The Arctic dipole anomaly is a pressure pattern characterized by high pressure on the arctic regions of North America and low pressure on those of Eurasia. 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.
In the 1990s and early 2000s, many studies of Arctic sea ice export focused on the Arctic and North Atlantic oscillations as the primary drivers of export. However, other studies, such as those by Watanabe and Hasumi and Vinje, suggested that the Arctic and North Atlantic oscillations 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. It is defined as the spatial distribution of the second leading empirical orthogonal functions 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, 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.
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The Arctic oscillation (AO) or Northern Annular Mode/Northern Hemisphere Annular Mode (NAM) is a weather phenomenon at the Arctic pole north of 20 degrees latitude. It is an important mode of climate variability for the Northern Hemisphere. The southern hemisphere analogue is called the Antarctic oscillation or Southern Annular Mode (SAM). The index varies over time with no particular periodicity, and is characterized by non-seasonal sea-level pressure anomalies of one sign in the Arctic, balanced by anomalies of opposite sign centered at about 37–45° N.
A circumpolar vortex, or simply polar vortex, is a large region of cold, rotating air that encircles both of Earth's polar regions. Polar vortices also exist on other rotating, low-obliquity planetary bodies. The term polar vortex can be used to describe two distinct phenomena; the stratospheric polar vortex, and the tropospheric polar vortex. The stratospheric and tropospheric polar vortices both rotate in the direction of the Earth's spin, but they are distinct phenomena that have different sizes, structures, seasonal cycles, and impacts on weather.
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