Arctic dipole anomaly

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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. [1] This pattern sometimes replaces the Arctic oscillation and the North Atlantic oscillation. [2] It was observed for the first time in the first decade of 2000s and is perhaps linked to recent climate change. [3] The Arctic dipole lets more southern winds into the Arctic Ocean resulting in more ice melting. [1] The summer 2007 event played an important role in the record low sea ice extent which was recorded in September. [2] 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. [2]

North America Continent entirely within the Northern Hemisphere and almost all within the Western Hemisphere

North America is a continent entirely within the Northern Hemisphere and almost all within the Western Hemisphere; it is also considered by some to be a northern subcontinent of the Americas. It is bordered to the north by the Arctic Ocean, to the east by the Atlantic Ocean, to the west and south by the Pacific Ocean, and to the southeast by South America and the Caribbean Sea.

Eurasia The combined continental landmass of Europe and Asia

Eurasia is the combined continental landmass of Europe and Asia. The term is a portmanteau of its constituent continents. Located primarily in the Northern and Eastern Hemispheres, it is bordered by the Atlantic Ocean to the west, the Pacific Ocean to the east, the Arctic Ocean to the north, and by Africa, the Mediterranean Sea, and the Indian Ocean to the south. The division between Europe and Asia as two different continents is a historical social construct, with no clear physical separation between them; thus, in some parts of the world, Eurasia is recognized as the largest of the six, five, or even four continents on Earth. In geology, Eurasia is often considered as a single rigid megablock. However, the rigidity of Eurasia is debated based on paleomagnetic data.

Arctic oscillation

The Arctic oscillation (AO) or Northern Annular Mode/Northern Hemisphere Annular Mode (NAM) is a weather phenomenon at the Arctic and Antarctic poles north of 20 degrees latitude. 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 37–45N.

Contents

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. [4] [5] [6] [7] [8] However, other studies, such as those by Watanabe and Hasumi [9] and Vinje, [10] 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. [11] 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%.

The NCEP/NCAR Reanalysis data set is a continually updated (1948–present) globally gridded data set that represents the state of the Earth's atmosphere, incorporating observations and numerical weather prediction (NWP) model output from 1948 to present. It is a joint product from the National Centers for Environmental Prediction (NCEP) and the National Center for Atmospheric Research (NCAR).

In statistics and signal processing, the method of empirical orthogonal function (EOF) analysis is a decomposition of a signal or data set in terms of orthogonal basis functions which are determined from the data. It is similar to performing a principal components analysis on the data, except that the EOF method finds both time series and spatial patterns. The term is also interchangeable with the geographically weighted PCAs in geophysics.

While the Arctic Oscillation has an annular structure centered over and covering the entire Arctic, [12] 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. [11] 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). [11]

Greenland autonomous country within the Kingdom of Denmark

Greenland is an autonomous constituent country of the Kingdom of Denmark between the Arctic and Atlantic oceans, east of the Canadian Arctic Archipelago. Though physiographically a part of the continent of North America, Greenland has been politically and culturally associated with Europe for more than a millennium. The majority of its residents are Inuit, whose ancestors began migrating from the Canadian mainland in the 13th century, gradually settling across the island.

Kara Sea A marginal sea of the Arctic Ocean north of Siberia between Novaya Zemlya and Severnaya Zemlya

The Kara Sea is part of the Arctic Ocean north of Siberia. It is separated from the Barents Sea to the west by the Kara Strait and Novaya Zemlya, and the Laptev Sea to the east by the Severnaya Zemlya archipelago. It is named after the Kara River, which is now relatively insignificant but which played an important role in the Russian conquest of northern Siberia. The Kara River name is derived from Nenets word meaning "hummocked ice".

Laptev Sea Marginal sea in the Arctic Ocean north of Siberia between the Kara Sea and the East Siberian Sea

The Laptev Sea is a marginal sea of the Arctic Ocean. It is located between the northern coast of Siberia, the Taimyr Peninsula, Severnaya Zemlya and the New Siberian Islands. Its northern boundary passes from the Arctic Cape to a point with co-ordinates of 79°N and 139°E, and ends at the Anisiy Cape. The Kara Sea lies to the west, the East Siberian Sea to the east.

Impacts on Arctic Sea Ice

Although the Arctic Oscillation is responsible for more of the total variance in mean sea level pressure over the Arctic, the meridional winds anomalies that arise as a result of the spatial structure of the Arctic dipole anomaly make it the primary driver of Arctic sea ice export variability. [13] During the positive phase of the Arctic dipole anomaly, anomalous winds drive sea ice from the central Arctic out the Fram Strait and into the Greenland Sea via the transpolar drift stream. In contrast, during the negative phase, anomalous winds reduce the removal of sea ice through the Fram Strait. This is supported by Watanabe et al., [13] as well as Wang et al., [14] which show that sea ice export is enhanced during the positive phase of the Arctic dipole anomaly, while export is reduced during the negative phase.

Fram Strait The passage between Greenland and Svalbard

The Fram Strait is the passage between Greenland and Svalbard, located roughly between 77°N and 81°N latitudes and centered on the prime meridian. The Greenland and Norwegian Seas lie south of Fram Strait, while the Nansen Basin of the Arctic Ocean lies to the north. Fram Strait is noted for being the only deep connection between the Arctic Ocean and the World Oceans. The dominant oceanographic features of the region are the West Spitsbergen Current on the east side of the strait and the East Greenland Current on the west.

Transpolar Drift Stream An ocean current of the Arctic Ocean

The Transpolar Drift Stream is a major ocean current of the Arctic Ocean, transporting sea ice from the Laptev Sea and the East Siberian Sea towards Fram Strait. Drift experiments with ships like Fram or Tara showed that the drift takes between two and four years.

However, the Arctic Oscillation cannot be ignored when considering sea ice export from the Arctic. By itself, circulation associated with a positive phase Arctic Oscillation results in an increase in sea ice export, while the negative phase of the Arctic Oscillation is associated with reduced Arctic sea ice export. [6] [8] When considering sea ice export in connection with the Arctic dipole anomaly, the Arctic Oscillation determines the sign of the dominant mean sea level pressure anomaly, while the Arctic dipole anomaly determines the location of the dominant mean sea level pressure anomaly (over the Canadian Arctic Archipelago and northern Greenland, or over the Kara and Laptev Seas). Therefore, while the Arctic dipole anomaly determines whether the overall export of sea ice will be promoted or restricted, the Arctic Oscillation will either enhance or diminish the influence of the Arctic dipole anomaly. [13]

Connection to Extreme Summer Sea Ice Minima

The Arctic dipole anomaly has also been suggested to play an important role in the occurrence of several extreme sea ice minima that have occurred since the mid-1990s, including the minimum in 2007. [14] Wang et al. [14] suggest that in addition to anomalous winds driving sea ice out of the Arctic through the Fram Strait, the positive phase of the Arctic dipole anomalies may also increase the flow of relatively warm waters from the North Pacific through the Bering Strait into the Arctic Ocean. Warmer waters, in addition to increased sea ice export, could result in reduced sea ice areal extent. Additionally, “preconditioning” of sea ice from the previous winter and summer seasons, as well as multidecadal trends, plays a role in determining the minimum sea ice extent for a given year. [14]

See also

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