The mean climatic features of pressure, wind and seasonal airflow regime provide only a partial picture of climatic conditions. Some patterns of circulation occur irregularly and yet, because of their tendency to persist for weeks or even months, form an essential element of the climate.
Blocking patterns are an important example. It was noted in Chapter 7 that the zonal circulation in mid-latitudes sometimes breaks down into a cellular pattern. This is commonly associated with a split of the jet stream into two branches over higher and lower mid-latitudes and the formation of a cut-off low (see Chapter 9H.4) south of a high-pressure cell. The latter is referred to as a blocking anticyclone since it prevents the normal eastward motion of depressions in the zonal flow. Figure 10.9 illustrates the frequency of blocking for part of the northern hemisphere with five major blocking centres shown (H). A major area of blocking is Scandinavia, particularly in spring. Cyclones are
Source: From Knox and Hay (1985), by permission of the Royal Meteorological Society.
diverted northeastward towards the Norwegian Sea or southeastward into southern Europe. This pattern, with easterly flow around the southern margins of the anticyclone, produces severe winter weather over much of northern Europe. In January to February 1947, for example, easterly flow across Britain as a result of blocking over Scandinavia led to extreme cold and frequent snowfall. Winds were almost continuously from the east between 22 January and 22 February and even daytime temperatures rose little above freezing point. Snow fell in some part of Britain every day from 22 January to 17 March 1947, and major snowstorms occurred as occluded Atlantic depressions moved slowly across the country. Other notably severe winter months - January 1881, February 1895, January 1940 and February 1986 - were the result of similar pressure anomalies with pressure well above average to the north of the British Isles and below average to the south, giving persistent easterly winds.
The effects of winter blocking situations over northwest Europe are shown in Figures 10.10 and 10.11. Precipitation amounts are above normal, mainly over Iceland and the western Mediterranean, as depressions are steered around the blocking high following the path of the upper jet streams. Over most of Europe, precipitation remains below average and this pattern is repeated with summer blocking. Winter temperatures are above average over the northeastern Atlantic and adjoining land areas, but below average over central and eastern Europe and the Mediterranean due to outbreaks of cP air (Figure 10.11). The negative temperature anomalies associated with cool northerly airflow in summer cover most of Europe; only northern Scandinavia has above-average values.
The exact location of the block is of the utmost importance. For instance, in the summer of 1954 a blocking anticyclone across eastern Europe and Scandinavia allowed depressions to stagnate over the British Isles, giving a dull, wet August, whereas in 1955 the blocking was located over the North Sea and a fine, warm summer resulted. Persistent blocking over northwestern Europe caused drought in Britain and the continent during 1975 to 1976. Another, less common location of blocking is Iceland. A notable example was the 1962 to 1963 winter, when persistent high pressure southeast of Iceland led to northerly and northeasterly airflow over Britain. Temperatures in central England were the lowest since 1740, with a mean of 0°C for December 1962 to February 1963. Central Europe was affected by easterly airstreams with mean January temperatures 6°C below average.
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