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Pallav Ray and Chidong Zhang

1. Introduction The mechanism of initiation of the Madden–Julian oscillation (MJO; Madden and Julian 1971 , 1972 ) remains a challenging problem for research. This is highly relevant to operational forecast of the MJO, since the MJO is connected to a variety of events around the globe [see references in Lau and Waliser (2005) and Zhang (2005) ]. The time scale of the MJO relates to the subseasonal predictability and affects medium- and extended-range weather forecasts in both the tropics

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Yanzhen Chi, Fuqing Zhang, Wei Li, Jinhai He, and Zhaoyong Guan

strengthening of the mean southerly wind due to the reversal of zonal land–sea thermal contrast ( Qi et al. 2008 ; Zhao et al. 2007 ; Zhu et al. 2012 ). Through all of this debate on mechanism, there is not yet a consensus criterion for defining the EASSM onset. In recent years, the impacts of intraseasonal oscillations in convective activity, such as the Madden–Julian oscillation (MJO) ( Madden and Julian 1971 ), on the EASM has attracted a lot of attention ( Wen et al. 2004 ; Ding and Chan 2005 ; Zhou

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Philip J. Klotzbach

1. Introduction The Madden–Julian oscillation (MJO) is a globally propagating mode of tropical atmospheric intraseasonal variability ( Madden and Julian 1972 , 1994 ). Associated with this convectively driven mode of variability are large-scale variations in upper- and lower-level winds, vertical motion, atmospheric moisture content, and sea surface temperatures ( Fig. 1 ). This global wave tends to propagate eastward at approximately 5 m s −1 , circling the globe in approximately 40–50 days

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Jennifer Gahtan and Paul Roundy

1. Introduction The Madden–Julian oscillation (MJO; Zhang 2005 ) is typically characterized by equatorial eastward-propagating circulative and deep convective anomalies on intraseasonal temporal and planetary zonal scales. While circulation signals extend globally, MJO deep convection is mainly restricted to tropical warm pool regions, such that convective onset often occurs over the western Indian Ocean. Though discharge–recharge-type hypotheses suggest that the moistening required for onset

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David J. Raymond and Željka Fuchs

1. Introduction The Madden–Julian oscillation (MJO) is notoriously difficult for global models to simulate ( Lin et al. 2006 ). Furthermore, there is no universally accepted theory of this phenomenon. Zhang (2005) describes the essential characteristics of the MJO in his comprehensive review. These can be summarized briefly as follows: (i) The MJO is a global-scale, convectively coupled, eastward-moving disturbance with roots in the tropics. The convective aspects are most obvious in the

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Guosen Chen and Bin Wang

1. Introduction The slow eastward propagation of convective anomalies over the Indo-Pacific warm pool region is the most fundamental feature of the Madden–Julian oscillation (MJO) ( Madden and Julian 1972 ). Explanation of this slow eastward propagation is a central issue in studies of the MJO. The observed MJO eastward propagation speed exhibits a broad range from about 2 to 9 m s −1 ( Knutson et al. 1986 ; Zhang and Ling 2017 ). Understanding what accelerates or retards the MJO propagation

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Brandon O. Wolding and Eric D. Maloney

1. Introduction Despite four decades of research, no consensus currently exists as to what method is “best” for objectively assessing the state, either past or present, of the Madden–Julian oscillation (MJO) ( Straub 2013 ). This lack of consensus results from the diverse needs of the various user groups, differing views of the pertinent features that describe the MJO, and the advantages and disadvantages of the numerous techniques used to assess the MJO. Most MJO indices result from the

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Stephen Baxter, Scott Weaver, Jon Gottschalck, and Yan Xue

1. Introduction The Madden–Julian oscillation (MJO) is the dominant mode of subseasonal climate variability in the tropics, consisting of propagating patterns of convection coupled with upper- and lower-level winds ( Madden and Julian 1971 , 1972 , 1994 ). Studies indicate that the MJO has a strong impact on climate variability in the extratropics through the propagation of a Rossby wave train associated with the anomalous tropical convection ( Kiladis and Weickmann 1992 ; Mo and Higgins

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B. Pohl, Y. Richard, and N. Fauchereau

1. Introduction It has been established for decades that the Madden–Julian oscillation (MJO) is the dominant mode of intraseasonal variability in the tropical atmosphere ( Madden and Julian 1994 ; Zhang 2005 ). It basically consists in a slow eastward propagation of large-scale convective clusters along the equator, from the Indian Ocean to the Maritime Continent and then to the western Pacific basin. The time taken by the MJO to rejuvenate over the Indian Ocean is typically between 40 and 45

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Seok-Woo Son, Yuna Lim, Changhyun Yoo, Harry H. Hendon, and Joowan Kim

convection over the western Pacific during boreal winter in the easterly phase of the QBO ( Collimore et al. 2003 ; Liess and Geller 2012 ). ENSO and the QBO regulate not only seasonal-mean tropical convection but also subseasonal convective activity, such as the Madden–Julian oscillation (MJO; Madden and Julian 1994 ), during boreal winter (e.g., Pang et al. 2016 ; Yoo and Son 2016 ). Both the spatial structure and amplitude of the MJO vary from year to year in response to ENSO and the QBO. These

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