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Found 81 results

  1. Dr. Yanai’s Contributions to the Discovery and Science of the MJO Authors: Eric D. Maloney and Chidong Zhang First Published: 5th May, 2016 Abstract: This chapter reviews Professor Michio Yanai’s contributions to the discovery and science of the Madden–Julian oscillation (MJO). Professor Yanai’s work on equatorial waves played an inspirational role in the MJO discovery by Roland Madden and Paul Julian. Professor Yanai also made direct and important contributions to MJO research. These research contributions include work on the vertically integrated moist static energy budget, cumulus momentum transport, eddy available potential energy and eddy kinetic energy budgets, and tropical–extratropical interactions. Finally, Professor Yanai left a legacy through his students, who continue to push the bounds of MJO research. Link to full paper: https://journals.ametsoc.org/doi/pdf/10.1175/AMSMONOGRAPHS-D-15-0003.1
  2. 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
  3. MJO Phase Speed and Blocking - Presentation A presentation at the AMS 97th Annual meeting: Conference on “Fifth Symposium on Prediction of the MJO: Processes, Prediction and Impact” held at Washington State Convention Center between 23rd and 25th January, 2017 Presenters: Paul E. Roundy and R. M. Setzenfand Presentation Date: 23rd January, 2017 Presentation Summary: The phase speed of the MJO might be regulated by many different factors. Previous works have suggested that moist processes govern the phase speed. Yet, convection and rainfall tend to be less intense in MJO events propagating more slowly than 5 ms−1 than for MJO events moving at around 5 ms−1. This presentation reflects on a dynamical feedback that might influence MJO phase speed: Rossby wave breaking and blocking. A wavelet filter is applied to extract time series characterized by selected zonal wavenumbers and frequencies at select equatorial base longitudes. Results show that anomalies of active convection characterized by wavenumber 2 (the dominant scale of MJO convection over the warm pool) are associated with meridional potential vorticity (PV) gradients across the tropics to the east of the active convection that are near climatology for events moving east at 5 ms−1. These gradients are much weaker for slower events. The slowest phase speed events have almost no meridional PV gradients across the tropics between the mean latitudes of the subtropical jet streams, suggesting that jet exit regions occur immediately east of the deep convection, dumping mass in the upper troposphere over the region of suppressed convection. In the absence of PV gradients, synoptic to planetary scale waves moving into that environment break or cease to propagate linearly. This analysis is part of broader study of the association of the global atmospheric circulation with the phase speed of MJO convection. Link to conference video presentation (15 minutes): https://ams.confex.com/ams/97Annual/videogateway.cgi/id/37087?recordingid=37087&uniqueid=Paper302240&entry_password=987428 Link to full conference agenda: https://ams.confex.com/ams/97Annual/webprogram/5MJO.html Credit goes to Eric @Webberweather for finding this presentation - thank you.
  4. Circulation Response to Fast and Slow MJO Episodes Authors: Priyanka Yadav and David M. Straus Published: 6th April, 2017 Abstract: Fast and slow Madden–Julian oscillation (MJO) episodes have been identified from 850- and 200-hPa zonal wind and outgoing longwave radiation (OLR) for 32 winters (16 October–17 March) 1980/81–2011/12. For 26 fast cases the OLR took no more than 10 days to propagate from phase 3 (convection over the Indian Ocean) to phase 6 (convection over the western Pacific). For 8 slow cases the propagation took at least 20 days. Fast episode composite anomalies of 500-hPa height (Z500) show a developing Rossby wave in the mid-Pacific with downstream propagation through MJO phases 2–4. Changes in the frequency of occurrence of the NAO+ weather regime are modest. This Rossby wave is forced by anomalous cooling over the Maritime Continent during phases 2 and 3 (seen in phase-independent wave activity flux). The upper-level anticyclonic response to phase-3 heating is a secondary source of wave activity. The Z500 slow episode composite response to MJO phases 1 and 2 is an enhanced Aleutian low followed by a North American continental high. Following phase 4 the development of an NAO+ like pattern is seen over the Atlantic, transitioning to a strong NAO− pattern by phase 8. A dramatic increase in frequency of the NAO+ weather regime follows phases 4 and 5, while a strong increase in NAO− regime follows phases 6 and 7. The responses to MJO-related heating and cooling over the Indian and western Pacific Oceans in phases 1–4 provide a source for wave activity propagating to North America, augmented by storm-track anomalies. Link to full paper: https://journals.ametsoc.org/doi/pdf/10.1175/MWR-D-16-0352.1 Credit goes to Eric @Webberweather for finding this paper - thank you.
  5. Modulation of equatorial Pacific westerly/easterly wind events by the MJO and convectively‑coupled Rossby waves Authors: Martin Puy, J. Vialard, M. Lengaigne and E. Guilyardi Published: 16th June, 2015 Abstract: Synoptic wind events in the equatorial Pacific strongly influence the El Niño/Southern Oscillation (ENSO) evolution. This paper characterizes the spatio-temporal distribution of Easterly (EWEs) and Westerly Wind Events (WWEs) and quantifies their relationship with intraseasonal and interannual large-scale climate variability. We unambiguously demonstrate that the Madden–Julian Oscillation (MJO) and Convectively-coupled Rossby Waves (CRW) modulate both WWEs and EWEs occurrence probability. 86 % of WWEs occur within convective MJO and/or CRW phases and 83 % of EWEs occur within the suppressed phase of MJO and/or CRW. 41 % of WWEs and 26 % of EWEs are in particular associated with the combined occurrence of a CRW/MJO, far more than what would be expected from a random distribution (3 %). Wind events embedded within MJO phases also have a stronger impact on the ocean, due to a tendency to have a larger amplitude, zonal extent and longer duration. These findings are robust irrespective of the wind events and MJO/CRW detection methods. While WWEs and EWEs behave rather symmetrically with respect to MJO/CRW activity, the impact of ENSO on wind events is asymmetrical. The WWEs occurrence probability indeed increases when the warm pool is displaced eastward during El Niño events, an increase that can partly be related to interannual modulation of the MJO/CRW activity in the western Pacific. On the other hand, the EWEs modulation by ENSO is less robust, and strongly depends on the wind event detection method. The consequences of these results for ENSO predictability are discussed. Link to full guide: https://www.researchgate.net/profile/Eric_Guilyardi/publication/278682276_Modulation_of_equatorial_Pacific_westerlyeasterly_wind_events_by_the_Madden-Julian_oscillation_and_convectively-coupled_Rossby_waves/links/568d30f608aef987e565dcb5.pdf
  6. The Role of Downward Infrared Radiation in Arctic Amplification - Presentation Workshop Presentation: Aspen Global Change Institute, Aspen, Colorado Workshop Programme: “Understanding the Causes and Consequences of Polar Amplification” - June 12th -16th, 2017 Presenters: Steven Feldstein (+ Tingting Gong and Sukyoung Lee) Presentation Date: 12th June, 2017 Abstract: None Link to full video presentation (27 minutes): https://www.agci.org/lib/17s1/role-downward-infrared-radiation-arctic-amplification Link to presentation (slides and charts only): https://www.agci.org/sites/default/files/pdfs/lib/main/Feldstein_2017_0611.pdf Link to full agenda and presentations: https://www.agci.org/event/17s1
  7. 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
  8. Importance of Late Fall ENSO Teleconnection in the Euro-Atlantic Sector Authors: Martin P. King, Ivana Herceg-Bulić, Ileana Bladé, Javier García-Serrano, Noel Keenlyside, Fred Kucharski, Camille Li and Stefan Sobolowski Published: 23rd July, 2018 Abstract: Recent studies have indicated the importance of fall climate forcings and teleconnections in influencing the climate of the northern mid- to high latitudes. Here, we present some exploratory analyses using observational data and seasonal hindcasts, with the aim of highlighting the potential of the El Niño–Southern Oscillation (ENSO) as a driver of climate variability during boreal late fall and early winter (November and December) in the North Atlantic–European sector, and motivating further research on this relatively unexplored topic. The atmospheric ENSO teleconnection in November and December is reminiscent of the east Atlantic pattern and distinct from the well-known arching extratropical Rossby wave train found from January to March. Temperature and precipitation over Europe in November are positively correlated with the Niño-3.4 index, which suggests a potentially important ENSO climate impact during late fall. In particular, the ENSO-related temperature anomaly extends over a much larger area than during the subsequent winter months. We discuss the implications of these results and pose some research questions. Link to full paper: https://journals.ametsoc.org/doi/pdf/10.1175/BAMS-D-17-0020.1
  9. 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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  10. Are Multiple Tropical Cyclone Events Similar among Basins? Authors: Benjamin A. Schenkel Published: 22nd May, 2017 Abstract: The present study intercompares multiple tropical cyclone event (MTCE) characteristics among each global tropical cyclone (TC) basin using best-track data. Specific focus is placed on examining the number of MTCEs and TCs during MTCEs, the zonal distance between TCs during MTCEs, and the spatiotemporal separation between genesis events during MTCEs. The results suggest that the ratio of MTCEs relative to single TCs is substantially higher in the eastern North Pacific (ENP), western North Pacific (WNP), and south Indian Ocean (SI) basins compared to the North Atlantic (NA) and South Pacific (SP). The prolific nature of ENP, WNP, and SI MTCE activity results in approximately half of TCs occurring during MTCEs. During new TC genesis, the majority of preexisting TCs are generally located westward at a consistent zonal distance from new TC genesis for MTCEs within each basin with median values between −1620 and −1961 km. TC-induced Rossby wave dispersion may set this zonal length scale as implied by its moderate-to-strong correlations (R = 0.38–0.85; p < 0.05) with the shallow-water zonal wavelength of TC-induced stationary Rossby waves. A substantial majority of TC genesis events occur progressively eastward during ENP, WNP, and SP MTCEs, whereas NA and SI MTCEs exhibit no such tendency. Last, the temporal separation between the genesis of preexisting and new TCs is generally similar among basins with median values between 3 and 4 days. Together, these results are indicative of unusual similarity in MTCE characteristics among basins despite differences in environmental and TC characteristics in each basin. Link to full paper: https://journals.ametsoc.org/doi/pdf/10.1175/JCLI-D-17-0088.1
  11. Kelvin Waves and Tropical Cyclogenesis: A Global Survey Authors: Carl J. Schreck III Published: 15th June, 2015 Abstract: Convectively coupled atmospheric Kelvin waves are among the most prominent sources of synoptic-scale rainfall variability in the tropics, but large uncertainties surround their role in tropical cyclogenesis. This study identifies the modulation of tropical cyclones relative to the passage of a Kelvin wave’s peak rainfall (i.e., its crest) in each basin. Tropical cyclogenesis is generally inhibited for 3 days before the crest and enhanced for 3 days afterward. Composites of storms forming in the most favorable lags illustrate the dynamical impacts of the waves. In most basins, the tropical cyclone actually forms during the convectively suppressed phase of the wave. The 850-hPa equatorial westerly anomalies provide the cyclonic vorticity for the nascent storm, and 200-hPa easterly anomalies enhance the outflow. The wind anomalies persist at both levels longer than the Kelvin wave’s period and are often related to the Madden–Julian oscillation (MJO). The onset of these wind anomalies occurs with the Kelvin wave passage, while the MJO apparently establishes their duration. Many of the composites also show evidence of an easterly wave from which the tropical cyclone develops. The composite easterly wave amplifies or even initiates within the Kelvin wave crest. These results show the importance of Kelvin waves interacting with the MJO and easterly waves during tropical cyclogenesis. Given that Kelvin waves often circumnavigate the globe, these results show promise for long-range forecasting of tropical cyclogenesis in all basins. Link to full paper: https://journals.ametsoc.org/doi/pdf/10.1175/MWR-D-15-0111.1
  12. Subseasonal Forecasts of Convectively Coupled Equatorial Waves and the MJO: Activity and Predictive Skill Authors: Matthew A. Janiga, Carl J. Schreck III, James A. Ridout, Maria Flatau, Neil P. Barton, E. Joseph Metzger and Carolyn A. Reynolds Published: 30th March, 2018 (published online; 16th July, 2018) Abstract: In this study, the contribution of low-frequency (>100 days), Madden–Julian oscillation (MJO), and convectively coupled equatorial wave (CCEW) variability to the skill in predicting convection and winds in the tropics at weeks 1–3 is examined. We use subseasonal forecasts from the Navy Earth System Model (NESM); NCEP Climate Forecast System, version 2 (CFSv2); and ECMWF initialized in boreal summer 1999–2015. A technique for performing wavenumber–frequency filtering on subseasonal forecasts is introduced and applied to these datasets. This approach is better able to isolate regional variations in MJO forecast skill than traditional global MJO indices. Biases in the mean state and in the activity of the MJO and CCEWs are smallest in the ECMWF model. The NESM overestimates cloud cover as well as MJO, equatorial Rossby, and mixed Rossby–gravity/tropical depression activity over the west Pacific. The CFSv2 underestimates convectively coupled Kelvin wave activity. The predictive skill of the models at weeks 1–3 is examined by decomposing the forecasts into wavenumber–frequency signals to determine the modes of variability that contribute to forecast skill. All three models have a similar ability to simulate low-frequency variability but large differences in MJO skill are observed. The skill of the NESM and ECMWF model in simulating MJO-related OLR signals at week 2 is greatest over two regions of high MJO activity, the equatorial Indian Ocean and Maritime Continent, and the east Pacific. The MJO in the CFSv2 is too slow and too weak, which results in lower MJO skill in these regions. Link to full paper: https://journals.ametsoc.org/doi/pdf/10.1175/MWR-D-17-0261.1
  13. Climate science in the tropics: waves, vortices and PDEs Authors: Boualem Khouider, Andrew J Majda and Samuel N Stechmann Published: 16th November, 2012 Abstract: Clouds in the tropics can organize the circulation on planetary scales and profoundly impact long range seasonal forecasting and climate on the entire globe, yet contemporary operational computer models are often deficient in representing these phenomena. On the other hand, contemporary observations reveal remarkably complex coherent waves and vortices in the tropics interacting across a bewildering range of scales from kilometers to ten thousand kilometers. This paper reviews the interdisciplinary contributions over the last decade through the modus operandi of applied mathematics to these important scientific problems. Novel physical phenomena, new multiscale equations, novel PDEs, and numerical algorithms are presented here with the goal of attracting mathematicians and physicists to this exciting research area. Link to full paper: https://www.math.nyu.edu/faculty/majda/pdfFiles/Nonlinearity_MajdaKhouiderStechmann_3.17.12.pdf
  14. Absolute and Convective Instability of the African Easterly Jet Authors: Michael Diaz and Anantha Aiyyer Published: 28th January, 2015 Abstract: The stability of the African easterly jet (AEJ) is examined using idealized numerical simulations. It is found that a zonally homogeneous representation of the AEJ can support absolute instability in the form of African easterly waves (AEWs). This finding is verified through a local energy budget, which demonstrates the presence of both upstream and downstream energy fluxes. These energy fluxes allow unstable wave packets to spread upstream and downstream relative to their initial point of excitation. This finding is further verified by showing that the ground-relative group velocity of these wave packets has both eastward and westward components. In contrast with normal-mode instability theory, which emphasizes wave growth through energy extraction from the basic state, the life cycle of the simulated AEWs is strongly governed by energy fluxes. Convergent fluxes at the beginning of the AEW storm track generate new AEWs, whereas divergent fluxes at the end of the storm track lead to their decay. It is argued that, even with small normal-mode growth rates and a short region of instability, the presence of absolute instability allows AEWs to develop through the mixed baroclinic–barotropic instability mechanism, because upstream energy fluxes allow energy extracted through baroclinic and barotropic conversion to be recycled between successive AEWs. Link to full paper: https://journals.ametsoc.org/doi/pdf/10.1175/JAS-D-14-0128.1
  15. The Genesis of African Easterly Waves by Upstream Development Authors: Michael Diaz and Anantha Aiyyer Published: 27th June, 2013 Abstract: A genesis mechanism for African easterly waves (AEWs) is proposed. In the same manner that new troughs and ridges in the midlatitudes form downstream of existing ones through a mechanism known as downstream development, it is proposed that new AEWs can be generated upstream of existing AEWs. A local eddy kinetic energy budget of the AEW that ultimately became Hurricane Alberto (2000) demonstrates that upstream development explains its genesis more convincingly than previous theories of AEW genesis. The energetics and ageostrophic secondary circulation of a composite AEW are consistent with a new AEW forming as a result of this mechanism. Some strengths and weaknesses of upstream development as a paradigm for AEW genesis are discussed with respect to other potential mechanisms. Link to full paper: http://www4.ncsu.edu/~aaiyyer/papers/diaz_aiyyer_2013b.pdf
  16. Diabatic Rossby Waves as a Model for Convectively Coupled African Easterly Waves - Presentation A presentation at the AMS 32nd Conference on “Hurricanes and Tropical Meteorology” held at the Condado Plaza Hilton in Puerto Rico between 17th and 22nd April, 2016. Presenters: James O. H. Russell and A. Aiyyer  Presentation Date: 18th April, 2016 Presentation Summary: African easterly waves (AEWs) are synoptic-scale disturbances associated with the African Easterly Jet (AEJ) that move westward across the Sahel region of Africa during the West African Monsoon season. It has been shown that the barotropic and baroclinic extraction of energy from the AEJ is not sufficient to sustain AEWs, and that moist convection may have an important role in their maintenance. It is, however, not clear how the synoptic-scale waves and mesoscale convection interact. Furthermore the conditions under which some waves are more closely coupled with convection compared to others is also not well understood. This study examines the hypothesis that convection and AEWs may interact through the Diabatic Rossby Wave (DRW) mechanism. We examine whether the conditions for DRW genesis, a preconditioning of the low-levels ahead of a wave trough, and the generation of potential vorticity (PV) due to the vertical gradient of diabatic heating produced by convection, occur in AEWs. The impact of diabatic PV generation on the amplification, propagation, and evolution of AEWs will be examined through PV and energy budgets calculated using gridded reanalysis and satellite derived products. Results from this will motivate numerical simulations using the weather research and forecasting (WRF) model to further examine the sensitivity of AEWs to moist convection. Link to conference video presentation (14 minutes): https://ams.confex.com/ams/32Hurr/videogateway.cgi/id/34129?recordingid=34129&amp;uniqueid=Paper293211&amp;entry_password=578878 Link to Manuscript: https://ams.confex.com/ams/32Hurr/webprogram/Paper293211.html (pdf file via conference presentation summary) Link to full conference agenda: https://ams.confex.com/ams/32Hurr/webprogram/32HURRICANES.html
  17. Direct and indirect impacts of Saharan dust acting as cloud condensation nuclei on tropical cyclone eyewall development Authors: Henian Zhang, Greg M. McFarquhar, William R. Cotton and Yi Deng Published: 20th March, 2009 Abstract: The mechanisms by which Saharan dust acting as cloud condensation nuclei (CCN) impact tropical cyclone (TC) evolution were examined by conducting numerical simulations of a mature TC with CCN added from lateral boundaries. CCN can affect eyewall development directly through release of latent heat when activated and subsequent growth of cloud droplets and indirectly through modulating rainband development. Convection in the rainbands was negatively correlated with that in the eyewall in all simulations. The development of rainbands tended to promote latent heat release away from the eyewall, block the surface inflow and enhance cold pools. The maximum impact of rainbands on the eyewall (or vice versa) occurred with a time lag of 3.5 to 5.5 hr. The convection in the eyewall and rainbands did not show a monotonic relationship to input CCN due to the non‐linear feedback of heating from a myriad of microphysical processes on storm dynamics. Link to full paper: https://agupubs.onlinelibrary.wiley.com/doi/epdf/10.1029/2009GL037276
  18. Planetary Scale Intraseasonal Disturbances - 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: Zeljka Fuchs, Socorro, NM and D. J. Raymond Presentation Date: 19th April, 2018  Presentation Summary: There are two planetary disturbances that by definition prefer long wavelengths. One moves eastward and is associated with the Madden-Julian oscillation, while another moves westward and is called the Rossby wave. Using the simple linear analytical model on an equatorial beta plane we model two unstable modes, eastward at meridional number n=-1 and westward propagating mode at meridional number n=1. Both of those modes are moisture modes, but the instability mechanism is not necessarily associated with the moisture mode instability, i.e. the negative gross-moist instability and cloud-radiation interactions. Instead the primary cause for the instability is mean easterlies. This makes sense when modeling the planetary disturbances as mean easterlies are present in the real atmosphere on a planetary scale due to the Hadley cell (GMS and CRI do not scale as planetary mechanisms). We speculate that our modeled eastward and westward propagating WISHE-moisture modes incorporate the basics of physics for the MJO and Rossby waves. Other effects as well as the nonlinearity play an important role, but the essence of the MJO and Rossby waves, according to our model, only requires mean easterlies and moisture mode physics.  Link to conference video presentation (15 minutes): https://ams.confex.com/ams/33HURRICANE/videogateway.cgi/id/46983?recordingid=46983&amp;uniqueid=Paper340106&amp;entry_password=166057 Link to full conference agenda: https://ams.confex.com/ams/33HURRICANE/webprogram/33HURRICANE.html
  19. Large-Scale Atmospheric Forcing of Vertical Wind Shear in the Tropics - Poster 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: Jhordanne J. Jones, M. M. Bell and P. J. Klotzbach Presentation Date: 17th April, 2018 Poster Presentation Summary: It has been well-established that vertical wind shear (VWS) plays a key role in modulating tropical cyclone activity, but there are multiple large-scale processes that can produce VWS in the tropics and their interactions are not fully understood. One of the strongest inter-annual drivers of tropical VWS is the El-Nino Southern Oscillation (ENSO) which modulates shear through variations in the Walker circulation. VWS may be further modulated through midlatitude intrusions of high potential vorticity (PV) streamers and Rossby wave breaking into lower latitudes. In this study, we investigate how both ENSO and PV streamer activity influence the seasonal and spatial variations in tropical VWS. The relative contributions from sources of ‘tropical’ ENSO and ‘extra-tropical’ high PV shear will be discussed, along with potential relationships between the two sources of VWS. Furthermore, we look at how tropical cyclone activity in the Atlantic basin is influenced when VWS is driven by ENSO, PV streamers, or combinations of both sources. Link to Handout: To download the handout, go the conference session and click on "Handout" - Link below: https://ams.confex.com/ams/33HURRICANE/webprogram/Paper339807.html Link to full conference agenda: https://ams.confex.com/ams/33HURRICANE/webprogram/33HURRICANE.html
  20. Modulation of the Northern Hemisphere Midlatitude Flow and Extreme Events By the Madden-Julian Oscillation - 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: Eric D. Maloney, E. A. Barnes, C. F. Baggett, K. C. Tseng, B. D. Mundhenk, S. Henderson and B. Wolding Presentation Date: 16th April, 2018 Presentation Summary: Recent work on teleconnections between the Madden-Julian oscillation (MJO) and northern Hemisphere midlatitude geopotential height anomalies, blocking, and atmospheric rivers (ARs) are discussed. It is first demonstrated using reanalysis fields and a linear baroclinic model that MJO teleconnections to higher latitudes are more robust during certain MJO phases due to the spatial configuration of MJO heating anomalies relative to the North Pacific jet. This robustness is also reflected in excellent ensemble agreement in prediction of North Pacific geopotential height anomalies in a leading numerical weather prediction model at 3-week lead times for certain MJO phases. It is demonstrated that climate and weather forecasting models can have difficulty simulating the spatial pattern and strength of such teleconnections not only due to poor MJO performance, but also due to biases in the spatial extent of the North Pacific jet that affect the pathway of Rossby wave propagation into high latitudes. The modulation of atmospheric blocking and atmospheric river (AR) activity associated with these MJO teleconnections are discussed. It is also shown that the nature of the MJO teleconnection to the Northern Hemisphere depends on the phase of the tropical quasi-biennial oscillation (QBO). The QBO phase-dependent modulation of AR activity by the MJO along the west coast of North America area is presented. A statistical prediction scheme for anomalous AR activity using the initial state of the MJO and QBO as the sole predictors is developed. When evaluated over 36 boreal winters, it is found that certain combinations of MJO and QBO phases produce predictive skill for anomalous AR activity up to 5 weeks in advance that exceeds that produced by a state-of-the-art numerical weather prediction model. Finally, recent modeling results suggest that the MJO teleconnection to higher latitudes may weaken in a warmer climate. The implications of these weaker teleconnections for subseasonal prediction of blocking and ARs will be discussed. Link to conference video presentation (15 minutes): https://ams.confex.com/ams/33HURRICANE/videogateway.cgi/id/46438?recordingid=46438&amp;uniqueid=Paper339784&amp;entry_password=812548 Link to full conference agenda: https://ams.confex.com/ams/33HURRICANE/webprogram/33HURRICANE.html
  21. Variability of African Easterly Wave Structures- 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: Yuan-Ming Cheng and C. D. Thorncroft Presentation Date: 16th April, 2018 Presentation Summary: In this study, we explore the structural variability of African easterly waves (AEWs) and their interaction with equatorial and extra-tropical waves. An Empirical Orthogonal Function (EOF) analysis was applied on appropriately-filtered fields to isolate the dominant wave patterns. Three variables were used with EOF analysis; brightness temperature derived from satellite observation (Tb EOF) and meridional wind from reanalysis data at 700 (v700 EOF) and 200 hPa (v200 EOF). The structure of the waves is extracted by projecting the fields of interest onto the principle components of the EOF patterns. The Tb EOF shows a confined AEW circulation centered around the ITCZ and characterized by more precipitation compared to climatology. In striking contrast to the Tb EOF, the v700 EOF is distinguished by a meridionally broad AEW circulation that appears to interact with equatorial and extratropical waves. In particular, while the peak in circulation is centered at 10°N, there is marked cross-equatorial flow that suggests the presence of a mixed Rossby-gravity wave (MRG) structure. A distinct upper-level MRG structure is also captured by the v200 EOF. The associated lower-level structure resembles an AEW-like flow pattern with an anomalously large wavelength. The upper-level MRG also appears to be closely tired to the mid-latitude Rossby waves in the Southern Hemisphere. Our results highlight the pronounced structural variability of AEWs and their interaction with other synoptic waves. This suggests the variability of AEW structure and activity could be modulated by, in addition to the large-scale environment, other synoptic waves in the region. Link to conference video presentation (13 minutes): https://ams.confex.com/ams/33HURRICANE/videogateway.cgi/id/46317?recordingid=46317&amp;uniqueid=Paper340031&amp;entry_password=660347 Link to full conference agenda: https://ams.confex.com/ams/33HURRICANE/webprogram/33HURRICANE.html
  22. The Challenge of Classifying Propagating Synoptic Disturbances in the African Tropics – Examples from the DACCIWA Field Campaign - 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: Peter Knippertz, A. H. Fink, A. Schlueter and M. Maranan Presentation Date: 16th April, 2018 Presentation Summary: Decades of meteorological research have produced a plethora of sophisticated conceptual models for midlatitude weather systems such as the Norwegian cyclone or the conveyor-belt models. For tropical climates, such models are generally more scattered. Arguably one of the most established and powerful concepts for low-latitude large-scale weather systems are equatorial waves based on linearized shallow-water systems as first discussed by Yanai, Matsuno and others in the 1960s. The simplest forms assume a weak horizontal temperature gradient and a resting basic state. All solutions decay away from the equator and can be separated into Kelvin, equatorial Rossby, mixed Rossby-gravity and inertia-gravity waves, all with their own characteristic patterns of geopotential, temperature, and wind perturbations, which typically oppose each other in the lower- and upper-troposphere. As the waves modulate convergence and divergence, they can couple with deep moist convection and the energy released maintains the waves against friction. This has led to the term convectively coupled equatorial waves (CCEWs). CCEWs can be identified from satellite-based outgoing longwave radiation (OLR) fields using space-time filtering. Interestingly, climatological results in wave frequency – zonal wavenumber space show two areas that are not solutions of the shallow water system: Large-scale, slow, eastward-moving signals associated with the Madden-Julian Oscillation (MJO) and relatively small, fast, and westward moving so-called “tropical disturbances” (TDs). Tropical cyclones and easterly waves are typically subsumed under the TD category. For the African tropics, the boreal summer season is characterized by the West African monsoon (WAM) system. Large differences in temperature between the hot Sahara and cooler southern West Africa lead to the thermal African easterly jet (AEJ). Baroclinic-barotropic instability supports the formation of synoptic-scale African easterly waves (AEWs), which are intimately coupled with convection, particularly over the Sahel. These are usually characterized by two cyclonic vortices at 850 hPa straddling the AEJ to the north and south. There are sporadic reports in the literature of single vortices or slow-moving cyclonic-anticyclonic vortex couplets, but the involved physical mechanisms are unclear. In June-July 2016 the DACCIWA (Dynamics-Aerosol-Chemistry-Cloud Interactions in West Africa) project organized a major international field campaign in southern West Africa including measurements from aircraft, ground, and radiosondes. Careful daily synoptic analyses were produced to guide flight-planning and post-analysis of observations. The results are a strong illustration of the richness of features encountered over the African tropics in summer. Three types of behaviors can be distinguished: The first type is classical AEWs with a northern and southern cyclonic vortex. They usually have a discernable signal in vorticity, wind, and precipitation fields. Particularly before the monsoon onset, a second type, single cyclonic vortices, occur at different latitudes with different propagation speeds. They fall into the TD filter window and are often related to long-lived mesoscale convective systems (MCSs) and thus modulate rainfall on the regional scale, but the exact dynamical reason for their existence is not entirely clear. The third type, which appears to be rarer and whose climatological and dynamical characteristics are barely covered in the literature, are jointly propagating cyclonic and anticyclonic vortices, which create an anomalous westerly flow in between them. The resulting conditions appear to depend on the origin of the involved air masses. If the westerly flow taps into moist air off the West African west coast, where high sea-surface temperatures are common, this can lead to anomalously moist conditions across the region. More work is needed to develop conceptual models for the latter two types of disturbances that share characteristics of both equatorial (e.g., mixed Rossby-gravity waves) and AEW features. Link to conference video presentation (16 minutes): https://ams.confex.com/ams/33HURRICANE/videogateway.cgi/id/46289?recordingid=46289&amp;uniqueid=Paper339155&amp;entry_password=728849 Link to full conference agenda: https://ams.confex.com/ams/33HURRICANE/webprogram/33HURRICANE.html
  23. African Easterly Waves - An Educational Guide Authors: Produced by the Comet Group, Boulder, Colorado Published: in 2007 and updated in 2012 This is an excellent "easy read" and comprehensive educational guide to African Easterly waves. Contents: Introduction Structure, Speed, and Frequency Seasonal Activity Lifecycle over Africa Formation of AEWs AEWs and Deep Convection Hazards Tracking AEWs Downstream Transformation Intraseasonal Variability of AEWs Summary Introduction: African easterly waves (AEWs) are the primary synoptic systems that occur over tropical North Africa and the tropical north Atlantic during the summer. These waves, which propagate westward, are important because they are linked with convective rainfall, the variability of which can have devastating societal impacts in Africa. They are also noted for being precursors to tropical cyclones in the tropical Atlantic and east Pacific Ocean basins. Easterly waves can be observed in satellite imagery because of their associated convection. They appear as circular or banded clouds in satellite images. The cloud pattern is usually poorly defined inland but becomes more organized and recognizable as they propagate towards the coast. This module explores easterly wave structure, movement, frequency, formation mechanisms, impacts, and downstream transformation as well as tools for monitoring waves. Summary: This module examined African easterly waves, the dominant synoptic systems of the summer in northern tropical Africa. They modulate rainfall and are precursors to tropical cyclones. AEWs: Are waves or cyclonic curvature maxima that develop along the AEJ Can occur from May to November but have peak activity in July to September Have wavelengths of 2000-4000 km Move westward at about 8 m s Occur every 3-5 days (intermittent waves north of 15°N have a 6-9 day period) Develop between 15°-30°E in the lee of high mountains Have maximum intensity near the West African coast (about 0°-10°W) Move along two tracks, one north and one south of the AEJ Have maximum amplitude at the AEJ level and low-level vorticity maxima near 850 hPa, with the latter predominating north of the jet Theories for their formation include: Barotropic-baroclinic instabilities along the AEJ Upstream forcing by diabatic heating from convection near AEJ entrance Orographic forcing by Ethiopian Highlands, which generates leeside low pressure in response to westward-moving upper-level disturbances As they move downstream from their formation region, they: Generally weaken over the eastern and central Atlantic Some intensify over the eastern Caribbean, particularly when aligned with upper-level divergence May be responsible for about half of Atlantic tropical cyclones and some in the Eastern Pacific Are precursors to the most intense tropical cyclones (Saffir-Simpson Category 3-5 hurricanes) Can interact with extratropical cyclones and cause moisture surges into the midlatitudes, including over Europe, North Africa, and North America They can be tracked by following: Convection on satellite images as banded or circular cloud patterns Meridional winds at 850 and 700 hPa; trough axis is the apex of the inverted trough between northerly and southerly winds Relative vorticity maxima at 850 and 700 hPa Potential vorticity maxima on potential temperature surfaces of 315 to 320K Time-height analysis of relative humidity, winds, and equivalent potential temperature Streamfunction troughs and ridges at 700 hPa Advection of the curvature vorticity of the stream function at 700 hPa Satellite microwave precipitable water or total water vapor content Their activity is intermittent and influenced by: The regional environment Coupling between convection and phase of the wave How the AEW is initiated The MJO Equatorial waves Extratropical interactions Upstream development, a new wave forms upstream of existing wave in a group of waves Link to full guide: https://www.meted.ucar.edu/tropical/synoptic/Afr_E_Waves/navmenu.php?tab=1&amp;page=2.0.0&amp;type=flash Link to print version: https://www.meted.ucar.edu/tropical/synoptic/Afr_E_Waves/print.htm Link to short poster paper: http://tornado.sfsu.edu/geosciences/classes/e365/EasterlyWaves/EasterlyWaves.html
  24. The Importance of Critical Layer in Differentiating Developing from Nondeveloping Easterly Waves Authors: Ali Asaadi, Gilbert Brunet and M.K. Yau Published: 16th October, 2016 Abstract: Recently Asaadi et al. found that an easterly wave (EW) train over the Atlantic and eastern Pacific is oriented in a southeast–northwest direction because of the observed tilt in the easterly jet. This tilt results in only one out of four (~25%) waves to be located at the cyclonic critical layer south of the jet axis in a comoving frame, and they subsequently developed into named storms. Asaadi et al. suggested a geometrical view for developing disturbances, which is the coexistence of a nonlinear critical layer and a region of weak meridional potential vorticity (PV) gradient over several days. Asaadi et al. focused on the developing waves and did not investigate the nondeveloping ones. To determine whether the nondeveloping EWs are not associated with a critical layer, a simple objective tracking technique is used to identify EWs. Composite views of the large-scale structure and characteristics of nondeveloping EWs show that ~91% of nondeveloping waves are not located on a critical layer, while the remaining ~9% indicate characteristics similar to the developing waves. Examination of the composite Okubo–Weiss parameter indicates that the nondeveloping waves are characterized by larger negative values, implying that they are dominated by deformation, unlike developing waves, which tend to be more immune from the deformation. Link to full paper: https://journals.ametsoc.org/doi/pdf/10.1175/JAS-D-16-0085.1
  25. Tropical cyclogenesis in a tropical wave critical layer: easterly waves Authors: T. J. Dunkerton, M. T. Montgomery and Z. Wang Published: 6th August 2009 Abstract: The development of tropical depressions within tropical waves over the Atlantic and eastern Pacific is usually preceded by a "surface low along the wave" as if to suggest a hybrid wave-vortex structure in which flow streamlines not only undulate with the waves, but form a closed circulation in the lower troposphere surrounding the low. This structure, equatorward of the easterly jet axis, is identified herein as the familiar critical layer of waves in shear flow, a flow configuration which arguably provides the simplest conceptual framework for tropical cyclogenesis resulting from tropical waves, their interaction with the mean flow, and with diabatic processes associated with deep moist convection. The recirculating Kelvin cat's eye within the critical layer represents a sweet spot for tropical cyclogenesis in which a proto-vortex may form and grow within its parent wave. A common location for storm development is given by the intersection of the wave's critical latitude and trough axis at the center of the cat's eye, with analyzed vorticity centroid nearby. The wave and vortex live together for a time, and initially propagate at approximately the same speed. In most cases this coupled propagation continues for a few days after a tropical depression is identified. For easterly waves, as the name suggests, the propagation is westward. It is shown that in order to visualize optimally the associated Lagrangian motions, one should view the flow streamlines, or stream function, in a frame of reference translating horizontally with the phase propagation of the parent wave. In this co-moving frame, streamlines are approximately equivalent to particle trajectories. The closed circulation is quasi-stationary, and a dividing streamline separates air within the cat's eye from air outside. The critical layer equatorward of the easterly jet axis is important to tropical cyclogenesis because its cat's eye provides (i) a region of cyclonic vorticity and weak deformation by the resolved flow, (ii) containment of moisture entrained by the developing gyre and/or lofted by deep convection therein, (iii) confinement of mesoscale vortex aggregation, (iv) a predominantly convective type of heating profile, and (v) maintenance or enhancement of the parent wave until the vortex becomes a self-sustaining entity and emerges from the wave as a tropical depression. The entire sequence is likened to the development of a marsupial infant in its mother's pouch. These ideas are formulated in three new hypotheses describing the flow kinematics and dynamics, moist thermodynamics and wave/vortex interactions comprising the "marsupial paradigm". A survey of 55 named tropical storms in 1998–2001 reveals that actual critical layers sometimes resemble the ideal east-west train of cat's eyes, but are usually less regular, with one or more recirculation regions in the co-moving frame. It is shown that the kinematics of isolated proto-vortices carried by the wave also can be visualized in a frame of reference translating at or near the phase speed of the parent wave. The proper translation speeds for wave and vortex may vary with height owing to vertical shear and wave-vortex interaction. Some implications for entrainment/containment of vorticity and moisture in the cat's eye are discussed from this perspective, based on the observational survey. Link to full paper: https://www.atmos-chem-phys.net/9/5587/2009/acp-9-5587-2009.pdf
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