Atmospheric Circulation

The long-term average pressure systems (discounting the short-lived cyclonic storms) reveal several important semipermanent features. The Icelandic Low and the Aleutian Low have a strong influence in winter,

Figure 2: Distribution of precipitation (mm/yr) in the Arctic. From AMAP Assessment Report: Arctic Pollution Issues, Arctic Monitoring and Assessment Program, 1998. Reproduced with permission causing traveling lows to intensify and develop into frequent strong cyclones between Iceland and southern Greenland and near the Aleutian Islands, respectively (see cyclone activity below). In winter, eastern Eurasia is dominated by the semipermanent Siberian High and persistent high pressure is also found over the Canadian Arctic Archipelago (the Beaufort High). These high-pressure centers are associated with stable and intense cold air that blocks cyclonic storms.

A distinctive characteristic of the Arctic atmosphere is the surface weather systems associated with a large-scale, cold-cored westerly circumpolar circulation in the middle and upper atmosphere (see Polar Vortex). Cyclones (low atmospheric pressure areas) and anticyclones (high pressure) are embedded in and steered by this flow, and variability in the path of these can radically affect the weather and climate beneath the vortex.

The synoptic (large-scale instantaneous state of the atmosphere) activity in the Arctic has changed since the 1950s, with winter cyclone activity being most

© AMAP 2003
Figure 3: Mean atmospheric sea-level pressure (mbar) in the Arctic in January and July. From AMAP Assessment Report: Arctic Pollution Issues, Arctic Monitoring and Assessment Program, 1998. Reproduced with permission

common near Iceland, between Svalbard and Scandinavia, the Norwegian and Kara seas, Baffin Bay, and the eastern Canadian Arctic. Cyclone tracks in winter most frequently enter the Arctic from the

Norwegian and Barents seas. Winter anticyclones are most frequent and strongest over Siberia and Alaska/Yukon, with additional but weaker systems over the central Arctic Ocean and Greenland.

During summer, cyclones are common in the same regions as in winter. Increased cyclonic activity over Siberia, the Canadian Arctic, and the central Arctic Ocean is related to increased cyclogenesis (cyclone generation) over northern parts of Eurasia and North America. These systems enter the Arctic Ocean from the Laptev Sea eastward to the Chukchi Sea. The Siberian and Alaska/Yukon anticyclones disappear, but anticyclones form over the Kara, Laptev, East Siberian and Beaufort seas, and southeastward across Canada.

North of 65° N, cyclone and anticyclone activity peaks during summer and is at a minimum during winter. Since the 1950s, cyclone numbers in winter, spring, and summer have increased significantly, as have anticyclone numbers in spring, summer, and fall. An interesting recent finding is that the atmospheric circulation over the Arctic Ocean alternates between "typical" anti-cyclonic and cyclonic circulation regimes approximately every 10-15 years. This has important implications for the Arctic's thermohaline (temperature and salinity) circulation, salinity anomalies observed in the Greenland Sea, and the variability of sea ice in the Arctic Ocean (see Arctic Ocean; Thermohaline Circulation).

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