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Arctic dipole anomaly
The Arctic dipole anomaly is a pressure pattern characterized by high pressure on the arctic regions of North America, and a low pressure on the Eurasia region.〔(【引用サイトリンク】url=http://nsidc.org/arcticseaicenews/2010/070610.html )〕 This pattern sometimes replaces the Arctic oscillation and the North Atlantic Oscillation. It was observed for the first time in the first decade of 2000s and is perhaps linked to recent climate change. The Arctic dipole lets more southern winds into the Arctic ocean resulting in more ice melting.〔 The summer 2007 event played an important role in the record low sea ice extent which was recorded in September.〔 The Arctic dipole has also been linked to changes in arctic circulation patterns that cause drier winters in Northern Europe, but much wetter winters in Southern Europe and colder winters in East Asia, Europe and the eastern half of North America.〔
== Description ==
In the 1990s and early 2000s, many studies of Arctic sea ice export focused on the Arctic Oscillation and North Atlantic Oscillation as the primary drivers of export.〔Kwok, R., 2000: Recent changes in Arctic Ocean sea ice motion associated with the North Atlantic Oscillation. ''Geophys. Res. Lett.'', 27, 775-778.〕〔Kwok, R., and D.A. Rothrock, 1999: Variability of Fram Strait ice flux and North Atlantic Oscillation. ''J. Geophys. Res.'', 104, 5177-5189.〕〔Rigor, I.G., J.M. Wallace, R.L. Colony, 2002: Response of sea ice to the Arctic Oscillation. ''J. Clim.'', 15, 2648-2663.〕〔Wang, J., and M. Ikeda, 2000: Arctic Oscillation and Arctic Sea-Ice Oscillation, ''Geophys. Res. Lett.'', 27, 1287-1290.〕〔Zhang, X., M. Ikeda, J.E. Walsh, 2003: Arctic sea ice and freshwater changes driven by the atmospheric leading mode in a coupled sea ice-ocean model. ''J. Clim.'', 16, 2159-2177.〕 However, other studies, such as those by Watanabe and Hasumi〔Watanabe, E., and H. Hasumi, 2005: Arctic sea ice response to wind stress variations. ''J. Geophys. Res.'', 110, C11007, doi:10.1029/2004JC002678.〕 and Vinje,〔Vinje, T, 2001: Fram Strait ice fluxes and atmospheric circulation: 1950-2000. ''J. Clim.'', 14, 3508-3517.〕 suggested that the Arctic Oscillation and North Atlantic Oscillation did not always explain the variability in sea ice export.
In 2006, the Arctic dipole anomaly was formally proposed by Bingyi Wu, Jia Wang, and John Walsh, using the NCEP/NCAR reanalysis datasets spanning 1960-2002.〔Wu, B., J. Wang, and J.E. Walsh, 2006: Dipole Anomaly in the Winter Arctic Atmosphere and Its Association with Sea Ice Motion. ''J. Clim.'', 19, 210-225, doi:10.1175/JCLI3619.1.〕 It is defined as the spatial distribution of the second leading EOF mode of monthly mean sea level pressure north of 70°N, where the first leading mode corresponds to the Arctic Oscillation. When defined for the winter season (October through March), the first leading mode (Arctic Oscillation) accounts for 61% of the total variance, while the second leading mode (Arctic dipole anomaly) accounts for 13%.
While the Arctic Oscillation has an annular structure centered over and covering the entire Arctic,〔Thompson, D., and J.M. Wallace, 1998: The Arctic Oscillation signature in the wintertime geopotential height and temperature fields. ''Geophys. Res. Lett.'', 25, 1297-1300.〕 the Arctic dipole anomaly has two poles of opposite sign: one over the Canadian Arctic Archipelago and northern Greenland, the other over the Kara and Laptev Seas.〔 This dipole structure leads to a pressure gradient with a zero isopleth oriented from the Bering Strait, across the Arctic to the Greenland and Barents Seas. As a result, anomalous winds are generally directed parallel to the zero isopleth either towards the Greenland and Barents Seas (positive Arctic dipole anomaly) or toward the Bering Strait (negative Arctic dipole anomaly).〔
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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