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Block (meteorology)
Blocks in meteorology are large-scale patterns in the atmospheric pressure field that are nearly stationary, effectively "blocking" or redirecting migratory cyclones. They are also known as blocking highs or blocking anticyclones.〔Glossary of Meteorology, Second Edition; American Meteorlogical Society, 2000; ISBN 1-878220-34-9.〕 These blocks can remain in place for several days or even weeks, causing the areas affected by them to have the same kind of weather for an extended period of time (e.g. precipitation for some areas, clear skies for others).〔(Brief page about Omega blocks )〕 In the Northern Hemisphere, extended blocking occurs most frequently in the spring over the eastern Pacific and Atlantic Oceans.〔
Similarly, in northern Europe anticyclonic blocks over western Russia and Scandinavia during the winter months can bring sub-zero easterly winds on their southern flanks, sometimes extending into the Atlantic Ocean and forcing the prevailing jet stream as far south as Portugal and Spain. Northern and Western European severe winters such as those of 1683–4, 1739–40, 1795, 1895, 1940, 1947, 1962–63, 1978–79, 1986, 2009–10 and December 2010 are caused by such blocks. Blocking highs were a key feature of the extreme winter droughts in southeastern Australia in 2006.〔() see p116〕
==Impact of the polar vortex==
(詳細はWesterlies increase in strength. When the polar cyclone is weak, the general flow pattern across mid-latitudes buckles and significant cold outbreaks occur. Extratropical cyclones which occlude and migrate into higher latitudes create cold-core lows within the polar vortex. Volcanic eruptions in the tropics lead to a stronger polar vortex during the winter for as long as two years afterwards. The strength and position of the cyclone shapes the flow pattern across the hemisphere of its influence. An index which is used in the northern hemisphere to gage its magnitude is the Arctic oscillation.〔Todd Mitchell (2004). (Arctic Oscillation (AO) time series, 1899 - June 2002. ) University of Washington. Retrieved on 2009-03-02.〕
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