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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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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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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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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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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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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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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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Atsuhiko Isobe and Robert C. Beardsley

interannual variations in the frequency and intensity of cold-air outbreaks may contribute significantly to biological variability in the Japan Sea. For the first time, this interannual variation in cold-air outbreak activity is investigated here using archived ocean and atmosphere datasets. A notable conclusion shown in section 5 is that cold-air outbreaks strengthen locally over the Japan Sea when the East Asian winter monsoon weakens in warm winters caused by the decadal Arctic Oscillation (AO

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Hiroyuki Ito, Nathaniel C. Johnson, and Shang-Ping Xie

remote forcing through El Niño–Southern Oscillation (ENSO) and associated Indian Ocean SST variations ( Wu et al. 2009 ; Xie et al. 2009 , 2010 ; Kosaka and Nakamura 2010 ; Chowdary et al. 2011 ; Hu et al. 2011 ; Zhou et al. 2011 ). Also, snowfall over the Tibetan Plateau and the phase of the Arctic Oscillation (AO) in spring leads circulation anomalies over East Asia in summer ( Seol and Hong 2009 ; Zhou el al. 2011 ). Important factors for the summer climate of East Asia are two atmospheric

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Chaoxia Yuan and Wenmao Li

Aleutian low, and the stronger EAWM (e.g., Yang et al. 2002 ; Kuang et al. 2008 ). The Arctic Oscillation (AO; Thompson and Wallace 1998 , 2000 ) is one of the most prominent patterns of the large-scale circulation anomalies in the Northern Hemisphere. In the winter, when the AO is in its negative phase, there are positive sea level pressure (SLP) anomalies in the polar region surrounded by negative ones in the midlatitudes. The weakened polar vortex leads to more southward intrusion of the cold

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