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Bo Christiansen

1. Introduction The atmospheric low-frequency variability is dominated by a relatively few large-scale modes. In the Northern Hemisphere extratropics, the most prominent include the North Atlantic Oscillation (NAO) and the Arctic Oscillation (AO), while the El Niño–Southern Oscillation (ENSO) dominates in the tropics. Although the variability of these modes is basically generated internally in the coupled atmosphere–ocean system the modes may also respond to external forcings. Volcanic

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Jan-Erik Tesdal, Ryan P. Abernathey, Joaquim I. Goes, Arnold L. Gordon, and Thomas W. N. Haine

might be linked to wind stress curl and climate indices such as the North Atlantic Oscillation (NAO) and the Arctic Oscillation (AO). Freshwater sources affecting salinity that were not explicitly considered in this study include those from the outflow from the Arctic Ocean, river discharge, and glacier melt from Greenland. Recent studies suggest that changes in freshwater outflow from the Arctic through the Fram and Davis Straits are not statistically significant ( Haine et al. 2015 ). 2. Methods

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K. M. de Beurs and G. M. Henebry

the increase in surface winter temperature in Europe and Asia (north of 40°N) ( Hurrell 1996 ; Hurrell and van Loon 1997 ; Thompson et al. 2000 ; Hurrell et al. 2003 ). A mode of climate variability with extensive effects in the Northern Hemisphere, is the northern annular mode (NAM) ( Thompson and Wallace 2001 ), which also goes by the name of the North Atlantic Oscillation (NAO) ( Hurrell 1995 ) or the Arctic Oscillation (AO) ( Thompson and Wallace 1998 ). Thompson and Wallace (2001

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Mario Sempf, Klaus Dethloff, Dörthe Handorf, and Michael V. Kurgansky

were initially motivated by the work of Sempf et al. (2005) , where the impact of orographic forcing and of zonal asymmetries in extratropical diabatic heating on the structure of the Arctic Oscillation (AO) was studied. There, it was necessary to force stationary waves in a physically correct manner, as far as possible. Therefore the nonzonal components of the radiative equilibrium temperature fields were adjusted in a way that, on the time mean, realistic patterns of nonzonal extratropical

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Nan Zhao, Sujie Liang, and Yihui Ding

1. Introduction The Arctic Oscillation (AO) (see Thompson and Wallace 1998 , 2000 ; Wallace 2000 ), also known as the Northern Hemisphere annular mode (NAM), is usually regarded as a result of wave–mean flow interaction ( Limpasuvan and Hartmann 1999 , 2000 ; DeWeaver and Nigam 2000 ; Eichelberger and Holton 2002 ; Lorenz and Hartmann 2003 ; Vallis et al. 2004 ; Riviere and Orlanski 2007 ; Benedict et al. 2004 ; Franzke et al. 2004 ; Feldstein 2003 ; Feldstein and Franzke 2006

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Renguang Wu and Shangfeng Chen

. Studies indicated that SAT variations over Eurasia are impacted by several atmospheric circulation patterns, including the Arctic Oscillation (AO)/North Atlantic Oscillation (NAO) and the Scandinavian pattern ( Thompson and Wallace 1998 ; Gong et al. 2001 ; Barnston and Livezey 1987 ; Zveryaev and Gulev 2009 ; Cheung et al. 2012 ; Chen et al. 2018a , 2019a ; Chen and Song 2019 ). During the positive (negative) phase of the NAO/AO, most parts of mid- to high-latitude Eurasia are dominated by

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Olivia Kellner and Dev Niyogi

warm (El Niño) and cold (La Niña) thresholds, respectively, with neutral events ranging from −0.4 to 0.4. Events are not further classified by −0.5 and +0.5 deviations into weak, moderate, or strong events for ease of use when product users are exploring the climatological data through the online tool interface. The same classification scheme is applied to the AO. The Arctic Oscillation is monitored through the application of an empirical orthogonal function to monthly mean sea level (1000 hPa

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Changhyun Yoo, Sukyoung Lee, and Steven B. Feldstein

winter that there is an increase in warm advection that takes place over West Antarctica via poleward-propagating Rossby wave trains that are responding to an increased sea surface temperature over the central tropical Pacific Ocean. Yoo et al. (2011 , hereafter YFL ) further investigated this relationship between tropical convection and Arctic SAT amplification, with their focus being on the interdecadal trend in the Madden–Julian oscillation (MJO) ( Madden and Julian 1971 , 1972 , 1994 ). They

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Nan Zhao, Sujie Liang, and Yihui Ding

1. Introduction Our previous studies ( Zhao and Takahashi 2006 ; Zhao et al. 2010 , 2012 ) on the origin of the Arctic Oscillation or Northern Hemisphere annular mode (hereafter AO/NAM) ( Thompson and Wallace 1998 , 2000 ) found that the three-dimensional spatial structure of the AO/NAM stems mainly from the linear normal modes of extratropical large-scale dynamics, although the temporal variability of this pattern may well be related to the nonlinear nature of the extratropical wave

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Joe M. Osborne, James A. Screen, and Mat Collins

; Semenov and Latif 2015 ). But can this state dependence actually be a help rather than a hindrance? For example, we know that certain background ocean–atmosphere states, while not strictly predictable, vary on (multi-) decadal time scales. Two dominant patterns of ocean–atmosphere variability in the Northern Hemisphere midlatitudes are the Pacific decadal oscillation (PDO) and the Atlantic multidecadal oscillation (AMO). Recently, Screen and Francis (2016) showed that Arctic warming is enhanced for

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