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Panxi Dai and Benkui Tan

1. Introduction The Arctic Oscillation (AO), also termed as the northern annular mode (NAM), is the leading mode of natural climate variability in the extratropics of the wintertime Northern Hemisphere from intraseasonal to decadal scales. The AO pattern consists of three centers of action: one is located over the North Pacific and the other two are located in the North Atlantic, which has a large projection onto the North Atlantic Oscillation (NAO). Since the initial work of Thompson and

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Sam B. Cornish, Yavor Kostov, Helen L. Johnson, and Camille Lique

Hemisphere, the Arctic Oscillation (AO; Thompson and Wallace 1998 ) captures the dominant variability in the large-scale wind forcing and therefore represents a natural starting point in identifying causal relationships between atmospheric and oceanic/cryospheric variability. Indeed, several studies have focused on determining the influence of the AO on patterns of sea ice drift ( Rigor et al. 2002 ; Kwok 2009 ; Kwok et al. 2013 ; Armitage et al. 2018 ) and surface geostrophic circulation ( Morison

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Hisanori Itoh

1. Introduction The Arctic Oscillation (AO) proposed by Thompson and Wallace (1998) is defined as the empirical orthogonal function (EOF) first mode of the monthly mean sea level pressure (SLP) in the Northern Hemisphere. The spatial pattern is a seesaw between the Arctic region and the midlatitude. In this sense, the pattern is also called the annular mode (AM). 1 Since the AO is related to many aspects in large-scale fields, it has been the subject of much attention and many research

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Hyo-Seok Park, Andrew L. Stewart, and Jun-Hyeok Son

). However, there were notable rebounds in sea ice extent, such as the 30% rebound in 2013 ( Tilling et al. 2015 ). It has previously been hypothesized that the positive Arctic Oscillation (AO)-induced sea ice thinning in winter can impact the summer sea ice extent ( Rigor et al. 2002 ; Holland and Stroeve 2011 ). A recent study also found that the years of anomalously small summer sea ice extent have often followed winter AO-induced ice drift away from the Arctic coast since 1991–92 ( Williams et al

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Fei Li, Huijun Wang, and Yongqi Gao

(AL), the East Asian trough (EAT) in the middle troposphere, and the East Asian jet stream (EAJS) in the upper troposphere. Factors that may influence the EAWM include El Niño–Southern Oscillation (ENSO; Webster and Yang 1992 ; Zhang et al. 1996 ; Lau and Nath 2000 ; Wang and He 2012 ; He and Wang 2013a ; Wang et al. 2013 ), the Arctic Oscillation (AO; Gong et al. 2001 ; Wu and Wang 2002 ; He and Wang 2013b ), the Antarctic Oscillation ( Fan and Wang 2004 , 2006 ), conditions on the

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Hainan Gong, Lin Wang, Wen Chen, and Debashis Nath

1. Introduction The Arctic Oscillation (AO) proposed by Thompson and Wallace (1998) is defined as the first empirical orthogonal function (EOF) mode of the sea level pressure (SLP) in the extratropical Northern Hemisphere. It reflects a seesaw in the SLP field between the Arctic and midlatitudes. In this sense, the AO is also referred to as the northern annular mode ( Thompson and Wallace 2001 ). Previous studies have demonstrated that the AO can strongly influence the global and regional

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Dingzhu Hu, Yi-Peng Guo, Zhe-Min Tan, and Zhaoyong Guan

mechanisms. As the dominant mode of NH extratropical variability, the boreal spring Arctic Oscillation (AO) is primarily driven by midlatitude eddies and has significant impacts on NH climate in both the tropics and extratropics (e.g., Thompson and Wallace 2000 ; Thompson et al. 2000 , 2003 ; Vallis et al. 2004 ; Nakamura et al. 2006 ; Gong et al. 2011 ; Chen et al. 2014a , 2017 ). The spring AO is connected to the climate variability such as precipitation, tropopause height, and total column

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Hongxu Zhao and G. W. K. Moore

1. Introduction The North Atlantic Oscillation (NAO) and the Arctic Oscillation (AO) are two of the most important modes characterizing climate variability especially during the boreal winter ( Hurrell 1995 ; Thompson and Wallace 1998 , hereafter TW98 ; Thompson and Wallace 2000 ; Wanner et al. 2001 ; Rigor et al. 2002 ). Both represent modes of variability associated with a meridional displacement of atmospheric pressure in the Northern Hemisphere (NH), with the NAO being more regionally

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Robert J. Allen and Charles S. Zender

1. Introduction The Arctic Oscillation (AO) ( Thompson and Wallace 1998 ) and its regional manifestation, the North Atlantic Oscillation (NAO) ( Wallace and Gutzler 1981 ), are associated with climate variability that affects much of North America and Eurasia (EA). This includes north–south shifts of zonal wind and storminess and large anomalies in surface temperature and precipitation. Although both appear to be fundamental, internal modes of the atmosphere, they may be modulated by external

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Jinqing Zuo, Hong-Li Ren, and Weijing Li

1. Introduction The Arctic Oscillation (AO) is the dominant mode of winter atmospheric low-frequency variability in the extratropical Northern Hemisphere (NH), and its typical structure is characterized by a pressure seesaw between the Arctic and middle latitudes of the NH ( Thompson and Wallace 1998 , 2000 ). The atmospheric circulation anomalies associated with the AO over the North Atlantic sector greatly resemble those associated with the North Atlantic Oscillation (NAO). The time series

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