3.1.5 The Arctic Air Mass and Tracks of Polar Cyclones
An overview of the circulation of air in the Arctic reveals that a lower, cold air mass moves south (outward from the Arctic region), while a higher jet stream moves toward the Arctic from the periphery. In the air 3-10km above the surface in the Arctic, the prevailing westerly winds flowing toward the center of the Arctic Ocean create a low-pressure circulation system. The general westerly air circulation around the cold-cored low pressure area is a result of the large temperature gradient between the equator and the poles, and of the Earth's rotation. Average annual air pressure at sea level in the center of the Arctic Ocean is 1,015.7hPa, with monthly averages of 1,011-1,020hPa exhibiting a continuing weak high-pressure area. Very little precipitation occurs in this region. High pressure prevails roughly one third of the time in the Arctic, with low pressure dominant in the remaining two-thirds of each year.
A look at the distribution of average air pressure at sea level for the month of January reveals that no high pressure occurs in the Arctic Ocean during this month. However, a high-pressure band extends through Siberia and Alaska from the center of the Arctic Ocean to the Bering Strait. In the diagram at right, the most common track for the low-pressure is indicated in a solid line and the secondary track is shown in a dotted line. The winter low-pressure (cyclone) travels from Iceland toward the Barents and Kara Seas. Another low-pressure mass spreads from northern Greenland to the Barents Sea, and sometimes a low-pressure appears traveling from the Bering Strait into the Arctic Ocean. The precipitation brought in by these low-pressure systems in winter is far less than the precipitation observed in summer.
In the diagram of the distribution of average air pressure at sea level for the month of July, air pressure for the entire Arctic Ocean is remarkably uniform, varying in a narrow range between 1,010hPa and 1,012hPa. Occasionally low-pressure masses enter from the periphery, and those from the North Atlantic, Russia (west of the Urals) and Siberia, carry a lower average air pressure of about 995hPa. The other low-pressure masses, arriving from Alaska, the Bering Strait and Canada, are somewhat weaker and feature air pressure of roughly 1,002hPa. Although summer in the Arctic Ocean tends to be free of strong winds, clear skies are rare and fog is common.