THE WEATHER AND CIRCULATION A MONTH WITH LARGE WEEKLY FLUCTUATIONS 1. ZONAL INDEX AND GENERAL CIRCULATION
December's mean circulation in the Northern Hemisphere was quite asymmetrical about the North Pole, as shown by the fields of mean 700-mb. height and its departure from normal (fig. l). The large positive anomaly over northern Siberia, for instance, contrasted with negative values at similar la...
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| Format: | Text |
| Language: | English |
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1956
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| Online Access: | http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.394.7665 http://docs.lib.noaa.gov/rescue/mwr/084/mwr-084-12-0444.pdf |
| Summary: | December's mean circulation in the Northern Hemisphere was quite asymmetrical about the North Pole, as shown by the fields of mean 700-mb. height and its departure from normal (fig. l). The large positive anomaly over northern Siberia, for instance, contrasted with negative values at similar latitudes in the Western Hemisphere. It is not unusual for adjacent regional circulations to be different in form, in the sense that one may exhibit highindex characteristics while low index is observed in t,he other [I, 2, 31. Such was the case in December 1956. It is perhaps more unusual for the entire Western Hemisphere to fit into the high-index category as well as it ' did this month. While no quantitative measure of circulation indices is routinely available for the Eastern Hemisphere, the departure from normal (DN) pattern in figure 1 leaves little doubt that a low zonal index and high-latitude blocking were dominant features of the circulation in that region. The positive anomaly center ($550 ft.) over northern Siberia was t'he largest 700-mb. height departure from normal in the Northern Hemisphere. An extension of the surrounding positive DN field eastward to the Bering Sea was accompanied by a large area of negative anomaly in the west-central Pacific. Eastward from the central Pacific, however, the pattern exhibited a number of features typically associated with high index, among them an unusually deep Icelandic Low and abnormally strong subtropical ridges in the eastern Pacific and western Atlantic Oceans. Some westerly component of mean 700-mb. DN flow was observed over the entire extensive area bounded by 40 ' and 60 ' N. hi,, and 140 ' W. long. eastward to central Europe. An index cycle of some 5 weeks ' duration was terminated with December's high values shown in figure 2. The cycle was remarkable because of its amplitude, with a range of more than 9 meters per second in &day mean values, from a low of 6.5 m. p. s. in late November to a high of 16 m. p. s. in December. Namias [l] indicated t'hat November ... |
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