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Olivia Martius and Heini Wernli

1. Introduction Upper-level westerly jet streams are ubiquitous and important features of the subtropical atmospheric flow. On relatively short time scales (up to a few days) these jets serve as waveguides for synoptic-scale Rossby waves, which in turn can trigger high impact weather events (e.g., Martius et al. 2008 ; Feldstein and Dayan 2008 ). With regard to the transport of chemical constituents, the subtropical jets can act as local barriers for meridional exchange (e.g., Shuckburgh et

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Michael Riemer, Marlene Baumgart, and Sven Eiermann

1. Cyclogenesis downstream of extratropical transition Recurving tropical cyclones, in particular those that undergo extratropical transition (ET; Jones et al. 2003 ), exert a profound impact on the midlatitude circulation. Several recent studies have investigated this impact in some detail and documented a distinct modification of the upper-level jet stream (e.g., Riemer et al. 2008 ; Riemer and Jones 2010 , 2014 ; Archambault et al. 2013 ; Grams et al. 2011 , 2013a ; Pantillon et al

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Simon T. K. Lang, Sarah C. Jones, Martin Leutbecher, Melinda S. Peng, and Carolyn A. Reynolds

experiments cover the period from before recurvature to ET (see section 3 ). Fig . 1. Track of Helene (2006) from 0000 UTC 16 Sep to 1200 UTC 24 Sep 2006. Black circles mark Helene’s position at 1200 UTC (the initialization times for the experiments). Between 17 and 19 September 2006, Helene intensified and developed a strong asymmetric outflow jet, a so-called outflow channel ( Figs. 2a,b ). At this time Helene moved northwestward, while to the northwest Hurricane Gordon (located around 35°N, 54°W

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Franziska Teubler and Michael Riemer

1. Introduction Rossby waves are a fundamental ingredient of atmospheric dynamics. For synoptic-scale Rossby waves, the strong gradient of potential vorticity (PV) associated with the midlatitude jet effectively constitutes the waveguide ( Platzman 1949 ; Schwierz et al. 2004 ; Martius et al. 2010 ). A local perturbation of this waveguide may lead to the formation of distinct wave packets (e.g., Simmons and Hoskins 1979 ; Hakim 2003 ; Riemer et al. 2008 ). The significance of such Rossby

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Jana Čampa and Heini Wernli

–105 . Uccellini , L. W. , R. A. Petersen , K. F. Brill , P. J. Kocin , and J. J. Tuccillo , 1987 : Synergistic interactions between an upper-level jet streak and diabatic processes that influence the development of a low-level jet and a secondary coastal cyclone . Mon. Wea. Rev. , 115 , 2227 – 2261 . Wang , C.-C. , and J. C. Rogers , 2001 : A composite study of explosive cyclogenesis in different sectors of the North Atlantic. Part I: Cyclone structure and evolution . Mon. Wea. Rev

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