phase 4 (Figs. 6a,b). The above processes are reversed for MJO phase 7. We now use Rossby wave ray tracing to further trace the trajectory of the Rossby wave trains described above. The ray paths of waves with wavenumbers 1–3 at 200 hPa generated by the perturbed circulation over the region 208S–208N, 708–1508E in winter are shown in Fig. 8. The wave ray paths represent the climate teleconnections (i.e., the propagation of stationary waves in realistic flows). The method for calculating the wave ray paths and application of the barotropic model are described in detail by Li et al. (2015) and Zhao et al. (2015). We found that some planetary waves generated by the perturbed circulation over this region, where there are strong convection anomalies in MJO phase 4, travel north to the northern Pacific and North America, and then turn south toward the African continent. This suggests the possibility that Rossby waves generated by a convection anomaly in the tropics related to MJO phase 4 may travel along the ray trajectories to the Northern Hemisphere middle and high latitudes. Thus, it can be expected that some wave ray paths are in agreement with the composite patterns of the geopotential height anomalies in Fig. 7. Figure 9 shows the corresponding vertical structures of the MJO-induced planetary waves that propagate into the stratosphere. The geopotential height anomalies in the northern extratropics during the high (low) occurrence-frequency years of MJO phase 4 generally tilt to the west with height and are in the opposite (same) phase as the climatological wavenumber 1, and this generates destructive (constructive) interference between the MJO induced waves and the background stationary waves (Figs. 9a,b). Thus, the weakened (enhanced) wave activity in the northern extratropical stratosphere corresponding to the high (low) occurrence frequency of MJO phase 4 can be expected. This result is associated with anomalous upward wave flux and E–P flux divergence at middle and high latitudes in the Northern Hemisphere middle and upper stratosphere and accompanied by the temperature anomalies during FIG. 8. Ray paths (green lines) at 200 hPa in winter. Black crosses denote wave sources in the region 208S–208N, 708–1508E. Rays with wavenumbers 1–3 are shown. The shading indicates meridional gradient of quasigeostrophic potential vorticity (K kg21ms21). FIG. 9. Longitude–height cross sections of the spatially averaged (458–758N) geopotential height anomalies (contours; gpm) during (a),(c) high- and (b),(d) low-occurrence-frequency years of MJO phases (a),(b) 4 and (c), (d) 7 with winter-averaged stationary waves of wave number 1 (shaded) from the NCEP–NCAR reanalysis data. Solid contours are positive, dashed contours are negative, and zero contours are thickened. Contour interval for the winter seen in Figs. 6a and 6b. MJO phase 7 has the opposite effect on the background wave number 1 to MJO phase 4 (Figs. 9c,d). We also examined the responses of wave numbers 2 and 3 to high (low) occurrence frequency of MJO phases 4 and 7 (Fig. 10). The anomalous waves of wave number 2 in geopotential height anomalies during the high and low occurrence frequency years of MJO phase 4 do not overall superpose on the climatological stationary waves of wave number 2 in the vertical in the northern extratropics. There is an overall in-phase superposition between the anomalous wave number 3 waves and climatological stationary waves of wave number 3; however, the phases are opposite to those of wave number 1 during the high and low occurrence frequency years of MJO phase 4. In MJO phase 7, the anomalous waves of wave number 2 and 3 in geopotential height anomalies during the high and low occurrence frequency years do not overall superpose on climatological stationary waves of wave number 2 and 3 in the vertical in the northern extratropics. Therefore, only the wave number 1 responses to MJO in geopotential height anomalies are able to explain the stratospheric wave activity anomalies during the high and low occurrence frequency years of MJO phases 4 and 7. At this stage, a question is raised: Why do only the occurrence frequencies of MJO phase 4 and MJO phase 7 have a significant influence on wave activity in the northern extratropical stratosphere. Here, we further examine the effects of other MJO phases on wave activity in the extratropics. Figure 11 shows the geopotential height anomalies at 200 hPa during the high and low occurrence frequency years of MJO phases 1, 2, 3, 5, 6, and 8. It is apparent that the PNA like wave trains that propagate poleward to middle and high latitudes also develop during the high and low occurrence frequency years of these MJO phases. However, it can be seen from Fig. 12 that the geopotential height anomalies during the high and low occurrence frequency years of MJO phases 1, 2, 3, 5, 6, and 8 do not overall superpose on stationary waves of wave number 1 in the vertical in the extratropics. This illustrates that anomalous waves caused by these MJO phases do not efficiently interfere with the stationary waves of wave number 1; consequently, the occurrence frequency of MJO phases 1, 2, 3, 5, 6, and 8 have no significant influence on the wave activity of wave number 1 in the northern extratropical stratosphere. Figure 13 shows the wave number 2 geopotential height anomalies during the high and low occurrence frequencies of MJO phases 1, 2, 3, 5, 6, and 8. FIG. 10. As in Fig. 9, but for the wave numbers (a),(b),(e),(f) 2 and (c),(d),(g),(h) 3. The anomalous waves of wave number 2 during the high and low occurrence frequency years of MJO phases 2, 3, 5, and 6 do not overall superpose on the climatological stationary waves of wave number 2 in the northern extratropics; nonetheless, the influence of MJO phases 1 and 8 on wave number 2 cannot be neglected. This suggests that wave number 2 can be weakened (strengthened) in the high (low) occurrence frequency years of MJO phases 1 and 8. Figure 14 is the same as Fig. 13, but for wave number 3. The anomalous wave number 3 geopotential height anomalies during the high and low occurrence frequency years of MJO phases 1, 2, 6, and 8 do not overall superpose on the climatological stationary waves of wave number 3 in the northern extratropics. However, the geopotential height anomalies in the northern extratropics during the high (low) occurrence frequency years of MJO phase 3 are in the opposite (same) phase as the background wave number 3, weakening (enhancing) the strength of wave number 3. MJO phase 5 has the opposite effect on the background wave number 3 to MJO phase 3. Figures 13 and 14 suggest that the impacts of wave numbers 2 and 3 on interannual variations in stratospheric FIG. 11. As in Fig. 7, but for (top)–(bottom) MJO phases 1, 2, 3, 5, 6, and 8. wave activity can be triggered by MJO phases 1, 3, 5, or 8. However, the correlation coefficients between the E–P flux divergence anomalies and the occurrence frequency of MJO phases 1, 3, 5, and 8 are small and not significant (Fig. 2). In addition, previous studies have recognized that wave number 1 disturbances caused by the MJO are the dominant waves that propagate into the winter stratosphere and subsequently weaken the polar vortex (Garfinkel et al. 2012b, 2014). Thus, the wave number 1 responses to MJO phases can explain a large part of the variability in the stratospheric wave activity during the high and low occurrence frequency years of eight MJO phases. 5. Conclusions The effect of the MJO on wave activity in the extratropical stratosphere has been reported in several previous studies (Garfinkel et al. 2012b, 2014). However, these studies focused mainly on the relationship between these two processes over intraseasonal time scales. The present study has investigated the relationship between the occurrence frequency of the individual phases of the MJO and the interannual variability of stratospheric high frequency wave activity in Northern Hemisphere middle and high latitudes during winter over the period 1979–2013. We have found a significant positive correlation between the occurrence frequency of MJO phase 4 and E–P flux divergence anomalies; that is, higher (lower) occurrence frequency of MJO phase 4 corresponds to weaker (stronger) upward wave fluxes and increased (decreased) E–P flux divergence anomalies at middle and high latitudes in the middle and upper stratosphere. This implies depressed (enhanced) wave activity accompanied by a stronger (weaker) polar vortex in this region. During MJO phase 4, an anomalous PNA-like Rossby wave train is generated that travels north to the middle and high latitudes in the northern extratropical troposphere, and the geopotential height anomalies in FIG. 12. As in Fig. 9, but for (top)–(bottom) MJO phases 1, 2, 3, 5, 6, and 8. FIG. 13. As in Fig. 12, but for wavenumber 2. FIG. 14. As in Fig. 12, but for wavenumber 3. the high (low) occurrence frequency years of MJO phase 4 are of the opposite (same) phase as the background wave number 1 in the vertical in the northern extratropics. The pattern of convection anomaly during MJO phase 7 is approximately opposite to that during MJO phase 4, consequently, the responses of wave number 1 to high and low occurrence frequencies of MJO phase 7 are opposite to that of MJO phase 4. As MJO phase 7 has weaker convection anomalies, the effect of MJO phase 7 on the wave activity in the northern extratropical stratosphere is weaker than that of MJO phase 4. The anomalous waves in the geopotential height field caused by MJO phases 1, 2, 3, 5, 6, and 8 are not overally superposed on the climatological waves of wave number 1, and the wave interference between them is inefficient. Therefore, MJO phases 1, 2, 3, 5, 6, and 8 have no significant influence on the wave activity in the northern extratropical stratosphere. Acknowledgments. This work was supported by the National Science Foundation of China (41630421, 41575038, and 41575039). We thank the Australian Bureau of Meteorology for providing the MJO index, and NOAA for providing the OLR data and OOMI. We would also thank the NCEP–NCAR and ECMWF for providing the reanalysis data. We thank the three anonymous reviewers for their helpful comments, which significantly improved the quality of the paper. REFERENCES Andrews, D. G., J. R. Holton, and C. B. Leovy, 1987: Middle At- mosphere Dynamics. Academic Press, 489 pp. Butchart, N., and Coauthors, 2006: Simulations of anthropogenic change in the strength of the Brewer–Dobson circulation. Climate Dyn., 27, 727–741, https://doi.org/10.1007/s00382-006-0162-4. Cagnazzo, C., and E. Manzini, 2009: Impact of the stratosphere on the winter tropospheric teleconnections between ENSO and the North Atlantic and European region. J. Climate, 22, 1223– 1238, https://doi.org/10.1175/2008JCLI2549.1. ——, and Coauthors, 2009: Northern winter stratospheric tem- perature and ozone responses to ENSO inferred from an en- semble of chemistry climate models. Atmos. Chem. Phys., 9, 8935–8948, https://doi.org/10.5194/acp-9-8935-2009. Calvo Fernández, N., R. R. García, R. García Herrera, D. Gallego Puyol, L. Gimeno Presa, E. Hernández Martín, and P. Ribera Rodríguez, 2004: Analysis of the ENSO signal in tropospheric and stratospheric temperatures observed by MSU, 1979–2000. J. Climate, 17, 3934–3946, https://doi.org/10.1175/1520- 0442(2004)017,3934:AOTESI.2.0.CO;2. ——, M. A. Giorgetta, R. G. Herrera, and E. Manzini, 2009: Nonlinearity of the combined warm ENSO and QBO effects on the Northern Hemisphere polar vortex in MAECHAM5 simulations. J. Geophys. Res., 114, D13109, https://doi.org/ 10.1029/2008JD011445. Camp, C. D., and K.-K. Tung, 2007: Stratospheric polar warming by ENSO in winter: A statistical study. Geophys. Res. Lett., 34, L04809, https://doi.org/10.1029/2006GL028521. Cassou, C., 2008: Intraseasonal interaction between the Madden– Julian oscillation and the North Atlantic Oscillation. Nature, 455, 523–527, https://doi.org/10.1038/nature07286. Chen, X., J. Ling, and C. Li, 2016: Evolution of the Madden–Julian oscillation in two types of El Niño. J. Climate, 29, 1919–1934, https://doi.org/10.1175/JCLI-D-15-0486.1. Cohen, J., and J. Jones, 2011: A new index for more accurate winter predictions. Geophys. Res. Lett., 38, L21701, https://doi.org/ 10.1029/2011GL049626. Dee, D. P., and Coauthors, 2011: The ERA-Interim reanalysis: Con- figuration and performance of the data assimilation system. Quart. J. Roy. Meteor. Soc., 137, 553–597, https://doi.org/10.1002/qj.828. Deng, L., T. Li, J. Liu, and M. Peng, 2016: Factors controlling the interannual variations of MJO intensity. J. Meteor. Res., 30, 328–340, https://doi.org/10.1007/s13351-016-5113-3. Edmon, H. J., Jr., B. J. Hoskins, and M. E. McIntyre, 1980: Eliassen– Palm cross sections for the troposphere. J. Atmos. Sci., 37, 2600– 2616, https://doi.org/10.1175/1520-0469(1980)037,2600: EPCSFT.2.0.CO;2. Garcia, R. R., and W. J. Randel, 2008: Acceleration of the Brewer– Dobson circulation due to increases in greenhouse gases. J. Atmos. Sci., 65, 2731–2739, https://doi.org/10.1175/2008JAS2712.1. ——, D. R. Marsh, D. E. Kinnison, B. A. Boville, and F. Sassi, 2007: Simulation of secular trends in the middle atmosphere, 1950– 2003. J. Geophys. Res., 112, D09301, https://doi.org/10.1029/ 2006JD007485. Garfinkel, C. I., and D. L. Hartmann, 2008: Different ENSO tele- connections and their effects on the stratospheric polar vortex. J. Geophys. Res., 113, D18114, https://doi.org/10.1029/ 2008JD009920. ——, T. A. Shaw, D. L. Hartmann, and D. W. Waugh, 2012a: Does the Holton–Tan mechanism explain how the quasi-biennial oscillation modulates the Arctic polar vortex? J. Atmos. Sci., 69, 1713–1733, https://doi.org/10.1175/JAS-D-11-0209.1. ——, S. B. Feldstein, D. W. Waugh, C. Yoo, and S. Lee, 2012b: Observed connection between stratospheric sudden warmings and the Madden–Julian oscillation. Geophys. Res. Lett., 39, L18807, https://doi.org/10.1029/2012GL053144. ——, J. J. Benedict, and E. D. Maloney, 2014: Impact of the MJO on the boreal winter extratropical circulation. Geophys. Res. Lett., 41, 6055–6062, https://doi.org/10.1002/2014GL061094. ——, M. M. Hurwitz, and L. D. Oman, 2015: Effect of recent sea surface temperature trends on the Arctic stratospheric vortex. J. Geophys. Res. Atmos., 120, 5404–5416, https://doi.org/10.1002/ 2015JD023284. Gill, A. E., 1980: Some simple solutions for heat-induced tropical circulation. Quart. J. Roy. Meteor. Soc., 106, 447–462, https:// doi.org/10.1002/qj.49710644905. Holland, M. M., D. A. Bailey, B. P. Briegleb, B. Light, and E. Hunke, 2012: Improved sea ice shortwave radiation physics in CCSM4: The impact of melt ponds and aerosols on Arctic sea ice. J. Climate, 25, 1413–1430, https://doi.org/10.1175/JCLI-D-11-00078.1. Holton, J. R., and H.-C. Tan, 1980: The influence of the equa- torial quasi-biennial oscillation on the global circulation at 50 mb. J. Atmos. Sci., 37, 2200–2208, https://doi.org/10.1175/ 1520-0469(1980)037,2200:TIOTEQ.2.0.CO;2. ——, and ——, 1982: The quasi-biennial oscillation in the Northern Hemisphere lower stratosphere. J. Meteor. Soc. Japan, 60, 140–148, https://doi.org/10.2151/jmsj1965.60.1_140. Hoskins, B. J., and D. J. Karoly, 1981: The steady linear response of a spherical atmosphere to thermal and orographic forc- ing. J. Atmos. Sci., 38, 1179–1196, https://doi.org/10.1175/ 1520-0469(1981)038,1179:TSLROA.2.0.CO;2. ——, and T. Ambrizzi, 1993: Rossby wave propagation on a realistic longitudinally varying flow. J. Atmos. Sci., 50, 1661–1671, https:// doi.org/10.1175/1520-0469(1993)050,1661:RWPOAR.2.0.CO;2. Hu, Y., and K. K. Tung, 2003: Possible ozone-induced long-term changes in planetary wave activity in late winter. J. Climate, 16, 3027–3038, https://doi.org/10.1175/1520-0442(2003)016,3027: POLCIP.2.0.CO;2. Hurrell, J. W., and Coauthors, 2013: The Community Earth Sys- tem Model: A framework for collaborative research. Bull. Amer. Meteor. Soc., 94, 1339–1360, https://doi.org/10.1175/ BAMS-D-12-00121.1. Hurwitz, M. M., P. A. Newman, and C. I. Garfinkel, 2012: On the influence of North Pacific sea surface temperature on the Arctic winter climate. J. Geophys. Res., 117, D19110, https:// doi.org/10.1029/2012JD017819. Inness, P. M., and J. M. Slingo, 2003: Simulation of the Madden– Julian oscillation in a coupled general circulation model. Part I: Comparison with observations and an atmosphere- only GCM. J. Climate, 16, 345–364, https://doi.org/10.1175/ 1520-0442(2003)016,0345:SOTMJO.2.0.CO;2. IPCC, 2001: Climate Change 2001: The Scientific Basis. Cambridge University Press, 881 pp. Johnson, N. C., and S. B. Feldstein, 2010: The continuum of North Pacific sea level pressure patterns: Intraseasonal, interannual, and interdecadal variability. J. Climate, 23, 851–867, https:// doi.org/10.1175/2009JCLI3099.1. Kalnay, E., and Coauthors, 1996: The NCEP/NCAR 40-Year Re- analysis Project. Bull. Amer. Meteor. Soc., 77, 437–471, https:// doi.org/10.1175/1520-0477(1996)077,0437:TNYRP.2.0.CO;2. Kang, W., and E. Tziperman, 2017: More frequent sudden strato- spheric warming events due to enhanced MJO forcing ex- pected in a warmer climate. J. Climate, 30, 8727–8743, https:// doi.org/10.1175/JCLI-D-17-0044.1. Karpechko, A. Y., and E. Manzini, 2012: Stratospheric influence on tropospheric climate change in the Northern Hemi- sphere. J. Geophys. Res., 117, D05133, https://doi.org/10.1029/ 2011JD017036. Kim, B.-M., S.-W. Son, S.-K. Min, J.-H. Jeong, S.-J. Kim, X. Zhang, T. Shim, and J.-H. Yoon, 2014: Weakening of the stratospheric polar vortex by Arctic sea-ice loss. Nat. Com- mun., 5, 4646, https://doi.org/10.1038/ncomms5646. Kuroda, Y., and K. Kodera, 1999: Role of planetary waves in the stratosphere–troposphere coupled variability in the Northern Hemisphere winter. Geophys. Res. Lett., 26, 2375–2378, https://doi.org/10.1029/1999GL900507. Kushner, P. J., and L. M. Polvani, 2004: Stratosphere– troposphere coupling in a relatively simple AGCM: The role of eddies. J. Climate, 17, 629–639, https://doi.org/10.1175/ 1520-0442(2004)017,0629:SCIARS.2.0.CO;2. Lean, J., G. Rottman, J. Harder, and G. Kopp, 2005: SORCE contributions to new understanding of global change and solar variability. Sol. Phys., 230, 27–53, https://doi.org/10.1007/ s11207-005-1527-2. L’Heureux, M. L., and R. W. Higgins, 2008: Boreal winter links be- tween the Madden–Julian oscillation and the Arctic Oscillation. J. Climate, 21, 3040–3050, https://doi.org/10.1175/2007JCLI1955.1. Li, K.-F., B. Tian, K. K. Tung, L. Kuai, J. R. Worden, Y. L. Yung, and B. L. Slawski, 2013: A link between tropical intraseasonal variability and Arctic stratospheric ozone. J. Geophys. Res. Atmos., 118, 4280–4289, https://doi.org/10.1002/jgrd.50391. Li, Y., and J. Li, 2012: Propagation of planetary waves in the hori- zontal non-uniform basic flow (in Chinese). Chin. J. Geophys., 55, 361–371. ——, ——, F. F. Jin, and S. Zhao, 2015: Interhemispheric propa- gation of stationary Rossby waves in a horizontally non- uniform background flow. J. Atmos. Sci., 72, 3233–3256, https://doi.org/10.1175/JAS-D-14-0239.1. Lin, H., G. Brunet, and J. Derome, 2009: An observed connection between the North Atlantic Oscillation and the Madden– Julian oscillation. J. Climate, 22, 364–380, https://doi.org/ 10.1175/2008JCLI2515.1. ——, ——, and B. Yu, 2015: Interannual variability of the Madden– Julian oscillation and its impact on the North Atlantic Oscil- lation in the boreal winter. Geophys. Res. Lett., 42, 5571–5576, https://doi.org/10.1002/2015GL064547. Liu, C., B. Tian, K.-F. Li, G. L. Manney, N. J. Livesey, Y. L. Yung, and D. E. Waliser, 2014: Northern Hemisphere mid-winter vortex-displacement and vortex-split stratospheric sudden warmings: Influence of the Madden–Julian oscillation and quasi-biennial oscillation. J. Geophys. Res. Atmos., 119, 12 599–12 620, https://doi.org/10.1002/2014JD021876. ——, Y. Liu, and Y.-L. Zhang, 2015: Simulation of the Madden– Julian oscillation in wintertime stratospheric ozone over the Tibetan Plateau and East Asia: Results from the Specified Dynamics version of the Whole Atmosphere Community Climate Model. Atmos. Ocean. Sci. Lett., 8, 264–270, https:// doi.org/10.3878/AOSL20150020. Lu, H., T. J. Bracegirdle, T. Phillips, A. Bushell, and L. Gray, 2014: Mechanisms for the Holton–Tan relationship and its decadal variation. J. Geophys. Res. Atmos., 119, 2811–2830, https://doi.org/ 10.1002/2013JD021352. Madden, R. A., and P. R. Julian, 1971: Detection of a 40–50 day oscillation in the zonal wind in the tropical Pacific. J. Atmos. Sci., 28, 702–708, https://doi.org/10.1175/1520-0469(1971)028,0702: DOADOI.2.0.CO;2. ——, and ——, 1972: Description of global-scale circulation cells in the tropics with a 40–50 day period. J. Atmos. Sci., 29, 1109–1123, https://doi.org/10.1175/1520-0469(1972)029,1109: DOGSCC.2.0.CO;2. ——, and ——, 1994: Observations of the 40–50-day tropical oscil- lation—A review. Mon. Wea. Rev., 122, 814–837, https://doi.org/ 10.1175/1520-0493(1994)122,0814:OOTDTO.2.0.CO;2. Manzini, E., M. A. Giorgetta, M. Esch, L. Kornblueh, and E. Roeckner, 2006: The influence of sea surface temperatures on the northern winter stratosphere: Ensemble simulations with the MAECHAM5 model. J. Climate, 19, 3863–3881, https://doi.org/10.1175/JCLI3826.1. Marsh, D. R., M. J. Mills, D. E. Kinnison, J.-F. Lamarque, N. Calvo, and L. M. Polvani, 2013: Climate change from 1850 to 2005 simulated in CESM1(WACCM). J. Climate, 26, 7372– 7391, https://doi.org/10.1175/JCLI-D-12-00558.1. Matsuno, T., 1966: Quasi-geostrophic motions in the equatorial area. J. Meteor. Soc. Japan, 44, 25–43, https://doi.org/10.2151/ jmsj1965.44.1_25. Matthews, A. J., B. J. Hoskins, and M. Masutani, 2004: The global response to tropical heating in the Madden–Julian oscillation during the northern winter. Quart. J. Roy. Meteor. Soc., 130, 1991–2011, https://doi.org/10.1256/qj.02.123. Mori, M., and M. Watanabe, 2008: The growth and triggering mechanism of the PNA: A MJO–PNA coherence. J. Meteor. Soc. Japan, 86, 213–236, https://doi.org/10.2151/jmsj.86.213. Neale, R. B., J. Richter, S. Park, P. H. Lauritzen, S. J. Vavrus, P. J. Rasch, and M. H. Zhang, 2013: The mean climate of the Community Atmosphere Model (CAM4) in forced SST and fully coupled experiments. J. Climate, 26, 5150–5168, https:// doi.org/10.1175/JCLI-D-12-00236.1. Newman, M., and P. D. Sardeshmukh, 2008: Tropical and strato- spheric influences on extratropical short-term climate vari- ability. J. Climate, 21, 4326–4347, https://doi.org/10.1175/ 2008JCLI2118.1. Newman, P. A., and E. R. Nash, 2000: Quantifying the wave driving of the stratosphere. J. Geophys. Res., 105, 12 485–12 497, https://doi.org/10.1029/1999JD901191. Perlwitz, J., and H.-F. Graf, 2001: Troposphere–stratosphere dy- namic coupling under strong and weak polar vortex condi- tions. Geophys. Res. Lett., 28, 271–274, https://doi.org/10.1029/ 2000GL012405. ——, and N. Harnik, 2004: Downward coupling between the stratosphere and troposphere: The relative roles of wave and zonal mean processes. J. Climate, 17, 4902–4909, https://doi.org/ 10.1175/JCLI-3247.1. Polvani, L. M., and D. W. Waugh, 2004: Upward wave activity flux as a precursor to extreme stratospheric events and subsequent anom- alous surface weather regimes. J. Climate, 17, 3548–3554, https:// doi.org/10.1175/1520-0442(2004)017,3548:UWAFAA.2.0.CO;2. Randel, W. J., F. Wu, and R. Stolarski, 2002: Changes in column ozone correlated with the stratospheric EP flux. J. Meteor. Soc. Japan, 80, 849–862, https://doi.org/10.2151/jmsj.80.849. Ren, R.-C., M. Cai, C. Xiang, and G. Wu, 2012: Observational evidence of the delayed response of stratospheric polar vortex variability to ENSO SST anomalies. Climate Dyn., 38, 1345– 1358, https://doi.org/10.1007/s00382-011-1137-7. Schwartz, C., and C. I. Garfinkel, 2017: Relative roles of the MJO and stratospheric variability in North Atlantic and European winter climate. J. Geophys. Res. Atmos., 122, 4184–4201, https://doi.org/10.1002/2016JD025829. Seo, K.-H., and S.-W. Son, 2012: The global atmospheric circula- tion response to tropical diabatic heating associated with the Madden–Julian oscillation during northern winter. J. Atmos. Sci., 69, 79–96, https://doi.org/10.1175/2011JAS3686.1. Subramanian, A. C., M. Jochum, A. J. Miller, R. Murtugudde, R. Neale, R. B. Neale, and D. E. Waliser, 2011: The Madden– Julian oscillation in CCSM4. J. Climate, 24, 6261–6282, https:// doi.org/10.1175/JCLI-D-11-00031.1. Sun, C., J. Li, and S. Zhao, 2015: Remote influence of At- lantic multidecadal variability on Siberian warm season precipitation. Sci. Rep., 5, 16853, https://doi.org/10.1038/ srep16853. ——, ——, R. Ding, and Z. Jin, 2017: Cold season Africa–Asia multidecadal teleconnection pattern and its relation to the Atlantic multidecadal variability. Climate Dyn., 48, 3903– 3918, https://doi.org/10.1007/s00382-016-3309-y. Thompson, D. W. J., and J. M. Wallace, 2001: Regional climate impacts of the Northern Hemisphere annular mode. Science, 293, 85–89, https://doi.org/10.1126/science.1058958. Wheeler, M. C., and H. H. Hendon, 2004: An all-season real- time multivariate MJO index: Development of an index for monitoring and prediction. Mon. Wea. Rev., 132, 1917–1932, https://doi.org/10.1175/1520-0493(2004)132,1917: AARMMI.2.0.CO;2. Wu, Z., X. Li, Y. Li, and Y. Li, 2016: Potential influence of Arctic sea ice to the interannual variations of East Asian spring precipitation. J. Climate, 29, 2797–2813, https://doi.org/10.1175/ JCLI-D-15-0128.1. Xie, F., J. Li, W. Tian, J. Feng, and Y. Huo, 2012: Signals of El Niño Modoki in the tropical tropopause layer and stratosphere. Atmos. Chem. Phys., 12, 5259–5273, https://doi.org/10.5194/ acp-12-5259-2012. ——, and Coauthors, 2016: A connection from Arctic stratospheric ozone to El Niño–Southern Oscillation. Environ. Res. Lett., 11, 124026, https://doi.org/10.1088/1748-9326/11/12/124026. Xu, H., J. Li, J. Feng, and J. Mao, 2013: The asymmetric relation- ship between the winter NAO and the precipitation in southwest China. Acta Meteor. Sin., 70, 1276–1291. Yang, C., T. Li, A. K. Smith, and X. Dou, 2017: The response of the Southern Hemisphere middle atmosphere to the Madden–Julian oscillation during austral winter using the Specified-Dynamics Whole Atmosphere Community Cli- mate Model. J. Climate, 30, 8317–8333, https://doi.org/10.1175/ JCLI-D-17-0063.1. Yoo, C., S. Feldstein, and S. Lee, 2011: The impact of the Madden– Julian oscillation trend on the Arctic amplification of surface air temperature during the 1979–2008 boreal winter. Geophys. Res. Lett., 38, L24804, https://doi.org/10.1029/2011GL049881. ——, S. Lee, and S. B. Feldstein, 2012: Mechanisms of Arctic surface air temperature change in response to the Madden– Julian oscillation. J. Climate, 25, 5777–5790, https://doi.org/ 10.1175/JCLI-D-11-00566.1. Zhang, C., M. Dong, S. Gualdi, H. H. Hendon, E. D. Maloney, A. Marshall, K. R. Sperber, and W. Wang, 2006: Simulations of the Madden–Julian oscillation in four pairs of coupled and uncoupled global models. Climate Dyn., 27, 573–592, https:// doi.org/10.1007/s00382-006-0148-2. Zhang, J., W. Tian, F. Xie, Y. Li, F. Wang, J. Huang, and H. Tian, 2015a: Influence of the El Niño Southern Oscillation on the total ozone column and clear-sky ultraviolet radiation over China. Atmos. Environ., 120, 205–216, https://doi.org/10.1016/ j.atmosenv.2015.08.080. ——, ——, Z. Wang, F. Xie, and F. Wang, 2015b: The influence of ENSO on northern midlatitude ozone during the winter to spring transition. J. Climate, 28, 4774–4793, https://doi.org/ 10.1175/JCLI-D-14-00615.1. ——, ——, M. P. Chipperfield, F. Xie, and J. Huang, 2016: Per- sistent shift of the Arctic polar vortex towards the Eurasian continent in recent decades. Nat. Climate Change, 6, 1094– 1099, https://doi.org/10.1038/nclimate3136. Zhao, S., J. Li, and Y. Li, 2015: Dynamics of an interhemispheric teleconnection across the critical latitude through a southerly duct during boreal winter. J. Climate, 28, 7437–7456, https:// doi.org/10.1175/JCLI-D-14-00425.1. Zheng, F., J. Li, Y. Li, S. Zhao, and D. Deng, 2016: Influence of the summer NAO on the spring-NAO-based predictability of the East Asian summer monsoon. J. Appl. Meteor. Climatol., 55, 1459–1476, https://doi.org/10.1175/JAMC-D-15-0199.1. Zhou, S., and A. J. Miller, 2005: The interaction of the Madden– Julian oscillation and the Arctic Oscillation. J. Climate, 18, 143–159, https://doi.org/10.1175/JCLI3251.1.
  13. Breaking planetary waves in the stratosphere Authors: M. E. McIntyre and T. N. Palmer Published: 13th October, 1983 Abstract: Satellite-borne IR radiometers are turning the Earth's stratosphere into one of the best available outdoor laboratories for observing the large-scale dynamics of a rotating, heterogeneous fluid under gravity. New insight is being gained not only into stratospheric dynamics as such, with its implications for pollutant behaviour and the ozone layer, but also indirectly into the dynamics of the troposphere, with its implications for weather forecasting. Similar dynamical regimes occur in the oceans and in stellar interiors. A key development has been the construction of coarse-grain maps of the large-scale distribution of potential vorticity in the middle stratosphere. Potential vorticity is a conservable quantity which has a central role in the dynamical theory, but is difficult to calculate accurately from observational data. We present the first mid-stratospheric potential vorticity maps which appear good enough to make visible the ‘breaking’ of planetary or Rossby waves, a phenomenon ubiquitous in nature and arguably one of the most important dynamical processes affecting the stratosphere as a whole. Link to full paper: http://www-atm.damtp.cam.ac.uk/people/mem/mp83-scanned.pdf
  14. Effects of stratospheric variability on El Niño teleconnections Authors: J H Richter, C Deser and L Sun Published: 17th December, 2015 Abstract: The effects of the tropical Pacific El Niño Southern Oscillation (ENSO) phenomenon are communicated to the rest of the globe via atmospheric teleconnections. Traditionally, ENSO teleconnections have been viewed as tropospheric phenomena, propagating to higher latitudes as Rossby waves. Recent studies, however, suggest an influence of the stratosphere on extra-tropical ENSO teleconnections. The stratosphere is highly variable: in the tropics, the primary mode of variability is the quasi-biennial oscillation (QBO), and in the extra-tropics sudden stratospheric warmings (SSWs) regularly perturb the mean state. Here, we conduct a 10-member ensemble of simulations with a stratosphere-resolving atmospheric general circulation model forced with the observed evolution of sea surface temperatures during 1952–2001 to examine the effects of the QBO and SSWs on the zonal-mean circulation and temperature response to El Niño, with a focus on the northern extra-tropics during winter. We find that SSWs have a larger impact than the QBO on the composite El Niño responses. During El Niño winters with SSWs, the polar stratosphere shows positive temperature anomalies that propagate downward to the surface where they are associated with increased sea-level pressure over the Arctic. During El Niño winters without SSWs, the stratosphere and upper troposphere show negative temperature anomalies but these do not reach the surface. The QBO modulates the El Niño teleconnection primarily in winters without SSWs: the negative temperature anomalies in the polar stratosphere and upper troposphere are twice as large during QBO West compared to QBO East years. In addition, El Niño winters that coincide with the QBO West phase show stronger positive sea-level pressure anomalies over the eastern Atlantic and Northern Europe than those in the QBO East phase. The results imply that the stratosphere imparts considerable variability to ENSO teleconnections. Link to full paper: http://iopscience.iop.org/article/10.1088/1748-9326/10/12/124021
  15. Progress in research of stratosphere-troposphere interactions: Application of isentropic potential vorticity dynamics and the effects of the Tibetan Plateau Authors: Rongcai Ren, Guoxiong Wu, Ming Cai, Shuyue Sun, Xin Liu and Weiping Li Published: 19th October, 2014 Abstract: This paper reviews recent progress in understanding isentropic potential vorticity (PV) dynamics during interactions between the stratosphere and troposphere, including the spatial and temporal propagation of circulation anomalies associated with the winter polar vortex oscillation and the mechanisms of stratosphere-troposphere coupling in the global mass circulation framework. The origins and mechanisms of interannual variability in the stratospheric circulation are also reviewed. Particular attention is paid to the role of the Tibetan Plateau as a PV source (via its thermal forcing) in the global and East Asian atmospheric circulation. Diagnosis of meridional isentropic PV advection over the Tibetan Plateau and East Asia indicates that the distributions of potential temperature and PV over the east flank of the Tibetan Plateau and East Asia favor a downward and southward isentropic transport of high PV from the stratosphere to the troposphere. This transport manifests the possible influence of the Tibetan Plateau on the dynamic coupling between the stratosphere and troposphere during summer, and may provide a new framework for understanding the climatic effects of the Tibetan Plateau. Link to full paper: http://www.lasg.ac.cn/staff/gxwu/docs/2014/7. Ren Wu etc JMR_QXXB.pdf
  16. Linking stratospheric circulation extremes and minimum Arctic sea ice extent Workshop Presentation: Aspen Global Change Institute, Aspen, Colorado Workshop Programme: “Understanding the Causes and Consequences of Polar Amplification” - June 12th -16th, 2017 Presenter: Karen Smith, Lorenzo Polvani and Bruno Tremblay Presentation Date: 14th June, 2017 Link to full presentation (32 minute video): https://www.agci.org/lib/17s1/linking-stratospheric-circulation-extremes-and-minimum-arctic-sea-ice-extent Link to presentation (slides and charts only): https://www.agci.org/sites/default/files/pdfs/lib/main/KSmith_Aspen2017.pdf Link to full agenda and presentations: https://www.agci.org/event/17s1
  17. Sudden Stratospheric Warmings and Anomalous Upward Wave Activity Flux Authors: Thomas Birner, John R. Albers Published: 28th, June, 2017 Abstract: Abrupt breakdowns of the polar winter stratospheric circulation such as sudden stratospheric warmings (SSWs) are a manifestation of strong two-way interactions between upward propagating planetary waves and the mean flow. The importance of sufficient upward wave activity fluxes from the troposphere and the preceding state of the stratospheric circulation in forcing SSW-like events have long been recognized. Past research based on idealized numerical simulations has suggested that the state of the stratosphere may be more important in generating extreme stratospheric events than anomalous upward wave fluxes from the troposphere. Other studies have emphasized the role of tropospheric precursor events. Here reanalysis data are used to define events of extreme stratospheric mean flow deceleration (SSWs being a subset) and events of extreme lower tropospheric upward planetary wave activity flux. While the wave fluxes leading to SSW-like events ultimately originate near the surface, the anomalous upward wave activity fluxes associated with these events primarily occur within the stratosphere. The crucial dynamics for forcing SSW-like events appear to take place in the communication layer just above the tropopause. Anomalous upward wave fluxes from the lower troposphere may play a role for some events, but seem less important for the majority of them. Link to full paper: https://www.jstage.jst.go.jp/article/sola/13A/Special_Edition/13A_13A-002/_pdf/-char/en
  18. Gravity Wave Effects on Polar Vortex Geometry During Split-Type Sudden Stratospheric Warmings Authors: J. R. Albers and T. Birner Conference: SPARC (Stratosphere-troposphere Processes And their Role in Climate) General Assembly at Queenstown, New Zealand Conference Date: 12th-17th January, 2014 Poster Session B: "Stratosphere-Troposphere-Ocean Dynamics and Predictability of Regional Climate" Full Poster Presentation: Sudden stratospheric warmings represent one of the most compelling tests of our ability to explain and predict the dynamical circulation of the stratosphere. Yet despite significant recent progress in the classification of the geometric structure and evolution of sudden warmings (e.g. split versus displacement events), our understanding of the dynamics underlying these events remains elusive. This fact is especially glaring in light of the fact that stratosphere-troposphere coupling is particularly strong during and after sudden warming events, and therefore our ability to predict tropospheric climate and weather could be improved by gaining a more clear understanding of the dynamics underlying sudden warming events. The traditional sudden stratospheric warming paradigm states that an anomalously strong pulse of planetary wave activity from the troposphere is required in order to trigger an event. However, the results of Matthewman and Esler (JAS 2011) contradict this hypothesis by pointing out that resonant excitation and vortex breakdown can occur for relatively weak wave forcing. With this idea in mind, we utilize reanalysis data to revisit and revise the traditional hypothesis of the necessary conditions for triggering a sudden warming. In this study, reanalysis (JRA25) data, including parameterized gravity wave drag, which extends upwards into the lower mesosphere (~55 km) is used to explore the relationship between gravity wave and planetary wave variability and polar vortex geometry prior to sudden stratospheric warming events. In particular, an attempt is made to identify the key geometric features of the polar vortex that are common to the development of all split-type warming events. Comparison of planetary and gravity wave activity suggests that anomalous gravity wave drag prior to split-type sudden warmings plays an important role in ʻtuningʼ the vortex towards its resonant, barotropic excitation point by minimizing the vortex area, elongating the vortex in the meridional-zonal plane, and aligning the vortex so that is barotropic throughout the stratosphere. The robustness of the role of gravity wave drag in vortex ʻtuningʼ is examined by contrasting the results obtained via JRA25 reanalysis data versus analogous results obtained from the ERA-Interim and MERRA reanalysis data sets. The results presented in this study have important implications for stratosphere-troposphere research by providing an important new view on the geometric state of the polar vortex prior to split-type warmings that may help researchers attempting to understand and predict sudden stratospheric warming events and tropospheric weather and climate. Link to full paper: There is no full paper available and this often applies to these short poster presentations. Some of them lead on to further research and publications by the same author(s). Where there are relevant contributions, we shall endeavour to locate these papers and place them in this portal in due course. Link to Conference report: http://www.apn-gcr.org/resources/files/original/b7553eec5a55fd26ec273c8b23a38eb2.pdf
  19. Preconditioning of Arctic Stratospheric Polar Vortex Shift Events Authors: Jinlong Huang and Wenshou Tian Published: 23rd March, 2018 Abstract: This study examines the preconditioning of events in which the Arctic stratospheric polar vortex shifts toward Eurasia (EUR events), North America (NA events), and the Atlantic (ATL events) using composite analysis. An increase in blocking days over northern Europe and a decrease in blocking days over the Bering Strait favor the movement of the vortex toward Eurasia, while the opposite changes in blocking days over those regions favor the movement of the vortex toward North America. An increase in blocking days over the eastern North Atlantic and a decrease in blocking days over the Bering Strait are conducive to movement of the stratospheric polar vortex toward the Atlantic. These anomalous precursor blocking patterns are interpreted in terms of the anomalous zonal wave-1 or wave-2 planetary wave fluxes into the stratosphere that are known to influence the vortex position and strength. In addition, the polar vortex shift events are further classified into events with small and large polar vortex deformation, since the two types of events are likely to have a different impact at the surface. A significant difference in the zonal wave-2 heat flux into the lower stratosphere exists prior to the two types of events and this is linked to anomalous blocking patterns. This study further defines three types of tropospheric blocking events in which the spatial patterns of blocking frequency anomalies are similar to the blocking patterns prior to EUR, NA, and ATL events, respectively, and our reanalysis reveals that the polar vortex is indeed more likely to shift toward Eurasia, North America, and the Atlantic in the presence of the above three defined tropospheric blocking events. These shifts of the polar vortex toward Eurasia, North America, and the Atlantic lead to statistically significant negative height anomalies near the tropopause and corresponding surface cooling anomalies over these three regions. Link to full paper: Please note that the full paper is currently behind an AMS paywall. Link to the AMS website: https://journals.ametsoc.org/doi/abs/10.1175/JCLI-D-17-0695.1
  20. On the Relationship between ENSO, Stratospheric Sudden Warmings and Blocking Authors: David Barriopedro and Natalia Calvo Published: 8th March, 2014 Abstract: This paper examines the influence of El Niño–Southern Oscillation (ENSO) on different aspects of major stratospheric sudden warmings (SSWs), focusing on the precursor role of blocking events. The results reveal an ENSO modulation of the blocking precursors of SSWs. European and Atlantic blocks tend to precede SSWs during El Niño (EN), whereas eastern Pacific and Siberian blocks are the preferred precursors of SSWs during La Niña (LN) winters. This ENSO preference for different blocking precursors seems to occur through an ENSO effect on regional blocking persistence, which in turn favors the occurrence of SSWs. The regional blocking precursors of SSWs during each ENSO phase also have different impacts on the upward propagation of planetary-scale wavenumbers 1 and 2; hence, they determine ENSO differences in the wavenumber signatures of SSWs. SSWs occurring during EN are preceded by amplification of wavenumber 1, whereas LN SSWs are predominantly associated to wavenumber-2 amplification. However, there is not a strong preference for splitting or displacement SSWs during any ENSO phase. This is mainly because during EN, splitting SSWs do not show a wavenumber-2 pattern. Link to full paper: https://journals.ametsoc.org/doi/pdf/10.1175/JCLI-D-13-00770.1
  21. Extraordinary Features of Planetary Wave Propagation During the Boreal Winter 2013/2014 & Zonal Wave Number 2 Predominance Authors: Y. Harada and T. Hirooka Published: 12th October, 2017 Abstract: Observational features of the winter 2013/2014 are investigated using of the Japanese 55‐year Reanalysis data. This winter can be characterized by the continuous predominance of planetary waves of zonal wave number two (WN2) that did not cause major sudden stratospheric warming (SSW) events. It is found that the vertical component of the Eliassen‐Palm flux of WN2 for the winter 2013/2014 is almost equal to the highest value of the winter 2008/2009. The longitudinal distribution of vertical components of Plumb wave activity flux for this winter shows marked downward propagation around 100°W and upward propagation around 60°E, both of which are the strongest of their type among the 56 winters since 1958/1959. The convergence of wave packets propagating from around 60°E contributes to the development and continuance of the quasi‐barotropic Aleutian High, which is associated with the extension of negative extended refractive index (Ks) region. The extension of negative Ks region is related to the convergence or reflection of the wave packets emanating from tropospheric blocking highs developing in the North Pacific Ocean; the development and continuance of the quasi‐barotropic Aleutian High is considered to be one of plausible reasons for the lack of major SSWs in the winter 2013/2014. In addition to these results, we revealed the significant contribution of smaller scale waves (with a zonal wave number of three or more) to the structure of localized wave packet propagation in the stratosphere. Link to full paper (workshop presentation): https://events.oma.be/indico/event/6/material/slides/6.pdf
  22. The preconditioning of major sudden stratospheric warmings Authors: S. Bancalá, K. Krüger and M. Giorgetta Published: 16th February, 2012 Abstract: The preconditioning of major sudden stratospheric warmings (SSWs) is investigated with two long time series using reanalysis (ERA‐40) and model (MAECHAM5/MPI‐OM) data. Applying planetary wave analysis, we distinguish between wavenumber‐1 and wavenumber‐2 major SSWs based on the wave activity of zonal wavenumbers 1 and 2 during the prewarming phase. For this analysis an objective criterion to identify and classify the preconditioning of major SSWs is developed. Major SSWs are found to occur with a frequency of six and seven events per decade in the reanalysis and in the model, respectively, thus highlighting the ability of MAECHAM5/MPI‐OM to simulate the frequency of major SSWs realistically. However, from these events only one quarter are wavenumber‐2 major warmings, representing a low (∼0.25) wavenumber‐2 to wavenumber‐1 major SSW ratio. Composite analyses for both data sets reveal that the two warming types have different dynamics; while wavenumber‐1 major warmings are preceded only by an enhanced activity of the zonal wavenumber‐1, wavenumber‐2 events are either characterized by only the amplification of zonal wavenumber‐2 or by both zonal wavenumber‐1 and zonal wavenumber‐2, albeit at different time intervals. The role of tropospheric blocking events influencing these two categories of major SSWs is evaluated in the next step. Here, the composite analyses of both reanalysis and model data reveal that blocking events in the Euro‐Atlantic sector mostly lead to the development of wavenumber‐1 major warmings. The blocking–wavenumber‐2 major warming connection can only be statistical reliable analyzed with the model time series, demonstrating that blocking events in the Pacific region mostly precede wavenumber‐2 major SSWs. Link to full paper: https://pdfs.semanticscholar.org/5d99/9c51d49c694a91bf2ec092927ee60920e529.pdf
  23. Role of gravity waves in vertical coupling during sudden stratospheric warmings Authors: Erdal Yiğit and Alexander S. Medvedev Published: 24th August, 2016 Abstract: Gravity waves are primarily generated in the lower atmosphere, and can reach thermospheric heights in the course of their propagation. This paper reviews the recent progress in understanding the role of gravity waves in vertical coupling during sudden stratospheric warmings. Modeling of gravity wave effects is briefly reviewed, and the recent developments in the field are presented. Then, the impact of these waves on the general circulation of the upper atmosphere is outlined. Finally, the role of gravity waves in vertical coupling between the lower and the upper atmosphere is discussed in the context of sudden stratospheric warmings. Link to full paper: https://geoscienceletters.springeropen.com/articles/10.1186/s40562-016-0056-1
  24. Snow–(N)AO Teleconnection and Its Modulation by the Quasi-Biennial Oscillation Authors: Y. Peings Published: 29th November, 2017 Abstract: This study explores the wintertime extratropical atmospheric response to Siberian snow anomalies in fall, using observations and two distinct atmospheric general circulation models. The role of the quasi-biennial oscillation (QBO) in modulating this response is discussed by differentiating easterly and westerly QBO years. The remote influence of Siberian snow anomalies is found to be weak in the models, especially in the stratosphere where the “Holton–Tan” effect of the QBO dominates the simulated snow influence on the polar vortex. At the surface, discrepancies between composite analyses from observations and model results question the causal relationship between snow and the atmospheric circulation, suggesting that the atmosphere might have driven snow anomalies rather than the other way around. When both forcings are combined, the simulations suggest destructive interference between the response to positive snow anomalies and easterly QBO (and vice versa), at odds with the hypothesis that the snow–North Atlantic Oscillation/Arctic Oscillation [(N)AO] teleconnection in recent decades has been promoted by the QBO. Although model limitations in capturing the relationship exist, altogether these results suggest that the snow–(N)AO teleconnection may be a stochastic artifact rather than a genuine atmospheric response to snow-cover variability. This study adds to a growing body of evidence suggesting that climate models do not capture a robust and stationary snow–(N)AO relationship. It also highlights the need for extending observations and/or improving models to progress on this matter. Link to full paper: https://escholarship.org/uc/item/1hd557ss
  25. Coupling of Stratospheric Warmings with Mesospheric Coolings in Observations and Simulations Authors: Christoph Zülicke, Erich Becker, Vivien Matthias, and Dieter H. W. Peters Published: 19th January, 2018 Abstract: The vertical coupling between the stratosphere and the mesosphere is diagnosed from polar cap temperatures averaged over 60°–90°N with a new method: the joint occurrence of a warm stratosphere at 10 hPa and a cold mesosphere at 0.01 hPa. The investigation of an 11-yr-long dataset (2004–15) from Aura-MLS observations shows that such mesospheric coupling days appear in 7% of the winter. During major sudden stratospheric warming events mesospheric couplings are present with an enhanced average daily frequency of 22%. This daily frequency changes from event to event but broadly results in five of seven major warmings being classified as mesospheric couplings (2006, 2008, 2009, 2010, and 2013). The observed fraction of mesospheric coupling events (71%) is compared with simulations of the Kühlungsborn Mechanistic Circulation Model (KMCM), the Hamburg Model of the Neutral and Ionized Atmosphere (HAMMONIA), and the Whole Atmosphere Community Climate Model (WACCM). The simulated fraction of mesospheric coupling events ranges between 57% and 94%, which fits the observations. In searching for causal relations weak evidence is found that major warming events with strong intensity or split vortices favor their coupling with the upper mesosphere. More evidence is found with a conceptual model: an effective vertical coupling between 10 and 0.01 hPa is provided by deep zonal-mean easterlies at 60°N, which are acting as a gravity-wave guide. The explained variance is above 40% in the four datasets, which indicates a near-realistic simulation of this process. Link to full paper: Please note that the full paper is currently behind an AMS paywall Link to the AMS website: https://journals.ametsoc.org/doi/abs/10.1175/JCLI-D-17-0047.1
×