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Croix E. Christenson, Jonathan E. Martin, and Zachary J. Handlos

1. Introduction Narrow, rapidly flowing currents of air located near the tropopause are known as jet streams or jets. These jets, often found nearly girdling the globe while exhibiting large meridional meanders, are among the most ubiquitous structural characteristics of Earth’s atmosphere and are known to play a substantial role in the production of sensible weather in the midlatitudes. As the primary phenomena at the interface between synoptic-scale weather systems and the large

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Adam H. Monahan and John C. Fyfe

anomalies in the low and high latitudes (the black curve in Fig. 1 ). The associated time series is denoted the zonal index, and together with the dipole spatial pattern is typically interpreted as describing fluctuations in the central latitude of the eddy-driven jet (e.g., Robinson 1996 ; Feldstein and Lee 1998 ; DeWeaver and Nigam 2000 ; Lorenz and Hartmann 2001 ; Codron 2005 ; Wittman et al. 2005 ). In this study, the zonal index time series and its associated spatial pattern in zonal

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Tim Woollings, Elizabeth Barnes, Brian Hoskins, Young-Oh Kwon, Robert W. Lee, Camille Li, Erica Madonna, Marie McGraw, Tess Parker, Regina Rodrigues, Clemens Spensberger, and Keith Williams

1. Introduction Shifts of the jet streams are the dominant source of variability in weather patterns across much of the midlatitudes (e.g., Hurrell and Deser 2009 ). Jet shifts are associated with altered storm-track paths and with changes in the regions that experience a mild oceanic influence. Several recent seasons of extreme weather were driven by jet shifts as a proximate cause. In the 2009/10 winter, for example, severe cold over both North America and Eurasia was caused by an extreme

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Adam H. Monahan and John C. Fyfe

a dipole centered at approximately the latitude of the core of the time-mean jet. This structure is generally interpreted as representing meridional displacements of the eddy-driven jet (the so-called zonal index), while higher-order EOFs (when they are considered) are interpreted as reflecting changes in jet strength or width (e.g., Feldstein and Lee 1998 ; Feldstein 2000 ; DeWeaver and Nigam 2000 ; Codron 2005 ; Vallis et al. 2004 ). Wittman et al. (2005) consider numerical simulations

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Qiang Fu and Pu Lin

1. Introduction By examining atmospheric temperature trends since 1979 based on satellite-borne microwave sounding unit (MSU) data, Fu et al. (2006) identified the enhanced stratospheric cooling and tropospheric warming in the 15°–45° latitude belts in both hemispheres. The changes in meridional tropospheric temperature gradients in the vicinity of the jets provide evidence that the subtropical jets have been shifting poleward ( Fu et al. 2006 ). However, no interpretation has been presented

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Bing Pu and Kerry H. Cook

1. Introduction It has long been recognized that the eastern tropical Atlantic is an important moisture source for West Africa, and that low-level westerlies transport moisture onto the continent in boreal summer ( Lamb 1983 ; Koster et al. 1986 ; Cadet and Nnoli 1987 ; Druyan and Koster 1989 ; Grist and Nicholson 2001 ; Fontaine et al. 2003 ). The westerly flow near 10°N along the West African coast was identified as a jet in satellite-based observations by Grodsky et al. (2003) . Here

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Clemens Spensberger and Thomas Spengler

1. Introduction Jets in the upper troposphere signify regions of strong baroclinicity, a prerequisite for the development of extratropical cyclones (e.g., Lorenz 1955 ; Lindzen and Farrell 1980 ) and storm tracks ( Chang et al. 2002 ; Wirth et al. 2018 ). Jets also demarcate the Rossby waveguide, as they are usually associated with strong gradients in potential vorticity ( Hoskins and Ambrizzi 1993 ; Martius et al. 2010 ). In climate dynamics (e.g., Wallace and Gutzler 1981 ; Limpasuvan

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Xiaowei Hong, Riyu Lu, and Shuanglin Li

1. Introduction The summer Asian westerly jet elongates along midlatitude Eurasia over the upper troposphere and is an important component of the atmospheric circulation system. The year-to-year variation of the summer Asian jet reflects the adjustment and interaction of atmospheric circulation, and exerts great influences on climate, particularly over the Eurasian continent. The year-to-year variation of upper-tropospheric zonal winds over Eurasia is inhomogeneous along the zonal direction

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Joseph Kidston, G. K. Vallis, S. M. Dean, and J. A. Renwick

1. Introduction a. Jet stream movement There is evidence of a poleward shift of the eddy-driven jet streams in recent decades, particularly in the Southern Hemisphere ( Feldstein 2002 ; Marshall 2003 ; Ostermeier and Wallace 2003 ; Thompson et al. 2000 ; Trenberth et al. 2007 ). A large portion of the observed mid- to high-latitude temperature trends bear the signature of such a shift ( Hurrell 1996 ; Thompson et al. 2000 ; Thompson and Solomon 2002 ; Trenberth et al. 2007 ). Events such

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Yazhou Zhang, Peiwen Yan, Zhijie Liao, Danqing Huang, and Yaocun Zhang

1. Introduction In general, there are two branches of westerly jet streams in the upper troposphere over East Asia around the year, the East Asian polar-front jet (EAPJ) and the East Asian subtropical jet (EASJ). The EAPJ is located in the baroclinic zone, which is mainly formed by the eddy momentum flux convergence ( Panetta 1993 ; Lee 1997 ). The EASJ is an important component of the global subtropical jet, which is driven by the angular momentum transport along the poleward shift of the

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