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Found 77 results

  1. The Corresponding Tropospheric Environments during Downward-extending and Non-downward-extending Events of Stratospheric Northern Annular Mode Anomalies Authors: Zhang, Wenshou Tian, Jiankai Zhang, Jinlong Huang, Fei Xie and Mian Xu Published: 25th January, 2019 Abstract: Using the NCEP/NCAR reanalysis dataset, this study classifies stratospheric Northern Annular Mode (NAM) anomalies during negative/positive phase into two categories—anomalies extending into the troposphere (referred as negative/positive TEs) and those not extending into the troposphere (referred as negative/positive NTEs), and the corresponding tropospheric environments during the TEs and NTEs are identified. Compared with that for the negative NTEs, the upward wave fluxes entering the stratosphere are stronger and more persistent during the negative TEs. Furthermore, the stronger and more persistent upward wave fluxes during the negative TEs are due to more favorable conditions for upward wave propagation, which is manifested by fewer occurrences of negative refractive index squared in the mid-high latitude troposphere and stronger wave intensity in the mid-high latitude troposphere. However, the tropospheric wave intensity plays a more important role than the tropospheric conditions of planetary wave-propagation in modulating the upward wave fluxes into the stratosphere. Stronger and more persistent upward wave fluxes in the negative TEs, particularly wave-1 fluxes, are closely related to the negative geopotential height anomalies over the North Pacific and positive geopotential height anomalies over the Euro-Atlantic sectors. These negative/positive geopotential height anomalies over the North Pacific/Euro-Atlantic are related to the positive/negative diabatic heating anomalies and the decreased/increased blocking activities in the mid-high latitudes. The subtropical diabatic heating could also impact on the strength of the mid-high latitude geopotential height anomalies through modulating horizontal wave fluxes. For positive NAM events, the results are roughly similar to those for negative NAM events, but with opposite signal. Link to full paper: https://journals.ametsoc.org/doi/pdf/10.1175/JCLI-D-18-0574.1 https://agupubs.onlinelibrary.wiley.com/doi/epdf/10.1029/2018JD029368
  2. Observed and Simulated Teleconnections Between the Stratospheric Quasi‐Biennial Oscillation and Northern Hemisphere Winter Atmospheric Circulation Authors: Martin B. Andrews , Jeff R. Knight, Adam A. Scaife, Yixiong Lu, Tongwen Wu, Lesley J. Gray and Verena Schenzinger Published: 15th January, 2019 Abstract: The Quasi‐Biennial Oscillation (QBO) is the dominant mode of interannual variability in the tropical stratosphere, with easterly and westerly zonal wind regimes alternating over a period of about 28 months. It appears to influence the Northern Hemisphere winter stratospheric polar vortex and atmospheric circulation near the Earth's surface. However, the short observational record makes unequivocal identification of these surface connections challenging. To overcome this, we use a multicentury control simulation of a climate model with a realistic, spontaneously generated QBO to examine teleconnections with extratropical winter surface pressure patterns. Using a 30‐hPa index of the QBO, we demonstrate that the observed teleconnection with the Arctic Oscillation (AO) is likely to be real, and a teleconnection with the North Atlantic Oscillation (NAO) is probable, but not certain. Simulated QBO‐AO teleconnections are robust, but appear weaker than in observations. Despite this, inconsistency with the observational record cannot be formally demonstrated. To assess the robustness of our results, we use an alternative measure of the QBO, which selects QBO phases with westerly or easterly winds extending over a wider range of altitudes than phases selected by the single‐level index. We find increased strength and significance for both the AO and NAO responses, and better reproduction of the observed surface teleconnection patterns. Further, this QBO metric reveals that the simulated AO response is indeed likely to be weaker than observed. We conclude that the QBO can potentially provide another source of skill for Northern Hemisphere winter prediction, if its surface teleconnections can be accurately simulated. Link to full paper: https://agupubs.onlinelibrary.wiley.com/doi/epdf/10.1029/2018JD029368
  3. Seasonal and Regional Variations of Long-Term Changes in Upper-Tropospheric Jets from Reanalyses Authors: Gloria L. Manney and Michaela I. Hegglin First Published: September 15th, 2017 Published on line: December 19th, 2017 Abstract: Long-term changes in upper-tropospheric jet latitude, altitude, and strength are assessed for 1980–2014 using five modern reanalyses: MERRA, MERRA-2, ERA-Interim, JRA-55, and NCEP CFSR. Changes are computed from jet locations evaluated daily at each longitude to analyze regional and seasonal variations. The changes in subtropical and polar (eddy driven) jets are evaluated separately. Good agreement among the reanalyses in many regions and seasons provides confidence in the robustness of the diagnosed trends. Jet shifts show strong regional and seasonal variations, resulting in changes that are not robust in zonal or annual means. Robust changes in the subtropical jet indicate tropical widening over Africa except during Northern Hemisphere (NH) spring, and tropical narrowing over the eastern Pacific in NH winter. The Southern Hemisphere (SH) polar jet shows a robust poleward shift, while the NH polar jet shifts equatorward in most regions/seasons. Both subtropical and polar jet altitudes typically increase; these changes are more robust in the NH than in the SH. Subtropical jet wind speeds have generally increased in winter and decreased in summer, whereas polar jet wind speeds have weakened (strengthened) over Africa and eastern Asia (elsewhere) during winter in both hemispheres. The Asian monsoon has increased in area and appears to have shifted slightly westward toward Africa. The results herein highlight the importance of understanding regional and seasonal variations when quantifying long-term changes in jet locations, the mechanisms for those changes, and their potential human impacts. Comparison of multiple reanalyses is a valuable tool for assessing the robustness of jet changes. Link to Paper: https://journals.ametsoc.org/doi/pdf/10.1175/JCLI-D-17-0303.1
  4. The key role of background sea surface temperature over the cold tongue in asymmetric responses of the Arctic stratosphere to El Niño–Southern Oscillation Authors: Fei Xie, Xin Zhou, Jianping Li, Cheng Sun, Juan Feng and Xuan Ma Published: Nov 2018 Abstract: The response of the Arctic stratosphere to El Niño activity is strong but the response to La Niña activity is relatively weak. The asymmetric responses of Arctic stratosphere to El Niño and La Niña events are thought to be caused by asymmetric El Niño–Southern Oscillation (ENSO) teleconnections. Here, we suggest that the background sea surface temperature (SST) over cold tongue of tropical eastern Pacific may be an important contributor to the asymmetric ENSO teleconnections. The atmosphere is very sensitive to tropical SST variations in the range of 26 °C–30 °C. During El Niño events, the background SST over cold tongue plus El Niño SST anomalies typically falls into the range. Under these conditions, the atmospheric response to El Niño SST anomalies is strong. During La Niña events, the background SST plus La Niña SST anomalies is typically below the range, which leads to a weak response of the atmosphere to SST anomalies. The proposed mechanism is well supported by simulations. Link to full paper: http://iopscience.iop.org/article/10.1088/1748-9326/aae79b/meta
  5. Why CO2 cools the middle atmosphere – a consolidating model perspective Authors: Helge F. Goessling and Sebastian Bathiany Published: 29th August, 2016 Abstract: Complex models of the atmosphere show that increased carbon dioxide (CO2) concentrations, while warming the surface and troposphere, lead to lower temperatures in the stratosphere and mesosphere. This cooling, which is often referred to as “stratospheric cooling”, is evident also in observations and considered to be one of the fingerprints of anthropogenic global warming. Although the responsible mechanisms have been identified, they have mostly been discussed heuristically, incompletely, or in combination with other effects such as ozone depletion, leaving the subject prone to misconceptions. Here we use a one-dimensional window-grey radiation model of the atmosphere to illustrate the physical essence of the mechanisms by which CO2 cools the stratosphere and mesosphere: (i) the blocking effect, associated with a cooling due to the fact that CO2 absorbs radiation at wavelengths where the atmosphere is already relatively opaque, and (ii) the indirect solar effect, associated with a cooling in places where an additional (solar) heating term is present (which on Earth is particularly the case in the upper parts of the ozone layer). By contrast, in the grey model without solar heating within the atmosphere, the cooling aloft is only a transient blocking phenomenon that is completely compensated as the surface attains its warmer equilibrium. Moreover, we quantify the relative contribution of these effects by simulating the response to an abrupt increase in CO2 (and chlorofluorocarbon) concentrations with an atmospheric general circulation model. We find that the two permanent effects contribute roughly equally to the CO2-induced cooling, with the indirect solar effect dominating around the stratopause and the blocking effect dominating otherwise. Link to full paper: https://www.earth-syst-dynam.net/7/697/2016/esd-7-697-2016.pdf Credit goes to Eric @Webberweather for finding this paper - thank you.
  6. 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
  7. Nonlinearity of the combined warm ENSO and QBO effects on the Northern Hemisphere polar vortex in MAECHAM5 simulations Authors: Natalia Calvo, Marco A. Giorgetta, Ricardo Garcia‐Herrera and Elisa Manzini Published: 14th July, 2009 Abstract: The influence of the quasi‐biennial oscillation (QBO) on the Northern Hemisphere (NH) polar vortex response to warm El Niño–Southern Oscillation (ENSO) events and the impact of the warm ENSO events on the QBO signal in the NH polar stratosphere have been analyzed using the Middle Atmosphere ECHAM5 model. The experiment setup was designed to include simulations of extended NH winter seasons for either strong easterly or strong westerly phases of the tropical QBO, forced with either sea surface temperatures (SSTs) from the strong ENSO event that occurred in 1997/1998 or with climatological SSTs. It has been found that the weakening and warming of the polar vortex associated with a warm ENSO are intensified at the end of the winter during both QBO phases. In addition, the westerly QBO phase delays the onset of the warm ENSO signal, while the easterly QBO phase advances it. Warm ENSO events also impact the extratropical signal of the QBO by intensifying (weakening) the QBO effects in early (late) winter. Therefore, it appears that during warm ENSO events the duration of QBO signal in the northern extratropics is shortened while its downward propagation accelerated. Our dynamical analysis has revealed that these results are due to changes in the background flow caused by the QBO combined with changes in the anomalous propagation and dissipation of extratropical waves generated by warm ENSO. In both cases, a nonlinear behavior in the response of the polar vortex is observed when both warm ENSO and the easterly phase of the QBO operate together. These results suggest that the Arctic polar vortex response to combined forcing factors, in our case warm ENSO and the QBO phenomena, is expected to be nonlinear also for other coexistent forcing factors able to affect the variability of the vortex in the stratosphere. Link to full paper: https://agupubs.onlinelibrary.wiley.com/doi/epdf/10.1029/2008JD011445
  8. Stratospheric role in interdecadal changes of El Niño impacts over Europe Authors: B. Ayarzagüena, J. López-Parages, M. Iza, N. Calvo and B. Rodríguez-Fonseca Published: 2nd April, 2018 Abstract: The European precipitation response to El Niño (EN) has been found to present interdecadal changes, with alternated periods of important or negligible EN impact in late winter. These periods are associated with opposite phases of multi-decadal sea surface temperature (SST) variability, which modifies the tropospheric background and EN teleconnections. In addition, other studies have shown how SST anomalies in the equatorial Pacific, and in particular, the location of the largest anomalous SST, modulate the stratospheric response to EN. Nevertheless, the role of the stratosphere on the stationarity of EN response has not been investigated in detail so far. Using reanalysis data, we present a comprehensive study of EN teleconnections to Europe including the role of the ocean background and the stratosphere in the stationarity of the signal. The results reveal multidecadal variability in the location of EN-related SST anomalies that determines different teleconnections. In periods with relevant precipitation signal over Europe, the EN SST pattern resembles Eastern Pacific EN and the stratospheric pathway plays a key role in transmitting the signal to Europe in February, together with two tropospheric wavetrains that transmit the signal in February and April. Conversely, the stratospheric pathway is not detected in periods with a weak EN impact on European precipitation, corresponding to EN-related SST anomalies primarily located over the central Pacific. SST mean state and its associated atmospheric background control the location of EN-related SST anomalies in different periods and modulate the establishment of the aforementioned stratospheric pathway of EN teleconnection to Europe too. Link to full paper: https://link.springer.com/content/pdf/10.1007%2Fs00382-018-4186-3.pdf
  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. A stratospheric pathway linking a colder Siberia to Barents-Kara Sea sea ice loss Authors: Pengfei Zhang, Yutian Wu, Isla R. Simpson, Karen L. Smith, Xiangdong Zhang, Bithi De and Patrick Callaghan Published: July 2018 Abstract: Previous studies have extensively investigated the impact of Arctic sea ice anomalies on the midlatitude circulation and associated surface climate in winter. However, there is an ongoing scientific debate regarding whether and how sea ice retreat results in the observed cold anomaly over the adjacent continents. We present a robust “cold Siberia” pattern in the winter following sea ice loss over the Barents-Kara seas in late autumn in an advanced atmospheric general circulation model, with a well-resolved stratosphere. Additional targeted experiments reveal that the stratospheric response to sea ice forcing is crucial in the development of cold conditions over Siberia, indicating the dominant role of the stratospheric pathway compared with the direct response within the troposphere. In particular, the downward influence of the stratospheric circulation anomaly significantly intensifies the ridge near the Ural Mountains and the trough over East Asia. The persistently intensified ridge and trough favor more frequent cold air outbreaks and colder winters over Siberia. This finding has important implications for improving seasonal climate prediction of mid-latitude cold events. The results also suggest that the model performance in representing the stratosphere-troposphere coupling could be an important source of the discrepancy between recent studies. Link to full paper: http://advances.sciencemag.org/content/4/7/eaat6025
  12. Upper-Tropospheric Static Stability in Tropical Cyclones: Observations and Modeling - Presentation A presentation at the AMS 33rd Conference on “Hurricanes and Tropical Meteorology” held at Ponte Vedra, Florida, USA between 16th and 20th April, 2018 Presenters: Patrick Duran and J. Molinari Presentation Date: 19th April, 2018  Presentation Summary: Upper-tropospheric thermodynamic processes play an important role in tropical cyclone (TC) structure and evolution. Until recently, however, few observations existed within this upper-level region of TCs. High-altitude dropsonde observations from two field campaigns – the Office of Naval Research Tropical Cyclone Intensity Experiment (TCI) and the NASA Hurricane and Severe Storm Sentinel (HS3) – have provided datasets of unprecedented resolution in the upper troposphere and lower stratosphere. These observations reveal large static stability variations with both space and time in the upper levels of TCs. The upper troposphere within TCs tends to be characterized by small static stability, with temperature lapse rates close to dry-adiabatic and squared Brunt-Vӓisälä frequency (N2) on the order of 10-5 s-2. Just above the cold-point tropopause, however, exists a distinct static stability maximum, with N2 in some places exceeding 10-3 s-2. This lower-stratospheric stable layer is particularly pronounced within the TC cirrus canopy. A secondary, typically weaker, static stability maximum also can appear 1-2 km below the tropopause in the vicinity of TCs. The variability observed in the dropsondes is simulated in an idealized, axisymmetric framework using Cloud Model 1 (CM1). As the simulated hurricane intensifies, static stability strengthens in the lower stratosphere, forming a shallow layer of particularly large N2 just above the cold-point tropopause. A budget analysis of N2 reveals that the strengthening of this lower-stratospheric stable layer is related to the upward growth of the TC circulation, which introduces strong vertical gradients of potential temperature tendencies due to radiative cooling and turbulent mixing. The secondary stability maximum 1-2 km below the tropopause appears to arise due to differential potential temperature advection within the upper-level outflow layer, combined with strong localized turbulent mixing in a shallow region of particularly strong vertical wind shear.  Link to conference video presentation (16 minutes): https://ams.confex.com/ams/33HURRICANE/videogateway.cgi/id/46871?recordingid=46871&uniqueid=Paper339247&entry_password=108870 Link to Manuscirpt: file:///C:/Users/David/Downloads/2018_Duran&Molinari_AMStropical_extended_abstract.pdf (you may need to open in a new tab) Link to full conference agenda: https://ams.confex.com/ams/33HURRICANE/webprogram/33HURRICANE.html
  13. Relationship Between Phase of the Quasi-Biennial Oscillation (QBO) and MJO Propagation Through the Maritime Continent - Presentation A presentation at the AMS 33rd Conference on “Hurricanes and Tropical Meteorology” held at Ponte Vedra, Florida, USA between 16th and 20th April, 2018 Presenters: Casey R Densmore, B. S. Barrett, E. R. Sanabia and P. Ray Presentation Date: 18th April, 2018 Presentation Summary: One region particularly favorable for enhanced convective activity during the Madden Julian Oscillation (MJO) active phase is the Maritime Continent (MC). As the ascending branch of the MJO envelope reaches the MC, it sometimes propagates eastward and reaches the Western Pacific Ocean (a propagating event). However, the convective envelope may also decouple from zonal wind anomalies and weaken over the MC, not reaching the Western Pacific Ocean (a non-propagating event). Propagation of the MJO across the MC is currently an active area of research. In this study, anomalies of specific humidity, lower and upper tropospheric zonal (u) and meridional (v) wind components, geopotential height, and temperature from the European Centre for Long-Range Weather Forecasts (ECMWF) ERA-Interim Reanalysis were compared for propagating and non-propagating MJO events. Equatorial zonal wind anomalies over the Maritime Continent at 50 hPa were used to define QBO phase. The Wheeler-Hendon Realtime Multivariate (RMM) MJO Index was used to classify and categorize the geographic location (e.g. phase) and intensity (e.g. amplitude). Of particular emphases were differences in the mean atmospheric states for easterly, westerly, and neutral QBO phases for different MJO propagating events. The goal of this work was to better understand the physical mechanisms that favor MJO propagation across the MC. Link to conference video presentation (15 minutes): https://ams.confex.com/ams/33HURRICANE/videogateway.cgi/id/46828?recordingid=46828&uniqueid=Paper339768&entry_password=942250 Link to full conference agenda: https://ams.confex.com/ams/33HURRICANE/webprogram/33HURRICANE.html
  14. Breaking planetary waves in the stratosphere Authors: M. E. McIntyre and T. N. Palmer Published: 13th October, 1983 Abstract: Satellite-borne IR radiometers are turning the Earth's stratosphere into one of the best available outdoor laboratories for observing the large-scale dynamics of a rotating, heterogeneous fluid under gravity. New insight is being gained not only into stratospheric dynamics as such, with its implications for pollutant behaviour and the ozone layer, but also indirectly into the dynamics of the troposphere, with its implications for weather forecasting. Similar dynamical regimes occur in the oceans and in stellar interiors. A key development has been the construction of coarse-grain maps of the large-scale distribution of potential vorticity in the middle stratosphere. Potential vorticity is a conservable quantity which has a central role in the dynamical theory, but is difficult to calculate accurately from observational data. We present the first mid-stratospheric potential vorticity maps which appear good enough to make visible the ‘breaking’ of planetary or Rossby waves, a phenomenon ubiquitous in nature and arguably one of the most important dynamical processes affecting the stratosphere as a whole. Link to full paper: http://www-atm.damtp.cam.ac.uk/people/mem/mp83-scanned.pdf
  15. Effects of stratospheric variability on El Niño teleconnections Authors: J H Richter, C Deser and L Sun Published: 17th December, 2015 Abstract: The effects of the tropical Pacific El Niño Southern Oscillation (ENSO) phenomenon are communicated to the rest of the globe via atmospheric teleconnections. Traditionally, ENSO teleconnections have been viewed as tropospheric phenomena, propagating to higher latitudes as Rossby waves. Recent studies, however, suggest an influence of the stratosphere on extra-tropical ENSO teleconnections. The stratosphere is highly variable: in the tropics, the primary mode of variability is the quasi-biennial oscillation (QBO), and in the extra-tropics sudden stratospheric warmings (SSWs) regularly perturb the mean state. Here, we conduct a 10-member ensemble of simulations with a stratosphere-resolving atmospheric general circulation model forced with the observed evolution of sea surface temperatures during 1952–2001 to examine the effects of the QBO and SSWs on the zonal-mean circulation and temperature response to El Niño, with a focus on the northern extra-tropics during winter. We find that SSWs have a larger impact than the QBO on the composite El Niño responses. During El Niño winters with SSWs, the polar stratosphere shows positive temperature anomalies that propagate downward to the surface where they are associated with increased sea-level pressure over the Arctic. During El Niño winters without SSWs, the stratosphere and upper troposphere show negative temperature anomalies but these do not reach the surface. The QBO modulates the El Niño teleconnection primarily in winters without SSWs: the negative temperature anomalies in the polar stratosphere and upper troposphere are twice as large during QBO West compared to QBO East years. In addition, El Niño winters that coincide with the QBO West phase show stronger positive sea-level pressure anomalies over the eastern Atlantic and Northern Europe than those in the QBO East phase. The results imply that the stratosphere imparts considerable variability to ENSO teleconnections. Link to full paper: http://iopscience.iop.org/article/10.1088/1748-9326/10/12/124021
  16. The Karakoram/Western Tibetan vortex: seasonal and year-to-year variability Authors: Xiao-Feng Li, Hayley J. Fowler, Nathan Forsythe, Stephen Blenkinsop and David Pritchard Published: 21st February, 2018 Abstract: The “Karakoram Vortex” (KV), hereafter also referred to as the “Western Tibetan Vortex” (WTV), has recently been recognized as a large-scale atmospheric circulation system related to warmer (cooler) near-surface and mid-lower troposphere temperatures above the Karakoram in the western Tibetan Plateau (TP). It is characterized by a deep, anti-cyclonic (cyclonic) wind anomaly associated with higher (lower) geopotential height in the troposphere, during winter and summer seasons. In this study, we further investigate the seasonality and basic features of the WTV in all four seasons, and explore its year-to-year variability and influence on regional climate. We find the WTV accounts for the majority of year-to-year circulation variability over the WTP as it can explain over 50% (R2⩾0.5R2⩾0.5) variance of the WTP circulation on multiple levels throughout the troposphere, which declines towards the eastern side of the TP in most seasons. The WTV is not only more (less) active but also has a bigger (smaller) domain area, with a deeper (shallower) structure, in winter and spring (summer and autumn). We find that the WTV is sensitive to both the location and intensity of the Subtropical Westerly Jet (SWJ), but the relationship is highly dependent on the climatological mean location of SWJ axes relative to the TP in different seasons. We also show that the WTV significantly modulates surface and stratospheric air temperatures, north–south precipitation patterns and total column ozone surrounding the western TP. As such, the WTV has important implications for the understanding of atmospheric, hydrological and glaciological variability over the TP. Link to full paper: https://link.springer.com/content/pdf/10.1007%2Fs00382-018-4118-2.pdf
  17. 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
  18. 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
  19. Influence of Arctic sea ice on the North Atlantic Oscillation Workshop Presentation: Aspen Global Change Institute, Aspen, Colorado Workshop Programme: “Understanding the Causes and Consequences of Polar Amplification” - June 12th -16th, 2017 Presenter: James Screen Presentation Date: 14th June, 2017 Link to full presentation (38 minute video): https://www.agci.org/lib/17s1/influence-arctic-sea-ice-north-atlantic-oscillation Link to presentation (slides and charts only): https://www.agci.org/sites/default/files/pdfs/lib/main/AGCI_2017_Screen.pdf Link to full agenda and presentations: https://www.agci.org/event/17s1
  20. Sudden Stratospheric Warmings and Anomalous Upward Wave Activity Flux Authors: Thomas Birner, John R. Albers Published: 28th, June, 2017 Abstract: Abrupt breakdowns of the polar winter stratospheric circulation such as sudden stratospheric warmings (SSWs) are a manifestation of strong two-way interactions between upward propagating planetary waves and the mean flow. The importance of sufficient upward wave activity fluxes from the troposphere and the preceding state of the stratospheric circulation in forcing SSW-like events have long been recognized. Past research based on idealized numerical simulations has suggested that the state of the stratosphere may be more important in generating extreme stratospheric events than anomalous upward wave fluxes from the troposphere. Other studies have emphasized the role of tropospheric precursor events. Here reanalysis data are used to define events of extreme stratospheric mean flow deceleration (SSWs being a subset) and events of extreme lower tropospheric upward planetary wave activity flux. While the wave fluxes leading to SSW-like events ultimately originate near the surface, the anomalous upward wave activity fluxes associated with these events primarily occur within the stratosphere. The crucial dynamics for forcing SSW-like events appear to take place in the communication layer just above the tropopause. Anomalous upward wave fluxes from the lower troposphere may play a role for some events, but seem less important for the majority of them. Link to full paper: https://www.jstage.jst.go.jp/article/sola/13A/Special_Edition/13A_13A-002/_pdf/-char/en
  21. 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
  22. 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
  23. On the Relationship between ENSO, Stratospheric Sudden Warmings and Blocking Authors: David Barriopedro and Natalia Calvo Published: 8th March, 2014 Abstract: This paper examines the influence of El Niño–Southern Oscillation (ENSO) on different aspects of major stratospheric sudden warmings (SSWs), focusing on the precursor role of blocking events. The results reveal an ENSO modulation of the blocking precursors of SSWs. European and Atlantic blocks tend to precede SSWs during El Niño (EN), whereas eastern Pacific and Siberian blocks are the preferred precursors of SSWs during La Niña (LN) winters. This ENSO preference for different blocking precursors seems to occur through an ENSO effect on regional blocking persistence, which in turn favors the occurrence of SSWs. The regional blocking precursors of SSWs during each ENSO phase also have different impacts on the upward propagation of planetary-scale wavenumbers 1 and 2; hence, they determine ENSO differences in the wavenumber signatures of SSWs. SSWs occurring during EN are preceded by amplification of wavenumber 1, whereas LN SSWs are predominantly associated to wavenumber-2 amplification. However, there is not a strong preference for splitting or displacement SSWs during any ENSO phase. This is mainly because during EN, splitting SSWs do not show a wavenumber-2 pattern. Link to full paper: https://journals.ametsoc.org/doi/pdf/10.1175/JCLI-D-13-00770.1
  24. Extraordinary Features of Planetary Wave Propagation During the Boreal Winter 2013/2014 & Zonal Wave Number 2 Predominance Authors: Y. Harada and T. Hirooka Published: 12th October, 2017 Abstract: Observational features of the winter 2013/2014 are investigated using of the Japanese 55‐year Reanalysis data. This winter can be characterized by the continuous predominance of planetary waves of zonal wave number two (WN2) that did not cause major sudden stratospheric warming (SSW) events. It is found that the vertical component of the Eliassen‐Palm flux of WN2 for the winter 2013/2014 is almost equal to the highest value of the winter 2008/2009. The longitudinal distribution of vertical components of Plumb wave activity flux for this winter shows marked downward propagation around 100°W and upward propagation around 60°E, both of which are the strongest of their type among the 56 winters since 1958/1959. The convergence of wave packets propagating from around 60°E contributes to the development and continuance of the quasi‐barotropic Aleutian High, which is associated with the extension of negative extended refractive index (Ks) region. The extension of negative Ks region is related to the convergence or reflection of the wave packets emanating from tropospheric blocking highs developing in the North Pacific Ocean; the development and continuance of the quasi‐barotropic Aleutian High is considered to be one of plausible reasons for the lack of major SSWs in the winter 2013/2014. In addition to these results, we revealed the significant contribution of smaller scale waves (with a zonal wave number of three or more) to the structure of localized wave packet propagation in the stratosphere. Link to full paper (workshop presentation): https://events.oma.be/indico/event/6/material/slides/6.pdf
  25. The preconditioning of major sudden stratospheric warmings Authors: S. Bancalá, K. Krüger and M. Giorgetta Published: 16th February, 2012 Abstract: The preconditioning of major sudden stratospheric warmings (SSWs) is investigated with two long time series using reanalysis (ERA‐40) and model (MAECHAM5/MPI‐OM) data. Applying planetary wave analysis, we distinguish between wavenumber‐1 and wavenumber‐2 major SSWs based on the wave activity of zonal wavenumbers 1 and 2 during the prewarming phase. For this analysis an objective criterion to identify and classify the preconditioning of major SSWs is developed. Major SSWs are found to occur with a frequency of six and seven events per decade in the reanalysis and in the model, respectively, thus highlighting the ability of MAECHAM5/MPI‐OM to simulate the frequency of major SSWs realistically. However, from these events only one quarter are wavenumber‐2 major warmings, representing a low (∼0.25) wavenumber‐2 to wavenumber‐1 major SSW ratio. Composite analyses for both data sets reveal that the two warming types have different dynamics; while wavenumber‐1 major warmings are preceded only by an enhanced activity of the zonal wavenumber‐1, wavenumber‐2 events are either characterized by only the amplification of zonal wavenumber‐2 or by both zonal wavenumber‐1 and zonal wavenumber‐2, albeit at different time intervals. The role of tropospheric blocking events influencing these two categories of major SSWs is evaluated in the next step. Here, the composite analyses of both reanalysis and model data reveal that blocking events in the Euro‐Atlantic sector mostly lead to the development of wavenumber‐1 major warmings. The blocking–wavenumber‐2 major warming connection can only be statistical reliable analyzed with the model time series, demonstrating that blocking events in the Pacific region mostly precede wavenumber‐2 major SSWs. Link to full paper: https://pdfs.semanticscholar.org/5d99/9c51d49c694a91bf2ec092927ee60920e529.pdf
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