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James Williams, Bruno Tremblay, Robert Newton, and Richard Allard

1.5 m was observed from repeated submarine transects between the coast of Alaska and the North Pole. The decrease in sea ice draft is attributed primarily to sea ice dynamics with thermodynamics playing a less influential role ( Tucker et al. 2001 ). The particular dynamical mechanisms leading to the reduction in thickness are an anomalously weak Beaufort Gyre, a broad Transpolar Drift Stream, and high export of multiyear ice (MYI) associated with a positive Arctic Oscillation (AO) index in the

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H. L. Tanaka and Hiroki Tokinaga

1. Introduction Arctic oscillation (AO) advocated by Thompson and Wallace (1998) 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 midlatitudes. It is defined as a primary mode of an empirical orthogonal function (EOF) for the sea level pressure field in the Northern Hemisphere (NH). The spatial pattern of the AO is characterized by its zonally symmetric or “annular

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Yongjia Lu, Wenshou Tian, Jiankai Zhang, Jinlong Huang, Ruhua Zhang, Tao Wang, and Mian Xu

1. Introduction The Arctic Oscillation (AO), also called the northern annular mode (NAM), shows a deep and nearly barotropic structure, extending from the troposphere to the stratosphere, and is closely associated with the stratospheric polar vortex (e.g., Thompson and Wallace 1998 ; Baldwin and Dunkerton 1999 , 2001 ; Mukougawa and Hirooka 2004 ; Luo et al. 2012 ; Cheng and Tan 2019 ). Overall, there is a possibility that stratospheric NAM signals can extend downward into the troposphere

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Hengchun Ye, Eric J. Fetzer, Ali Behrangi, Sun Wong, Bjorn H. Lambrigtsen, Crysti Y. Wang, Judah Cohen, and Brandi L. Gamelin

circulation on Arctic climate variability, represented by the Arctic Oscillation (AO; Thompson and Wallace 1998 ), which describes the fluctuation of atmospheric pressure differences between the central Arctic and the two weaker centers at about 45°N over the Atlantic and Pacific basins. A regional study over western Siberia suggested a large influence of the AO on air temperature and a lesser influence on precipitation during winter ( Frey and Smith 2003 ). Thus, to understand the association between

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Mitchell Bushuk, Dimitrios Giannakis, and Andrew J. Majda

between the Bering Sea and Sea of Okhotsk ( Deser et al. 2000 ). Regression of sea level pressure (SLP) onto the corresponding principal component (PC) yields a spatial pattern that closely resembles the Arctic Oscillation (AO; Thompson and Wallace 1998 ), the leading pattern of SLP variability north of 20°N. Deser et al. (2000) observe a connection between the low-frequency (interannual to decadal) variability of the atmosphere and the low-frequency variability of sea ice. In particular, they find

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Masashi Kohma, Seiya Nishizawa, and Shigeo Yoden

maximum phase. Asymmetry of the time variations of SSW and VI events is characterized by the difference in the acceleration process of the stratospheric westerly wind: deceleration due to wave driving can be rapid, while acceleration associated with radiative cooling is generally more gradual. As for the slow variations in the Northern Hemisphere troposphere, the Arctic Oscillation (AO) is one of the well-known slow variations. The term was coined by Thompson and Wallace (1998) as the leading

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Yuki Kanno, John E. Walsh, and Toshiki Iwasaki

), North Atlantic Oscillation (NAO), and Arctic Oscillation (AO) affect climate variability over NA during winter ( Rogers 1984 ; Thompson and Wallace 1998 ; Thompson and Wallace 2001 ; Walsh et al. 2001 ; Cellitti et al. 2006 ; Marinaro et al. 2015 ). Figure 1 shows the 500-hPa manifestations of these large-scale modes of variability. The PNA and TNH patterns are related to variations in sea surface temperature in the tropical Pacific Ocean, especially El Niño–Southern Oscillation (ENSO) ( Yu

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Y. Peings, D. Saint-Martin, and H. Douville

evidence that the Siberian snow cover, particularly in October, is a good precursor of the Arctic Oscillation (AO; Thompson and Wallace 1998 ), which is also described as the northern annular mode (NAM). The snow cover extent also correlates well with the North Atlantic Oscillation (NAO), which is sometimes considered as a regional signature of the annular mode ( Bojariu and Gimeno 2003 ). This lagged correlation has important implications for seasonal forecasting, as it could lead to improved

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Thomas Jung and Michael Hilmer

1. Introduction The North Atlantic oscillation (NAO)—a phenomenon that is well known for many decades—is the dominant mode of North Atlantic atmospheric variability and describes the simultaneous strengthening and weakening of the Azores high and Icelandic low (e.g., Defant 1924 ; Walker 1924 ; Cayan 1992 ; Hurrell 1995 ). Recently, the link between the NAO and Arctic sea ice export through Fram Strait during wintertime has attracted increasing scientific interest (e.g., Kwok and Rothrock

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Patricia DeRepentigny, L. Bruno Tremblay, Robert Newton, and Stephanie Pfirman

and the total volume of ice in the summer have been steadily decreasing since 1987. The large-scale pattern of sea level pressure in the Arctic can be characterized, to first order, by the Arctic Oscillation (AO) ( Thompson and Wallace 1998 ), the first EOF of Northern Hemisphere sea level pressure, or by the northern annular mode (NAM) ( Thompson and Wallace 2001 ), a closely related high-latitude pattern. Rigor et al. (2002) showed that the AO explains 52% of the variance of sea level pressure

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