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Jia Wang, Xuezhi Bai, Haoguo Hu, Anne Clites, Marie Colton, and Brent Lofgren

Tropical–North Hemisphere (TNH), the North Atlantic Oscillation (NAO) or the Arctic Oscillation (AO) ( Thompson and Wallace 1998 ; Wang and Ikeda 2000 , 2001 ; Wang et al. 2005 ), the Polar/Eurasian (POL), and the West Pacific (WP), etc., are associated with anomalous ice cover on the Great Lakes ( Assel and Rodionov 1998 , Assel et al. 2003 ; Rodionov and Assel 2000 , 2001 ). Combinations of threshold values (both positive and negative) of the POL, PNA, and TNH indices accounted for much of the

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Andrew W. Robertson

1. Introduction The Arctic oscillation (AO) emerges as the leading empirical mode of wintertime monthly sea level pressure (SLP) over the Northern Hemisphere (NH) ( Lorenz 1951 ; Kutzbach 1970 ; Trenberth and Paolino 1981 ; Wallace and Gutzler 1981 ; Thompson and Wallace 1998 ). Its spatial structure is characterized by anomalous SLP of one sign throughout the Arctic Basin, with anomalies of the opposite sign centered over the Azores and, more weakly, over the North Pacific (see Fig. 1f

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Lon Hood, Semjon Schimanke, Thomas Spangehl, Sourabh Bal, and Ulrich Cubasch

.g., HS12 ). This, combined with a strong positive Aleutian response, yields a pattern that resembles a positive mode of the Arctic Oscillation (AO; Thompson and Wallace 1998 ). As argued, for example, by Meehl et al. (2009) , model simulations of 11-yr climate responses with amplitudes comparable to those reported in observations probably require inclusion of indirect “top down” forcing via the stratosphere in addition to any “bottom up” forcing resulting from relatively small total solar

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H. L. Tanaka

1. Introduction The Arctic Oscillation (AO) postulated by Thompson and Wallace (1998 , 2000) has attracted more attention in recent years. The AO is a north–south seesaw of the atmospheric mass between the Arctic region poleward of 60°N and a surrounding zonal ring in the midlatitudes. It is defined as a primary mode of an empirical orthogonal function (EOF) for the sea level pressure field in the Northern Hemisphere. The spatial pattern of the AO is characterized by its zonally symmetric or

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Judah Cohen and Justin Jones

1. Introduction Prediction of the phase and magnitude of the dominant mode of Northern Hemisphere climate variability, referred to as the Arctic Oscillation (AO) or northern annular mode (NAM), is considered the next most important anticipated advance in seasonal winter climate prediction ( Cohen 2003 ). Studies have shown that, on synoptic time scales, the variability in phase of the AO is due to wave breaking, for example, Feldstein and Franzke (2006) . Nonetheless, most studies on longer

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Yuta Ando, Masayo Ogi, and Yoshihiro Tachibana

control over variations of air temperatures over Japan, focusing on the anomalous cold winter of 2012/13 in the Northern Hemisphere. One of most important components of atmospheric circulation in the winter over the Northern Hemisphere is the Arctic Oscillation (AO) as defined by Thompson and Wallace (1998) . The winter AO is strongly coupled with SAT fluctuations over midlatitudes ( Thompson and Wallace 2000 ). The negative phase of the AO directly influences the occurrence of cold surges over East

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Anthony J. Broccoli, Thomas L. Delworth, and Ngar-Cheung Lau

) temperature trends to temporal fluctuations in extratropical circulation. Of particular interest is the proposed relationship between NH extratropical mean temperature and the Arctic oscillation (AO). The AO, as defined by Thompson and Wallace (1998, 2000) , is a pattern of atmospheric variability characterized by a zonally symmetric redistribution of atmospheric mass between the Arctic and midlatitudes, extending from the lower stratosphere to the surface. The AO bears some similarity ( Deser 2000) to

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Jonathan Edwards-Opperman, Steven Cavallo, and David Turner

) and its potential impact on the Atlantic meridional overturning circulation (AMOC; Rahmstorf et al. 2015 ). As the Arctic continues to warm, it is imperative that a better understanding of Greenland’s climate and the meteorological factors relating to the melt of the GIS is achieved. Previous research concerning atmospheric influences on the GIS has noted the importance of the North Atlantic Oscillation (NAO). The negative phase of the NAO (−NAO) has been linked with the advection of warm, moist

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Neeraj Agarwal, Armin Köhl, Carlos Roberto Mechoso, and Detlaf Stammer

to the Integrated Climate Analysis and Prediction (CliSAP) Excellence Cluster. REFERENCES Ambaum , M. H. , B. J. Hoskins , and D. B. Stephenson , 2001 : Arctic Oscillation or North Atlantic Oscillation? J. Climate , 14 , 3495 – 3507 , doi: 10.1175/1520-0442(2001)014<3495:AOONAO>2.0.CO;2 . An , S.-I. , H. Kim , and B.-M. Kim , 2013 : Impact of freshwater discharge from the Greenland Ice Sheet on North Atlantic climate variability . Theor. Appl. Climatol. , 112, 29 – 43

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Masahiro Watanabe and Fei-Fei Jin

1. Introduction A concept of Arctic Oscillation (AO), or the northern annular mode, proposed by Thompson and Wallace (1998) has recently provided a different view of the low- frequency atmospheric variability from the classic teleconnection. The AO, defined by the leading empirical orthogonal function (EOF) to the Northern Hemisphere sea level pressure (SLP) anomalies, has a hemispheric extent dominated by a zonally uniform structure as shown by the AO-covariant height anomalies ( Fig. 1a

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