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Ryan Eastman and Stephen G. Warren

models is that the Arctic experiences a seasonal cycle in cloud cover and precipitation, with peaks of precipitation, cloud cover, and cloud liquid water content (LWC) during summertime and a minimum in cloud cover and precipitation in winter and early spring. Arctic climate variations and changes can be tied to changes in circulation, such as the Arctic Oscillation (AO). In its positive phase, the AO is characterized by an increase in midlatitude westerlies and, in the Arctic, decreased sea level

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Xuanji Wang and Jeffrey R. Key

( Curry et al. 1996 ; Wallace et al. 1996 ; Rigor et al. 2000 ; Chen et al. 2002 ; Groves and Francis 2002a , b ; Chapman and Walsh 1993 ; Myneni et al. 1997 ; Wang and Key 2003 ). A significant change in the climate system occurred in the late 1970s and early 1980s, as revealed in the Arctic Oscillation (AO) and other climate indices ( Thompson and Wallace 1998 ; Thompson and Solomon 2002 ; Wolter and Timlin 1993 ; Zhang et al. 1997 ; Mantua et al. 1997 ; Wallace and Gutzler 1981

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Xiangdong Zhang, John E. Walsh, Jing Zhang, Uma S. Bhatt, and Moto Ikeda

shown to be positive and statistically significant for the winter, spring, and summer; the trend for autumn was also positive but not statistically significant. The positive trend for winter was supported by McCabe et al.'s (2001) results for the 1959–97 period, which showed a substantial increase of wintertime cyclones around 1989 [the final year of Serreze et al.'s (1993) study period], which was also the time of a well-documented increase of the Arctic Oscillation (AO) index ( Thompson and

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Igor V. Polyakov, Roman V. Bekryaev, Genrikh V. Alekseev, Uma S. Bhatt, Roger L. Colony, Mark A. Johnson, Alexander P. Maskshtas, and David Walsh

multidecadal variability in the polar region compared with the lower latitudes. This may suggest that the origin of this variability may be hidden in complex interactions between the Arctic and North Atlantic. This suggestion is supported by the correlations between air temperature from Arctic coastal stations and the North Atlantic Oscillation (NAO) index, which is characterized by a north–south-oriented dipole in sea level pressure structure over the Atlantic. Maximum correlations are found in the near

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Nathanael Harwood, Richard Hall, Giorgia Di Capua, Andrew Russell, and Allan Tucker

tropical oceans, in particular with above-average sea surface temperatures (SSTs) in the Pacific, is known to impact midlatitude flow through intense convection and latent heat release, which generate planetary-scale Rossby waves ( Trenberth et al. 1998 ). El Niño–Southern Oscillation (ENSO) ( Scaife et al. 2017a ) and other tropical Rossby wave source regions ( Scaife et al. 2017b ) provide predictive skill in seasonal midlatitude circulation forecasting, and ENSO has a stronger role in winter. Arctic

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Tiina Nygård, Rune G. Graversen, Petteri Uotila, Tuomas Naakka, and Timo Vihma

variations of the Arctic Oscillation (AO) ( Eastman and Warren 2010 ; Devasthale et al. 2012 ; Jun et al. 2016 ). Recently, much research focus has been on moisture intrusions, which are narrow plumes of anomalously warm and moist air, typically accompanied by increased cloud amount and enhanced downward longwave radiation ( Woods et al. 2013 ; Johansson et al. 2017 ; Liu et al. 2018 ), penetrating to the Arctic. The intrusions are responsible for a major part of the moisture transport to the Arctic

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Xiangdong Zhang, Moto Ikeda, and John E. Walsh

1. Introduction Recent observational studies have demonstrated that the Arctic climate is undergoing dramatic changes. The Arctic anticyclone has weakened and cyclonic activity has strengthened over the central Arctic Ocean since 1988 ( Walsh et al. 1996 ). The large-scale atmospheric leading mode, the Arctic Oscillation (AO; Thompson and Wallace 1998 ) and the North Atlantic Oscillation (NAO; Hurrell 1995 ), have shown pronounced fluctuation. The positive phase of AO or

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I. V. Polyakov, G. V. Alekseev, L. A. Timokhov, U. S. Bhatt, R. L. Colony, H. L. Simmons, D. Walsh, J. E. Walsh, and V. F. Zakharov

(2000) ] is evident in various climatically important parameters of the Arctic air–sea–ice system ( Mysak et al. 1990 ; Yi et al. 1999 ; Venegas and Mysak 2000 ; Polyakov et al. 2002 , 2003a , b ) as well as in proxy records ( Delworth and Mann 2000 ). Understanding the mechanisms behind this variability is not trivial due to its evolving spectrum and changing relationship with large-scale climate parameters like the North Atlantic Oscillation (NAO). However, spectral analysis of the NAO index

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Kunhui Ye and Gabriele Messori

uncertainty concerning Arctic–midlatitudes linkages. A number of mechanisms for the impacts of AA and sea ice loss on the midlatitudes have been proposed, including the modulation of the storm tracks (e.g., Overland et al. 2011 ), of the eddy-driven jet stream ( Francis and Vavrus 2015 ), of planetary waves (e.g., Petoukhov and Semenov 2010 ; Tang et al. 2013 ), and of the North Atlantic Oscillation/Arctic Oscillation (NAO/AO; e.g., Alexander et al. 2004 ; Screen et al. 2014 ). However, large

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Sergey V. Shoutilin, Alexander P. Makshtas, Motoyoshi Ikeda, Alexey V. Marchenko, and Roman V. Bekryaev

-level air temperature rather than to those in the atmospheric circulation. In summary, the change in Arctic ice cover was referenced to the Arctic Oscillation/North Atlantic Oscillation (AO/NAO) through ice advection and air temperature ( Zhang et al. 1998 ), and a cyclonic frequency over the central Arctic ( Maslanik et al. 1996 ). Rothrock et al. (1999) discussed the possible thermodynamic processes that could produce the observed thinning, such as an increase in oceanic heat flux, poleward

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