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Justin J. Wettstein and Linda O. Mearns

/or event specific. However, all these effects underscore the importance of climatic extremes, particularly on a daily timescale, in damage to forests. In our region of concern, the northeastern United States and neighboring areas of Canada, local variability of climate is related to large-scale patterns of variability. The dominant mode of large-scale variability in midlatitude Northern Hemisphere temperature variability is the North Atlantic Oscillation–Arctic Oscillation (NAO–AO). A significant

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Mark C. Serreze and Andrew P. Barrett

1. Introduction The past decade has seen an explosion of literature concerning the atmospheric circulation of the north polar region. To a considerable degree, this stems from the recognition that rapid changes observed in the Arctic, including rises in surface air temperature and declining sea ice extent, can be explained in part by attendant shifts in atmospheric patterns. Most of this interest has focused on winter. Studies of links with the North Atlantic Oscillation (NAO) and its

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Yi Deng, Tae-Won Park, and Ming Cai

1. Introduction The northern annular mode (NAM) is characterized by a deep, nearly barotropic seesaw in the isobaric surface geopotential height field between the Arctic and surrounding midlatitudes (e.g., Thompson and Wallace 1998 , 2000 ). Manifesting itself also as a surface pressure dipole in the northern extratropics [i.e., the Arctic Oscillation (AO)], the NAM is closely tied to the subseasonal fluctuations in the strength of the meridional mass circulation ( Cai and Ren 2007 ). In the

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Svenya Chripko, Rym Msadek, Emilia Sanchez-Gomez, Laurent Terray, Laurent Bessières, and Marie-Pierre Moine

studies, a positive correlation was found between Arctic sea ice decline and cold winters over Eurasia and North America since the 1980s—the warm Arctic and cold continents (WACC) pattern ( Overland et al. 2011 ; Cohen et al. 2013 ), sometimes restricted to the warm Arctic and cold Eurasia (WACE) pattern ( Mori et al. 2014 , 2019 ). Observations also suggest a link between Arctic sea ice decline and the negative phase of the northern annular mode (NAM), also called the Arctic Oscillation (AO), which

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Maddalen Iza, Natalia Calvo, and Elisa Manzini

1. Introduction El Niño–Southern Oscillation (ENSO) is the main source of interannual variability in the tropics with relevant teleconnections in the Northern Hemisphere (NH) extratropics ( Horel and Wallace 1981 ). The stratospheric signal during the warm ENSO phase (El Niño) has been extensively documented ( García-Herrera et al. 2006 ; Manzini et al. 2006 ; Fletcher and Kushner 2011 ); during El Niño winters, upward wave activity toward the stratosphere is enhanced through constructive

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Daniel M. Mitchell, Lesley J. Gray, James Anstey, Mark P. Baldwin, and Andrew J. Charlton-Perez

et al. (2011) , similar dates can be achieved by using a simple threshold method in that splits are defined when the vortex aspect ratio is notably elliptical, and displacements when the centroid latitude is notably equatorward, adding confidence that the clustering algorithm is reliable in this case. b. Calculating the NAM The NAM (known as the Arctic oscillation at the surface) is the leading mode of wintertime variability in the Northern Hemisphere circulation ( Thompson and Wallace 1998

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Ian White, Chaim I. Garfinkel, Edwin P. Gerber, Martin Jucker, Valentina Aquila, and Luke D. Oman

.1175/JAS-D-14-0012.1 Hitchcock , P. , and P. H. Haynes , 2016 : Stratospheric control of planetary waves . Geophys. Res. Lett. , 43 , 11 884 – 11 892 , https://doi.org/10.1002/2016GL071372 . 10.1002/2016GL071372 Hitchcock , P. , T. G. Shepherd , and G. L. Manney , 2013 : Statistical characterization of Arctic polar-night jet oscillation events . J. Atmos. Sci. , 26 , 2096 – 2116 , https://doi.org/10.1175/JCLI-D-12-00202.1 . Jucker , M. , 2016 : Are sudden stratospheric

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Koji Yamazaki, Masayo Ogi, Yoshihiro Tachibana, Tetsu Nakamura, and Kazuhiro Oshima

1. Introduction The North Atlantic Oscillation (NAO), which is a simultaneous seesaw-like strengthening and weakening of the Icelandic low and the Azores high, has a great impact on the Northern Hemisphere climate, particularly on the European climate (e.g., Hurrell et al. 2003 ). The Arctic Oscillation (AO) or the northern annular mode (NAM) ( Thompson and Wallace 2000 , 2001 ) is the most dominant pattern in the variation of the northern extratropical circulation in winter and includes the

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Gina R. Henderson, Daniel J. Leathers, and Brian Hanson

level pressure in the North Atlantic, implying that excess snow over Siberia could lead to higher pressure over the North Atlantic and a subsequent negative North Atlantic Oscillation (NAO) phase. Gong et al. (2003) further proposed that a positive feedback of vertical propagation of stationary waves over Siberia could weaken the polar vortex in the stratosphere, resulting in a negative Arctic Oscillation (AO) anomaly in the troposphere. The certainty of this mechanism was limited by vertical

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Paul C. Loikith and Anthony J. Broccoli

(NAM), also referred to as the Arctic Oscillation, is characterized by an annular structure of latitudinally stratified geopotential height anomalies of opposing sign with one band at high northern latitudes and the other over the midlatitudes. When the NAM is in the positive phase, the midlatitude westerlies are anomalously strong, inhibiting the southward penetration of surges of cold air into the continental United States. This corresponds to anomalously warm temperatures in the midlatitudes and

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