800 MONTHLY WEATHER REVIEW VOI. 99, No. 10 WEATHER AND ClRCULATl UDC 561.613: 651.~6.1:551.524.36(73-15)"1971.07" irculaticsn Reversal Accompanied

• Main Author: A. James Wagner
• Other Authors: The Pennsylvania State University CiteSeerX Archives
• Format: Text
• Language: English
• Subjects: Hudson Bay; Gulf of Alaska; Canada; Pacific; Hudson; Spitzbergen; Alaska
• Online Access: http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.408.4707; http://docs.lib.noaa.gov/rescue/mwr/099/mwr-099-10-0800.pdf

July was characterized by a considerable reduction in the strength of high-latitude blocking that had reached a peak during June. The Western Hemisphere polar zonal index, a measure of the strength of the westerlies in the latitude band between 55 ' and 70°N, increased from-0.3 m/s (2.7 m/s below normal) in June to +2.1 m/s (0.8 m/s below normal) in July. The major portion of the block retrograded to northeast Siberirt, where a 700-mb High with a 52-m positive anomaly center was found (figs. 1, 2). Heights fell by more than 30 m over a large portion of the polar area, with greatest falls of 85 m near Spitzbergen (fig. 3). As storms deepened south of the Siberian block and moved to the western Aleutians with greater than normal intensity for the season, the midlatitude portion of the Pacific ridge was thrust eastward into the Gulf of Alaska where it joined with the remains of the June blocking in northwest Canada. The largest height anomaly change from June to July in the Western half of the Northern Hemisphere was centered near 50'N, 140'W where an increase of 101 m was observed (fig. 3). Amplification of the planetary wave train downstream from the strengthened western North American ridge, which was 47 m above normal, resulted in deepening to as much as 72 m below normal of a full-latitude trough extending from the eastern shore of Hudson Bay to the southern Appalachian Mountains and strengthening of the Bermuda High (figs. 1,2,3). Heights also rose strongly across Europe and northern Russia, with maximum June to July increases of 92 m over the British Isles and 106 m