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Youmin Tang, Hai Lin, Jacques Derome, and Michael K. Tippett

1. Introduction The Arctic Oscillation (AO) is the dominant mode of monthly mean sea level pressure variability over the Northern Hemisphere with an out-of-phase relation between the sea level pressure over the Arctic Basin and at the midlatitudes ( Thompson and Wallace 1998 ). The AO has a close association with the North Atlantic Oscillation (NAO) due to its strong manifestation over the Atlantic sector. The interannual and longer-term changes in the wintertime AO have an enormous impact on

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Tae-Won Park, Chang-Hoi Ho, and Song Yang

a wave train across the Eurasian continent and a blocking from the North Pacific, respectively. The wave train type and the blocking type have different mechanisms. The former is a growing baroclinic wave, whereas the latter is a dipole pattern related to slowly retrogressing blockings. Previous studies have also addressed the influences of large-scale climate phenomena such as the Arctic Oscillation (AO; Thompson and Wallace 1998 ), El Niño–Southern Oscillation (ENSO), and Madden

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Jun-Mei Lü, Seong-Joong Kim, Ayako Abe-Ouchi, Yongqiang Yu, and Rumi Ohgaito

( Gladstone et al. 2005 ; Braconnot et al. 2007a ; Brewer et al. 2007 ). Nevertheless, there are still significant discrepancies between different model results ( Braconnot et al. 2007b ; Weber et al. 2007 ; Zheng et al. 2008 ). The Arctic Oscillation (AO) is a dominant mode of atmospheric variability in the Northern Hemisphere (NH). A large number of studies have revealed that the AO is an important determinant of the winter climate at middle and high latitudes in the NH ( Kerr 1999 ; Thompson and

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Daniela I. V. Domeisen, Gualtiero Badin, and Inga M. Koszalka

1. Introduction The North Atlantic Oscillation (NAO) and the Arctic Oscillation (AO) are spatiotemporal patterns of airmass variability in the North Atlantic region and the extratropical Northern Hemisphere, respectively ( Hurrell 1995 ; Greatbatch 2000 ; Hurrell et al. 2001 ; Visbeck et al. 2001 ). The NAO and AO patterns dominate the variability of the atmosphere on time scales of days to decades ( Thompson and Wallace 2000 ; Thompson et al. 2000 ; Woollings et al. 2015 ). They arise as

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Lei Song and Renguang Wu

; Bueh et al. 2011 ; Shoji et al. 2014 ; Song and Wu 2017 ), the deepening of the East Asian trough ( Zhang et al. 1997 ; Jeong and Ho 2005 ; Bueh et al. 2011 ; Song et al. 2016 ; Song and Wu 2017 ), and Rossby wave trains propagating along the polar front jet and the subtropical jet ( Watanabe 2004 ; Takaya and Nakamura 2005a , b ; Song et al. 2016 ; Song and Wu 2017 ). The Arctic Oscillation (AO) ( Thompson and Wallace 1998 , 2000 ) is an important factor in the occurrence of cold events

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Shangfeng Chen, Wen Chen, Renguang Wu, and Linye Song

–Japan teleconnection ( Nitta 1987 ; Huang and Sun 1992 ), Eurasian snow cover ( Wu and Kirtman 2007 ; Zhao et al. 2007 ), and atmospheric systems over midhigh latitudes of Eurasia ( Li et al. 2001 ; Gong et al. 2002 ; Gong and Ho 2003 ; Ding and Wang 2005 ). The Arctic Oscillation (AO) is the first leading mode of atmospheric variability over extratropical Northern Hemisphere (NH) ( Thompson and Wallace 1998 , 2000 ). Studies demonstrated that boreal spring AO has a significant impact on the following EASM

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Xiao-Yi Yang, Xiaojun Yuan, and Mingfang Ting

the stratospheric polar vortex ( Kim et al. 2014 ). The atmospheric responses in all of the above studies tend to be related to the changes in the Arctic Oscillation (AO), or its regional manifestation, the North Atlantic Oscillation (NAO) mode. Nevertheless, the relatively short observational record and the low signal-to-noise ratio in the midlatitude make it difficult to interpret the physical linkage between Arctic sea ice and AO/NAO modes ( Screen et al. 2013 , 2014 ). Because of the

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Ho Nam Cheung, Wen Zhou, Hing Yim Mok, and Man Chi Wu

between the extratropics of the East Asian continent and the tropics of the Pacific Ocean. Therefore, an accurate forecast of the EAWM should consider climate factors from both sources. On one hand, climate variability over the extratropical region can be captured by the Arctic Oscillation (AO; Thompson and Wallace 1998 ). On interannual time scales, Gong et al. (2001) showed that it is negatively correlated with the intensity of the Siberian high such that the AO may exert an impact on the EAWM

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Tímea Haszpra, Dániel Topál, and Mátyás Herein

1. Introduction The most prominent warming during the recent epoch of global climate change has occurred in and around the Arctic ( Serreze and Francis 2006 ). The Arctic Oscillation (AO)—the leading mode of interannual atmospheric variability in the Northern Hemisphere winter—is designated as a key factor of Arctic atmospheric dynamics describing the hemispheric-scale meridional dipole structure of pressure anomalies ( Thompson and Wallace 1998 ). Most often the AO is calculated as the leading

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Young-Joon Kim and Maria Flatau

weeks and can last longer than 2 months ( Baldwin and Dunkerton 2001 ). This effect is often measured by an index of the Arctic Oscillation (AO; Thompson and Wallace 1998 ), which describes the dominant pattern of nonseasonal sea level pressure variations north of 20°N. It has been reported that the surface pressure signal shifts to a negative pattern of the AO after SSW events ( Limpasuvan et al. 2004 ). However, the negative phase of the AO, which signals a weak tropospheric polar vortex, does

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