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Yuki Kanno, John E. Walsh, and Toshiki Iwasaki

1. Introduction Cold air outbreaks (CAOs) are sudden intrusions of polar cold air masses into the midlatitudes. The associated cold conditions and heavy snowfalls in the midlatitudes can severely impact human activities. The frequencies and magnitudes of CAOs dominate climate variability in the winter extratropics. In addition, CAOs play a role in heat, energy, and angular momentum balances ( Garreaud 2001 ; Mailler and Lott 2010 ). Therefore, the dynamical processes underlying CAOs are among

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Muhammad Rais Abdillah, Yuki Kanno, and Toshiki Iwasaki

1. Introduction Intermittent cold air outbreak (CAO) events during the winter season often cause severe damage to human activities ( Chang et al. 1979 ; Chen et al. 2002 ; Tangang et al. 2008 ; Yang et al. 2009 ). CAO events are characterized by strong equatorward wind and a sudden drop of temperatures, which in turn significantly alters the weather in the affected region. In East Asia, a CAO event is known as the prominent feature of East Asian winter monsoon ( Zhang et al. 1997 ; Wang and

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David Medvigy, Robert L. Walko, and Roni Avissar

Tropical Rainfall Measuring Mission ( Kummerow et al. 1998 ). While Medvigy et al. (2011) focused on changes in precipitation mean, intensity, and frequency, they also suggested that the frequency of extreme cold events in South America could increase following deforestation. Although cold air incursions into southern South America are observed to occur year-round, they are stronger and more common during the austral winter, June–August (JJA) ( Marengo et al. 1997a ). Garreaud (2000) described the

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Lukas Papritz, Stephan Pfahl, Harald Sodemann, and Heini Wernli

1. Introduction The Antarctic continent and the belt of sea ice surrounding it provide a gigantic pool of radiatively cooled air, in particular during the cold season from early autumn to late spring. Associated with the pool of cold air is a strong temperature inversion in the interior of the continent, which over the most elevated regions exceeds 25 K in the winter mean ( Phillpot and Zillman 1970 ). Equatorward excursions of these cold air masses across the sea ice boundary over the

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Koutarou Takaya and Hisashi Nakamura

(cyclones), respectively. In (b), the area surrounded by thick lines indicates the region over which surface air temperature is averaged for the index shown in Fig. 2 . The East Asian winter monsoon has been known to vary on various time scales (e.g., Huang et al. 2012 ). Many previous studies on the intraseasonal variability of the East Asian winter monsoon focused on the atmospheric circulations and heat budgets in the lower troposphere associated with cold surges, recognizing the monsoon as a near

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Rebecca M. Westby, Yun-Young Lee, and Robert X. Black

introduces a “warming hole.” Geophys. Res. Lett. , 31 , L17109 , doi:10.1029/2004GL020528 . Park , T.-W. , C.-H. Ho , and S. Yang , 2011 : Relationship between the Arctic Oscillation and cold surges over East Asia . J. Climate , 24 , 68 – 83 . Portis , D. H. , M. P. Cellitti , W. L. Chapman , and J. E. Walsh , 2006 : Low-frequency variability and evolution of North American cold air outbreaks . Mon. Wea. Rev. , 134 , 579 – 597 . Portmann , R. W. , S. Solomon , and G

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James S. Risbey, Didier P. Monselesan, Terence J. O’Kane, Carly R. Tozer, Michael J. Pook, and Peter T. Hayman

, 379 – 402 , https://doi.org/10.1175/MWR-D-16-0291.1 . 10.1175/MWR-D-16-0291.1 Perrin , G. , and I. Simmonds , 1995 : The origin and characteristics of cold air outbreaks over Melbourne . Aust. Meteor. Mag. , 44 , 41 – 59 . Pezza , A. , and T. Ambrizzi , 2005 : Dynamical conditions and synoptic tracks associated with different types of cold surge over tropical South America . Int. J. Climatol. , 25 , 215 – 241 , https://doi.org/10.1002/joc.1080 . 10.1002/joc.1080 Pook , M. , J

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Mizuki Iida, Shusaku Sugimoto, and Toshio Suga

, 35 pp., https://public.wmo.int/en/resources/library/wmo-statement-state-of-global-climate-2017 . Yamaguchi , J. , Y. Kanno , G. Chen , and T. Iwasaki , 2019 : Cold air mass analysis of the record-breaking cold surge event over East Asia in January 2016 . J. Meteor. Soc. Japan , 97 , 275 – 293 , https://doi.org/10.2151/jmsj.2019-015 . 10.2151/jmsj.2019-015 Yamamoto , K. , Y. Tachibana , M. Honda , and J. Ukita , 2006 : Intraseasonal relationship between the Northern

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Chaim I. Garfinkel and Nili Harnik

wave train propagated poleward). Each extremum in the wave train has a characteristic meridional extent of more than 30°, and therefore can advect over the entire region in which there is a large temperature gradient in Fig. 6a . As for warm extremes, the equatorward propagation is driven (at least in part) by the steering of the midtropospheric flow ( Figs. 8g–i ). Sea level pressure anomalies of broad meridional scale that propagate equatorward are also present for cold-air outbreaks over East

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Shawn M. Milrad, Eyad H. Atallah, and John R. Gyakum

-level cyclone center starts to intensify over the ocean. A weak negative anomaly is visible at t = −36 h over the Great Lakes region ( Fig. 12d ) and approaches St. John’s from the south at t = 0 h ( Fig. 12g ). The suggestion here is that baroclinic instability is present, as the near-surface cyclone helps to intensify the upstream upper-level trough by advecting cold air beneath 500 hPa (not shown) into the base of the 500-hPa trough. In examining Figs. 12f–h , the 500-hPa negative height anomaly

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