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David W. Stahle, Edward R. Cook, Dorian J. Burnette, Max C. A. Torbenson, Ian M. Howard, Daniel Griffin, Jose Villanueva Diaz, Benjamin I. Cook, A. Park Williams, Emma Watson, David J. Sauchyn, Neil Pederson, Connie A. Woodhouse, Gregory T. Pederson, David Meko, Bethany Coulthard, and Christopher J. Crawford

, but the NAO teleconnection to instrumental and reconstructed DJFMA precipitation also weakened during the earliest period of instrumental GPCC precipitation observations from 1892 to 1927 (not shown). The Arctic Oscillation (AO) is an annular mode of zonal circulation between 35° and 55°N ( Ambaum et al. 2001 ). The NAO and PNA are related to the AO ( Ambaum et al. 2001 ), and the correlation between indices of the NAO and AO for the winter to midspring season (DJFMA) is r = 0.68 ( p < 0

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R. W. Higgins, V. B. S. Silva, W. Shi, and J. Larson

PDO on precipitation (specifically the frequency of extreme daily rainfall) over the contiguous United States is largely through its modulation of ENSO. The circulations of both hemispheres exhibit important ring-like (or annular) modes of variability encircling the poles that fluctuate on time scales ranging from a week to decades (e.g., Thompson and Wallace 2000 ). The Northern Hemisphere annular mode, often referred to as the Arctic Oscillation (AO) ( Thompson and Wallace 1998 ), is marked by

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Jin-Song von Storch

1. Introduction Thompson and Wallace (1998 , 2000) found that the primary modes of the geopotential height field in each hemisphere are remarkably similar. Both modes, referred to as the Antarctic and the Arctic Oscillations (AAO and AO), are related to meridional dipoles in zonal-mean zonal wind field and to seesaws in atmospheric mass in the polar regions and the low-latitude zonal rings. While the similarity between the primary modes in the two hemispheres became more apparent in the

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Ignatius G. Rigor, John M. Wallace, and Roger L. Colony

Oscillation (AO), whose index is defined as the leading principal component (PC) of Northern Hemisphere SLP ( Thompson and Wallace 1998 ). The AO can be characterized as an exchange of atmospheric mass between the Arctic Ocean and the surrounding zonal ring centered ∼45°N. The observed trend in the AO toward its “high index” polarity (i.e., toward stronger westerlies at subpolar latitudes and lower SLP over the Arctic) is a way of interpreting the observed decrease in SLP over the North Pole and the

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Asgeir Sorteberg and Børge Kvingedal

the second half of the twentieth century. McCabe et al. (2001) showed that high-latitude cyclone frequencies correlate well (0.69) with the Arctic Oscillation (AO), which is the dominant pattern of sea level pressure (SLP) variations north of 20°N ( Thompson and Wallace 1998 ) over the 1959–97 period. Many studies have focused on the connection between Arctic sea ice variability and the AO or the North Atlantic Oscillation (NAO; Hurrell 1995 ), which essentially describes the same airmass

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Courtenay Strong and Robert E. Davis

circulation variability has also been described in the context of a more hemispheric seesaw of atmospheric mass known as the Arctic Oscillation (AO) ( Thompson and Wallace 1998 ), the leading empirical orthogonal function (EOF) of sea level pressure poleward of 20°N. The AO partially overlaps the NAO in the Atlantic but features a zonally symmetric structure over the Pacific, covers more of the Arctic, and is discernible in geopotential height fields from the troposphere into the lower stratosphere

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Yuefeng Li and L. Ruby Leung

temperature and rainfall in the Yangtze and Huai Rivers and an increase of rainfall in southeastern China ( Zhao et al. 2007 ). More recently, the relationships between the Asian summer monsoon anomalies and the Arctic have become active research topics. However, our understanding of the causes and effects of the anomalies is far from complete. A few papers have investigated the influence of the Arctic ice concentration ( B.-Y. Wu et al. 2009a , b ) and the Arctic Oscillation (AO) on the East Asian summer

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Lejiang Yu, Shiyuan Zhong, Mingyu Zhou, Donald H. Lenschow, and Bo Sun

. 2015 ) have contributed to the decrease in the Arctic sea ice extent. Besides the increased greenhouse gas emissions, natural variability of the climate system may have also contributed to the Arctic sea ice depletion. The downward trend of the Arctic sea ice prior to the 1990s has been linked to a positive trend in the North Atlantic Oscillation (NAO) index ( Deser et al. 2000 ), and this linkage is expected to extend into the first decade of the twenty-first century ( Ogi et al. 2010 ). The

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Marika M. Holland

appear to be interrelated and to be associated with variability and trends in the North Atlantic Oscillation (NAO) or the closely related Arctic Oscillation (AO; Thompson and Wallace 1998 ). The North Atlantic Oscillation is the dominant mode of variability in the North Atlantic region and represents a redistribution of atmospheric mass between centers of action located near the Azores high and the Icelandic low. A high NAO phase indicates a strengthening of the Azores high and the Icelandic low

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Florence Colleoni, Simona Masina, Annalisa Cherchi, and Doroteaciro Iovino

between K115 and K229 (cf. Fig. 5l with Fig. 5f ). While GHG have a homogeneous impact on Northern Hemisphere mean annual temperature, the impact of orbital parameters shows a larger spatial variability ( Figs. 5g–i ). Groll et al. (2005) and Groll and Widmann (2006) suggest that the orbital configuration of the simulated time period determines the structure of temperature-related teleconnections in the northern high latitudes, specifically the Arctic Oscillation. According to Thompson and

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