MEAN CIRCULATION PATTERNS BASED ON 12 YEARS OF RECENT NORTHERN HEMISPHERIC DATA

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Abstract
A new set of long-period means for sea level pressure and 700-mb. height, based on 12 recent years of reliable upper-level and surface observations in the Northern Hemisphere, is constructed. Comparison with the normals currently in use, which are based on data prior to 1950, reveals additional information and some significant differences, principally at high and low latitudes. The three cells of the polar vortex at 700 mb. are not as deep as shown in the normals, the cells over Baffin Island and the Siberian Arctic are farther north, and the Kamchatka cell is found primarily over the Bering Sea rather than over land. The Pacific High at 700 mb. is tricellular on an annual basis, with the additional Philippine cell becoming the dominant cell ill late winter. Other new features in the 700-mb. means include troughs near Spitzbergen, Alaska, the Philippine Sea, and the Bay of Bengal; and Highs in northeastern Siberia and the Caspian Sea area. Patterns of height differences between the new means and the normals suggest that a long-period trend toward increased blocking has been in progress during the last decade, particularly in the Baffin Bay area.

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