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Li Dong and Stephen J. Colucci

1. Introduction The large-scale atmospheric circulation is commonly discussed in the quasigeostrophic (QG) framework ( Bluestein 1992 ; Holton 2004 ) because of its ability to capture the essential processes of the large-scale circulation and a straightforward interpretation of these processes. It is well known that QG processes qualitatively account for the formation and maintenance of blocking flows ( Shutts 1983 ; Illari 1984 ; Mullen 1987 ; Alberta et al. 1991 ; Lupo and Smith 1995

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Takuya Aikawa, Masaru Inatsu, Naoto Nakano, and Tetsuya Iwano

1. Introduction Atmospheric blocks are largely anomalous anticyclones that frequently emerge in the Pacific, in the Atlantic, and over Europe ( Tibaldi and Molteni 1990 ) and persist for a week or longer. They often bring prolonged abnormal weather in the extratropics. Many papers have been devoted to the physical mechanism of blocks. One impact of a Rossby wave breaking with a daily time scale typically causes blocking onset ( Nakamura and Wallace 1993 ; Nakamura et al. 1997 ). Ji and

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Etienne Dunn-Sigouin, Seok-Woo Son, and Hai Lin

1. Introduction Atmospheric blocking is one of the most striking features of extratropical low-frequency variability. A synoptic-scale high pressure system, often accompanied by low pressure system at lower latitudes, occasionally becomes quasi stationary for several days to a few weeks against the background flow. This quasi-stationary system, referred to as a block, interrupts the eastward propagation of synoptic disturbances by reversing the climatological zonal flow. As a blocking high is

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David Small, Eyad Atallah, and John R. Gyakum

1. Introduction The prediction of blocking, commonly regarded as the persistent breakdown of the extratropical westerly zonal flow, characterized by a split jet stream and an upper-level synoptic-scale ridge, remains a significant challenge for numerical weather and climate forecasting models (e.g., Pelly and Hoskins 2003b ; Dunn-Sigouin et al. 2013 ). Because of its potential for triggering catastrophic floods ( Galarneau et al. 2012 ), droughts ( Green 1977 ; Dole et al. 2011 ), and warm

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Paolo Davini, Chiara Cagnazzo, Silvio Gualdi, and Antonio Navarra

1. Introduction Atmospheric blocking is a midlatitude weather pattern that describes a quasi-stationary, long-lasting, high pressure system that modifies the westerly flow, “blocking” (or at least diverting) the eastward movement of the migratory cyclones ( Rex 1950 ). This usually occurs when a subtropical air mass of low vorticity is advected poleward, developing an anticyclonic circulation. Blocking occurs throughout the year, even if it is more frequent during winter and spring. It

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Lisa-Ann Quandt, Julia H. Keller, Olivia Martius, Joaquim G. Pinto, and Sarah C. Jones

1. Introduction In summer 2010, parts of eastern Europe and Russia were affected by a long-lasting “mega” heat wave from mid-June to mid-August ( Dole et al. 2011 ; Barriopedro et al. 2011 ; Lau and Kim 2012 ; Trenberth and Fasullo 2012 ). Moreover, Pakistan was affected by severe flooding in July ( Hong et al. 2011 ; Houze et al. 2011 ; Lau and Kim 2012 ; Martius et al. 2013 ). These high-impact weather events were associated with an atmospheric block ( Schneidereit et al. 2012 ; Lupo

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Adam A. Scaife, Tim Woollings, Jeff Knight, Gill Martin, and Tim Hinton

1. Introduction A number of different indices have been proposed and used to diagnose atmospheric blocking (e.g., Rex 1950 ; Lejenäs and Økland 1983 ; Tibaldi and Molteni 1990 ; Pelly and Hoskins 2003 ; Berrisford et al. 2007 ). By applying these indices to numerical models it has now become well established that these models underestimate blocking frequency (e.g., D’Andrea et al. 1998 ). Furthermore, this result is largely insensitive to the exact choice of blocking index (e.g., Doblas

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Lisa-Ann Quandt, Julia H. Keller, Olivia Martius, and Sarah C. Jones

1. Introduction As atmospheric blocking may cause high-impact weather like heat waves and flooding (e.g., Matsueda 2009 ), there is a special interest in its predictability and the physical processes that limit forecast quality. Blocking is a large-scale flow pattern that decelerates arriving eddies, as well as the background flow, and forces them to follow a more meridional direction (e.g., Rex 1950 ; Arakawa 1952 ; Sumner 1954 ). Atmospheric blocking is persistent and self-sustaining (e

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Mu Mu and Zhina Jiang

research is to address the nature of initial perturbations that can easiest develop into the weather events in which we are interested. This initial perturbation, under some conditions, is called as the optimal precursor (OPR) to the weather event. With regard to atmospheric blocking, it is important to recognize blocking patterns during initial development because of their profound effect on local and global weather and climate ( Rex 1950 ; Li et al. 2001 ; Wang et al. 2008 ). Blocking onset is a

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Dehai Luo, Xiaodan Chen, Aiguo Dai, and Ian Simmonds

circulations has become an important research topic because continental cold anomalies are often linked to the winter Arctic sea ice loss or Arctic warming through changes in atmospheric circulations ( Overland et al. 2011 ; Outten and Esau 2012 ; Screen and Simmonds 2013 ; Cohen et al. 2014 ; Mori et al. 2014 ; Vihma 2014 ; Simmonds and Govekar 2014 ; Kug et al. 2015 ; Luo et al. 2016a , b ; Shepherd 2016 ; Yao et al. 2017 ). Atmospheric blocking is a quasi-stationary midlatitude large

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