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Extratropical Highlights - October 2000

1. Northern Hemisphere
The circulation during October featured a persistent pattern of above-normal 500-hPa heights centered over south-central Canada, the mid-latitudes of the eastern North Atlantic, and the Barents Sea, and below-normal heights over high latitudes of the North Atlantic (Figs. E9, E11). This persistent pattern contributed to persistent, and in some cases extreme precipitation and temperature anomalies in many areas of North America and Europe.

a. United States

The circulation pattern over North America was dominated by a strong 500-hPa ridge over south-central Canada and the upper Midwest and troughs along the east and west coasts of the United States (Fig. E9). This pattern contributed to wetter-than normal conditions over the western United States, drier-than-normal conditions in the Mid-Atlantic and southeastern states, and warmer-than-normal conditions throughout the center of the country (Figs. E3, E1). Many areas in South Carolina, North Carolina, Virginia, and Maryland experienced their driest October on record. This is in marked contrast to September, when much of the East Coast experienced much wetter-than-normal conditions.

b. Europe and Asia

The 500-hPa height anomaly pattern over the North Atlantic was accompanied by anomalously strong westerlies between 45 and 60N (Fig. E10), which contributed to above-normal precipitation over the British Isles and portions of western Europe (Fig. E3) and above-normal temperatures over central and eastern Europe (Fig. E1). A series of powerful storms struck the British Isles and western Europe during the month with high winds and flooding reported in many countries in the region. In contrast, much drier-than-normal conditions were observed over eastern Europe.

2. Southern Hemisphere

The 500-hPa height anomaly pattern exhibited very weak anomalies during October. However, considerable intra-monthly variability was observed in the flow pattern from New Zealand eastward to the Atlantic. During the first week of October a strong blocking ridge was located near the Antarctic Peninsula, with a strong split in the jet stream evident over the eastern South Pacific. The subtropical branch was displaced well to the north over the western Atlantic and southern Brazil, accompanied by significant rains and cool conditions over Southeast Brazil. During the following two-three weeks, a blocking pattern developed over the South Pacific and a strong trough developed and persisted near the Antarctic Peninsula. During this period the westerlies were concentrated over Central Argentina, and abnormally warm and dry conditions dominated Southeast Brazil, while excessive rains and flooding occurred in southern Brazil. The monthly mean anomalous temperature and precipitation patterns over South America reflect these latter conditions (Figs. E1, E3).

3. Southern Hemisphere Stratosphere

Stratospheric ozone values were extremely low over Antarctica again during the Southern Hemispheric winter and spring of 2000, as has been the case in recent years. The area covered by low total ozone values of less than 220 Dobson Units, defined as the "ozone hole", was larger in early September 2000 than in early September of any previous year (Fig. A2.4). The size of the ozone hole remained larger in September 2000 than in 1999, but rapidly decreased throughout October 2000. Widespread negative total ozone anomalies, computed with respect to the 1979-1986 base period, were observed during October 2000 (Fig. A2.3). Anomalies of up to 39 percent below normal are shown over the Atlantic Ocean sector of Antarctica, with strong negative anomalies also over southern South America. Small positive anomalies are shown over the Pacific Antarctic sector. This pattern of total ozone anomalies is consistent, in sign and geographical location, with the pattern of 50 hPa geopotential height anomalies (Fig. A2.1).

Temperature in the lower stratosphere are closely coupled to ozone through dynamics and photochemistry. Extremely low stratospheric temperatures (lower than -78C) over the Antarctic region contribute to depletion of ozone, since these low temperatures lead to the presence of polar stratospheric clouds (PSCs). PSCs enhance the production and lifetime of reactive chlorine, leading to stratospheric ozone depletion. Daily minimum temperatures over the Antarctic region, 65S to 90S at 50 hPa (near 19 km) were lower than average for most of 2000 (Fig. A2.5), and were sufficiently low for polar stratospheric clouds to form and contribute to enhanced stratospheric ozone depletion. During October 2000 lower stratosphere minimum temperatures increased above the critical PSC temperature, which was associated with the decrease in size of the ozone hole.

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