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Found 36 results

  1. The Effect of Climate Change on the Variability of the Northern Hemisphere Stratospheric Polar Vortex Authors: Daniel M. Mitchell, Scott M. Osprey and Lesley J. Gray Neal Butchart and Steven C. Hardiman, Andrew J. Charlton-Perez and Peter Watson Date published: January 13th 2012 Abstract With extreme variability of the Arctic polar vortex being a key link for stratosphere–troposphere influences, its evolution into the twenty-first century is important for projections of changing surface climate in response to greenhouse gases. Variability of the stratospheric vortex is examined using a state-of-the-art climate model and a suite of specifically developed vortex diagnostics. The model has a fully coupled ocean and a fully resolved stratosphere. Analysis of the standard stratospheric zonal mean wind diagnostic shows no significant increase over the twenty-first century in the number of major sudden stratospheric warmings (SSWs) from its historical value of 0.7 events per decade, although the monthly distribution of SSWs does vary, with events becoming more evenly dispersed throughout the winter. However, further analyses using geometric-based vortex diagnostics show that the vortex mean state becomes weaker, and the vortex centroid is climatologically more equatorward by up to 2.5°, especially during early winter. The results using these diagnostics not only characterize the vortex structure and evolution but also emphasize the need for vortex-centric diagnostics over zonally averaged measures. Finally, vortex variability is subdivided into wave-1 (displaced) and -2 (split) components, and it is implied that vortex displacement events increase in frequency under climate change, whereas little change is observed in splitting events. Link to paper: https://journals.ametsoc.org/doi/full/10.1175/JAS-D-12-021.1
  2. Wave Activity Events and the Variability of the Stratospheric Polar Vortex Author: Abraham Solomon Published: Oct 2014 Abstract: During Northern Hemisphere winter, polar stratospheric winds and temperatures exhibit significant variability that is due to the vertical propagation of planetary-scale waves. The most dramatic intraseasonal variations in temperature are associated with sudden stratospheric warmings (SSWs), which are wave-breaking events that occur approximately every other year. This paper will introduce the concept of wave activity events (WAEs), which are periods of enhanced pseudomomentum density in the polar stratosphere that occur every year. It will be demonstrated that all SSWs are associated with WAEs; furthermore, minor warmings and many final warmings in the polar spring are also WAEs, and therefore a better understanding of these more frequent wave events can provide additional insights into stratospheric wave-induced variability. Employing the Interim European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-Interim) for 1979–2011, 119 WAEs are identified and their life cycle is compared with that of the 23 SSWs observed during this period. Link to full paper: https://journals.ametsoc.org/doi/10.1175/JCLI-D-13-00756.1
  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. Near-term Climate Predictions of the North Atlantic Region - YouTube Presentation Presenters/Authors: Dr. Nick Dunstone, Doug Smith, Adam Scaife, Leon Hermanson, Rosie Eade, Niall Robinson, Martin Andrews and Jeff Knight Presentation Team from: The UK Met Office, Hadley Centre Presentation Date: 6th April, 2017 (at St Andrews, Scotland) Abstract: None (but see below) Link to full YouTube presentation (57 minutes): https://www.youtube.com/watch?reload=9&v=jfKr6oRnn2k This brilliant "balanced" presentation was based on a paper published by the same authors (on the 17th October, 2016) with the title "Skilful predictions of the winter North Atlantic Oscillation one year ahead". The full paper is still behind a "Nature GeoScience" paywall on this site: https://www.nature.com/articles/ngeo2824 Abstract to paper: The winter North Atlantic Oscillation is the primary mode of atmospheric variability in the North Atlantic region and has a profound influence on European and North American winter climate. Until recently, seasonal variability of the North Atlantic Oscillation was thought to be largely driven by chaotic and inherently unpredictable processes. However, latest generation seasonal forecasting systems have demonstrated significant skill in predicting the North Atlantic Oscillation when initialized a month before the onset of winter. Here we extend skilful dynamical model predictions to more than a year ahead. The skill increases greatly with ensemble size due to a spuriously small signal-to-noise ratio in the model, and consequently larger ensembles are projected to further increase the skill in predicting the North Atlantic Oscillation. We identify two sources of skill for second-winter forecasts of the North Atlantic Oscillation: climate variability in the tropical Pacific region and predictable effects of solar forcing on the stratospheric polar vortex strength. We also identify model biases in Arctic sea ice that, if reduced, may further increase skill. Our results open possibilities for a range of new climate services, including for the transport, energy, water management and insurance sectors.
  6. Extratropical Atmospheric Predictability From the Quasi-Biennial Oscillation in Subseasonal Forecast Models Authors: Chaim I. Garfinkel, Chen Schwartz, Daniela I. V. Domeisen, Seok-Woo Son, Amy H. Butler, Ian P. White. Published: July 2018 Abstract: The effect of the Quasi-Biennial Oscillation (QBO) on the Northern Hemisphere wintertime stratospheric polar vortex is evaluated in five operational subseasonal forecasting models. Of these five models, the three with the best stratospheric resolution all indicate a weakened vortex during the easterly phase of the QBO relative to its westerly phase, consistent with the Holton-Tan effect. The magnitude of this effect is well captured for initializations in late October and November in the model with the largest ensemble size. While the QBO appears to modulate the extratropical tropospheric circulation in some of the models as well, the importance of a polar stratospheric pathway, through the Holton-Tan effect, for the tropospheric anomalies is unclear. Overall, knowledge of the QBO can contribute to enhanced predictability, at least in a probabilistic sense, of the Northern Hemisphere winter climate on subseasonal timescales. Plain Language Summary The Quasi-Biennial Oscillation (QBO) is perhaps the most regular atmospheric phenomena that is not directly controlled by solar radiation and can be predicted more than a year in advance. It is characterized by alternating westerly and easterly winds in the tropical stratosphere. Here we show that the QBO can be used to improve month-ahead prediction of the Northern Hemisphere wintertime stratospheric polar vortex, and perhaps even the extratropical tropospheric circulation. Link to full paper: https://agupubs.onlinelibrary.wiley.com/doi/epdf/10.1029/2018JD028724
  7. 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
  8. 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
  9. 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
  10. 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. 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  11. 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
  12. Consequences of Arctic Amplification: Role of the Stratosphere - A Discussion Workshop Presentation: Aspen Global Change Institute, Aspen, Colorado Workshop Programme: “Understanding the Causes and Consequences of Polar Amplification” - June 12th -16th, 2017 Workshop Discussion: Relating to their presentations (on the same morning) Presenters: Karen L Smith, Jinro Ukita and Yutian Wu Presentation Date: 14th June, 2017 Link to full discussion (10 minute video): https://www.agci.org/lib/17s1/part-vi-discussion-consequences-arctic-amplification-role-stratosphere Link to full agenda and presentations: https://www.agci.org/event/17s1
  13. 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
  14. 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
  15. 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
  16. Varying stratospheric responses to tropical Atlantic SST forcing from early to late winter Authors: Jian Rao and Rongcai Ren Published: 9th November, 2017 Abstract: Using multiple reanalysis datasets and model simulations, we begin in this study by isolating the tropical Atlantic Ocean (TAO) sea surface temperature (SST) signals that are independent from ENSO, and then investigate their influences on the northern winter stratosphere. It is revealed that TAO SST forcing does indeed have significant effects on the northern winter stratosphere, but these effects vary from early to late winter in a way that explains the overall insignificant effect when the seasonal average is considered. The stratospheric polar vortex is anomalously weaker/warmer in November–December, stronger/colder in January–March, and weaker/warmer again in April–May during warm TAO years. The varying impacts of the TAO forcing on the extratropical stratosphere are related to a three-stage response of the extratropical troposphere to the TAO forcing during cold season. The tropospheric circulation exhibits a negative North Atlantic Oscillation–like response during early winter, an eastward propagating Rossby wave pattern in mid-to-late winter, and a meridional dipole over North America in spring. Associated with this is varying planetary wave activity in the stratosphere, manifested as an increase in early winter, a decrease in mid-to-late winter, and an increase again in spring. The varying modulation of stratospheric circulation by TAO forcing is consistently confirmed in three reanalysis datasets, and model simulations (fully coupled model and its component AGCM). The exception to the robustness of this verification is that the circumpolar wind response in the fully coupled model is relatively weaker, and that in its component AGCM appears a month later than observed. Link to full paper: https://www.researchgate.net/profile/Jian_Rao/publication/320971750_Varying_stratospheric_responses_to_tropical_Atlantic_SST_forcing_from_early_to_late_winter/links/5a092a654585157013a7799f/Varying-stratospheric-responses-to-tropical-Atlantic-SST-forcing-from-early-to-late-winter.pdf
  17. 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
  18. On the reproducibility of the September 2002 vortex splitting event in the Antarctic stratosphere achieved without satellite observations Authors: Shunsuke Noguchi and Chiaki Kobayashi Published: 1st November , 2017 Abstract: To highlight the impact of satellite measurements, a comparison between the Japanese 55‐year reanalysis (JRA‐55) and its equivalent without the assimilation of satellite observations (JRA‐55C; C stands for ‘conventional’ observations) was conducted. As an illustrative example of the detectability problem of extreme events, we report on the reproducibility of a stratospheric sudden warming (SSW) event that occurred in late September 2002; this event represents the first observed unique vortex splitting event in the Antarctic stratosphere. Through the data assimilation system of JRA‐55, the initial tendency of this warming event and the following recovery process were well captured even when no satellite observations were used. However, the warming in JRA‐55C does not satisfy the criteria for a major SSW event besides the lack of splitting behaviour in the polar vortex. A prominent difference between JRA‐55 and JRA‐55C during the SSW event, which was characterized by the sudden appearance of a nearly barotropic structure from the upper stratosphere to the troposphere, was found over the Western Hemisphere reflecting the geographic distribution of observational sites. Moreover, several differences in the precursory state of the polar vortex and the observational anchoring effect are consistent with the proposal that this SSW was caused by the catastrophic breakdown of a highly deformed polar vortex as suggested by some recent works. Link to full paper: https://rmets.onlinelibrary.wiley.com/doi/epdf/10.1002/qj.3193
  19. Tropospheric Cooling as a Mechanism for Stratospheric Polar Vortex Disturbances Authors: Thomas S. Ehrmann and S. J. Colucci Presentation: 28th June, 2017 Abstract: Disturbances in the Northern Hemisphere wintertime stratospheric polar vortex and their connection with cold temperature anomalies in the mid-latitude troposphere are studied using MERRA Reanalysis data for the winter seasons of 1980-2014. By taking geometric moments of potential vorticity in the upper stratosphere, 55 disturbances of the polar vortex are identified during the 35 winter seasons either as splits or displacements. The position of the polar vortex during each disturbance event is averaged to generate an area averaging filter. A potential vorticity inversion method is used to show negative height tendencies in the stratosphere, driven by negative temperature tendencies in the troposphere and lower stratosphere, under the disturbed polar vortex preceding most disturbance events. This suggests that tropospheric cooling may help determine the orientation of the stratospheric polar vortex during at least some disturbance events. Link to full paper: Please note that the full paper is behind a paywall but this was presented at the "19th Conference on Middle Atmosphere" hosted by the University of California, Department of Atmospheric Sciences, UCLA, Los Angeles. Session 9 entitled "The Polar Vortices & Planetary Waves: Upward Coupling" contained six excellent presentations 28th June 2017. Link to Conference Presentation Video: https://ams.confex.com/ams/21Fluid19Middle/videogateway.cgi/id/38615?recordingid=38615&uniqueid=Paper318511&entry_password=949504
  20. Characterizing Stratospheric Polar Vortex Variability With Computer Vision Techniques - Paper and Presentation Authors: Zachary D. Lawrence and Gloria L. Manney First Published: December 2nd, 2017 Published on line: February 5th, 2018 Abstract: Computer vision techniques are used to characterize the Arctic stratospheric polar vortex in 38 years of reanalysis data. Such techniques are typically applied to analyses of digital images, but they represent powerful tools that are more widely applicable: basic techniques and considerations for geophysical applications are outlined herein. Segmentation, descriptive, and tracking algorithms are combined in the Characterization and Analysis of Vortex Evolution using Algorithms for Region Tracking (CAVE‐ART) package, which was developed to comprehensively describe dynamical and geometrical evolution of polar vortices. CAVE‐ART can characterize and track multiple vortex regions through time, providing an extensive suite of region, moments, and edge diagnostics for each. CAVE‐ART is valuable for identifying vortex‐splitting events including, but not limited to, previously cataloged vortex‐split sudden stratospheric warmings. An algorithm for identifying such events detects 52 potential events between 1980 and 2017; of these, 38 are subjectively classified as distinct “split‐like” events. The algorithm based on CAVE‐ART is also compared with moment‐based methods previously used to detect split events. Furthermore, vortex edge‐averaged wind speeds from CAVE‐ART are used to define extreme weak and strong polar vortex events over multiple vertical levels; this allows characterization of their occurrence frequencies and extents in time and altitude. Weak and strong events show distinct signatures in CAVE‐ART diagnostics: in contrast to weak events, strong vortices are more cylindrical and pole centered, and less filamented, than the climatological state. These results from CAVE‐ART exemplify the value of computer vision techniques for analysis of geophysical phenomena. Plain Language Summary: A large‐scale cyclone called the stratospheric polar vortex forms in the middle atmosphere over the pole every fall in each hemisphere and lasts until spring. The Arctic and Antarctic vortices share many characteristics, including that they consist of strong eastward winds, and they extend from roughly 14 km above the surface to beyond 50 km. Understanding the behavior of these vortices is important because they affect stratospheric ozone depletion and influence weather and climate. In general, the Arctic stratospheric vortex exhibits anomalous behavior more often than its Antarctic counterpart; understanding such behavior can be aided by examining vortex geometry–their size, shape, location, etc. Here we use computer vision techniques similar to those used to analyze digital images to analyze the geometry of the Arctic polar vortex over years from 1980 to 2017. With these techniques, we find a large number of split‐like events in which the Arctic polar vortex breaks apart into two separate vortices. We also find important differences in vortex geometry when the Arctic vortex is strong versus when it is weak. Our results draw attention to details of stratospheric vortex variability that motivate further investigation of the physics that can help scientists better understand middle atmosphere weather and climate. Link to full paper: https://pdfs.semanticscholar.org/a517/ff55b611945a63dab6d4749c278d58c51825.pdf Please note that the full paper was originally behind a paywall but was later made available for public viewing. Prior to publishing their paper the authors presented it at the "19th Conference on Middle Atmosphere" hosted by the University of California, Department of Atmospheric Sciences, UCLA, Los Angeles between June 25th and June 29th, 2017. Session 9 entitled "The Polar Vortices & Planetary Waves: Upward Coupling" contained six excellent presentations June 28th, 2017, including this one which was nominated as an "award winner". Conference Presentation Summary: The Northern Hemisphere (NH) stratospheric polar vortex exhibits a wide range of interannual variability that can influence polar chemical processing, ozone depletion, and tropospheric weather and climate. Recently there has been a renewed interest in understanding and utilizing the full spectrum of polar vortex variability, including sudden warmings, strong vortex events, etc., because of the connections to tropospheric weather and climate on seasonal and sub-seasonal timescales. Here we present results from the Characterization and Analysis of Vortex Evolution using Algorithms for Region Tracking (CAVE-ART) package, which uses computer vision techniques to comprehensively characterize the three dimensional evolution of the polar vortex throughout the stratosphere. We will give a summary of the techniques that allow us to identify specific geometrical and dynamical polar vortex characteristics, including the evolution of individual offspring vortices during vortex-split events. Furthermore, we will show examples and applications of CAVE-ART using the full record of NH winters in the Modern-Era Retrospective analysis for Research and Applications version 2 (MERRA-2). Link to Conference Presentation Video (17 minutes): https://ams.confex.com/ams/21Fluid19Middle/videogateway.cgi/id/38610?recordingid=38610&uniqueid=Paper318796&entry_password=564520 Link to full conference agenda: https://ams.confex.com/ams/21Fluid19Middle/webprogram/19MIDDLE.html
  21. Vortex Preconditioning due to Planetary and Gravity Waves prior to Sudden Stratospheric Warmings Authors: John R. Albers and Thomas Birner Published: 14th June, 2014 Abstract: Reanalysis data are used to evaluate the evolution of polar vortex geometry, planetary wave drag, and gravity wave drag prior to split versus displacement sudden stratospheric warmings (SSWs). A composite analysis that extends upward to the lower mesosphere reveals that split SSWs are characterized by a transition from a wide, funnel-shaped vortex that is anomalously strong to a vortex that is constrained about the pole and has little vertical tilt. In contrast, displacement SSWs are characterized by a wide, funnel-shaped vortex that is anomalously weak throughout the prewarming period. Moreover, during split SSWs, gravity wave drag is enhanced in the polar night jet, while planetary wave drag is enhanced within the extratropical surf zone. During displacement SSWs, gravity wave drag is anomalously weak throughout the extratropical stratosphere. Using the composite analysis as a guide, a case study of the 2009 SSW is conducted in order to evaluate the roles of planetary and gravity waves for preconditioning the polar vortex in terms of two SSW-triggering scenarios: anomalous planetary wave forcing from the troposphere and resonance due to either internal or external Rossby waves. The results support the view that split SSWs are caused by resonance rather than anomalously large wave forcing. Given these findings, it is suggested that vortex preconditioning, which is traditionally defined in terms of vortex geometries that increase poleward wave focusing, may be better described by wave events (planetary and/or gravity) that “tune” the geometry of the vortex toward its resonant excitation points. Link to full paper: https://journals.ametsoc.org/doi/pdf/10.1175/JAS-D-14-0026.1
  22. Upward Wave Activity Flux as a Precursor to Extreme Stratospheric Events and Subsequent Anomalous Surface Weather Regimes Authors: Lorenzo M. Polvani and Darryn W. Waugh Published: 12th January, 2004 Abstract: It has recently been shown that extreme stratospheric events (ESEs) are followed by surface weather anomalies (for up to 60 days), suggesting that stratospheric variability might be used to extend weather prediction beyond current time scales. In this paper, attention is drawn away from the stratosphere to demonstrate that the originating point of ESEs is located in the troposphere. First, it is shown that anomalously strong eddy heat fluxes at 100 hPa nearly always precede weak vortex events, and conversely, anomalously weak eddy heat fluxes precede strong vortex events, consistent with wave–mean flow interaction theory. This finding clarifies the dynamical nature of ESEs and suggests that a major source of stratospheric variability (and thus predictability) is located in the troposphere below and not in the stratosphere itself. Second, it is shown that the daily time series of eddy heat flux found at 100 hPa and integrated over the prior 40 days, exhibit a remarkably high anticorrelation (−0.8) with the Arctic Oscillation (AO) index at 10 hPa. Following Baldwin and Dunkerton, it is then demonstrated that events with anomalously strong (weak) integrated eddy heat fluxes at 100 hPa are followed by anomalously large (small) surface values of the AO index up to 60 days following each event. This suggests that the stratosphere is unlikely to be the dominant source of the anomalous surface weather regimes discussed in Thompson et al. Link to full paper: https://journals.ametsoc.org/doi/pdf/10.1175/1520-0442(2004)017<3548%3AUWAFAA>2.0.CO%3B2
  23. Defining Sudden Stratospheric Warming in Climate Models: Accounting for Biases in Model Climatologies Authors: Junsu Kima et al Published: 4th March, 2017 Abstract: A sudden stratospheric warming (SSW) is often defined as zonal-mean zonal wind reversal at 10 hPa and 60°N. This simple definition has been applied not only to the reanalysis data but also to climate model output. In the present study, it is shown that the application of this definition to models can be significantly influenced by model mean biases (i.e., more frequent SSWs appear to occur in models with a weaker climatological polar vortex). To overcome this deficiency, a tendency-based definition is proposed and applied to the multimodel datasets archived for phase 5 of the Coupled Model Intercomparison Project (CMIP5). In this definition, SSW-like events are defined by sufficiently strong vortex deceleration. This approach removes a linear relationship between SSW frequency and intensity of the climatological polar vortex in the CMIP5 models. The models’ SSW frequency instead becomes significantly correlated with the climatological upward wave flux at 100 hPa, a measure of interaction between the troposphere and stratosphere. Lower stratospheric wave activity and downward propagation of stratospheric anomalies to the troposphere are also reasonably well captured. However, in both definitions, the high-top models generally exhibit more frequent SSWs than the low-top models. Moreover, a hint of more frequent SSWs in a warm climate is found in both definitions. Link to full paper: https://pdfs.semanticscholar.org/c68f/4d487633429caecdaba8eba41c5151557a6f.pdf?_ga=2.215728272.644931620.1527702945-1326629836.1527702945
  24. Planetary‐scale wave activity as a source of varying tropospheric response to stratospheric sudden warming events: A case study Authors: Patrick Martineau and Seok‐Woo Son Published: 3rd October, 2013 Abstract: Stratospheric Sudden Warming (SSW) events are typically, but not always, accompanied by negative Northern Annular Mode anomalies in the troposphere. However, large uncertainties remain as to which dynamical processes are responsible for those anomalies. In order to highlight sources of variability in stratosphere‐troposphere coupling among SSW events, we present a case study of three selected events and show detailed Transformed Eulerian Mean diagnostics for momentum changes in the stratosphere and troposphere in the course of those events. Our results suggest that planetary‐scale waves, especially the zonal wave number 2 component, may play an important role not only for the onset of tropospheric anomalies in response to SSW events but also for introducing variability in the vertical coupling, i.e., whether the tropospheric circulation anomalies lag, lead, or occur simultaneous to the weakening of the vortex. Particularly, the meridional propagation of those waves in the upper troposphere could be an important factor that determines whether SSW events lag or lead tropospheric anomalies. Link to full paper: https://agupubs.onlinelibrary.wiley.com/doi/epdf/10.1002/jgrd.50871
  25. Different ENSO teleconnections and their effects on the stratospheric polar vortex Authors: C. I. Garfinkel and D. L. Hartmann Published: 30th September, 2008 Abstract: Reanalysis data are used to study the El Niño–Southern Oscillation (ENSO) signal in the troposphere and stratosphere during the late fall to midwinter period. Warm ENSO events have extratropical tropospheric teleconnections that increase the wave 1 eddies and reduce the wave 2 eddies, as compared to cold ENSO. The increase in wave 1 overwhelms the decrease in wave 2, so the net effect is a weakened vortex. This modification in tropospheric wave forcing is induced by a deepening of the wintertime Aleutian low via the Pacific–North America pattern (PNA). Model results are also used to verify that the PNA is the primary mechanism through which ENSO modulates the vortex. During easterly Quasi‐Biennial Oscillation (EQBO), warm ENSO does not show a PNA response in the observational record. Consequently, the polar vortex does not show a strong response to the different phases of ENSO under EQBO, nor to the different phases of QBO under WENSO. It is not clear whether the lack of a PNA response to warm ENSO during EQBO is a real physical phenomenon or a feature of the limited data record we have. Link to full paper: https://atmos.washington.edu/~dennis/Garfinkel&Hartmann_JGR_2008.pdf
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