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Extratropical Highlights - September 1999

1. Northern Hemisphere

A strong inter-hemispheric symmetry of anomalous upper-level circulation features was again evident during September across the lower latitudes of the Pacific and Atlantic basins (Fig. T22, bottom). This pattern featured anomalous anticyclonic circulations across the eastern Pacific eastward to Africa, and anomalous cyclonic circulation anom-alies across the low latitudes of the western Pacific. Anticyclonic circulation anomalies also persisted across the middle latitudes of the Northern Hemisphere during the month. All of these features have prevailed during the June-September period, and are consistent with the ongoing pattern of anomalous tropical rainfall and tropical convective activity that has persisted during the period. This pattern of anomalous tropical rainfall included suppressed convection over the central equatorial Pacific (Fig. T25), an amplified south-east Asian monsoon system, above-normal rains across central America, and increased precipitation across the African Sahel region.

North America

Much of the United States was influenced by a large-scale, upper-level trough during September (Fig. E9), which contributed to significantly above-normal temperatures over the northeastern United States and southeastern Canada and to below-normal temperatures over the Plains states (Fig. E1). This circulation pattern, together with Tropical Storm Dennis and Hurricane Floyd contributed to extremely heavy rainfall along the entire eastern seaboard from the Carolinas to the maritime provinces of Canada (Fig. E3). In fact, eastern North Carolina experienced record flooding in association with extreme rainfall totals (reaching 500 - 750 mm) linked to landfalling Hurricane Floyd. In contrast, the region near and west of the trough axis, which included most of the midwestern United States, experienced drier-than-normal conditions (Figs. E3, E5).

This circulation pattern is in marked contrast to the pronounced upper-level ridge which dominated most of the country during August, and which contributed to significantly drier-than-normal conditions and severe drought conditions over large portions of the eastern United States.

b. Europe/ Asia

Most of Europe experienced significantly above-normal temperatures during September (Fig. E1), with temperatures above the 90th percentile observed throughout the region. In southern and central Europe, this warmth represents a continuation of extremely warm conditions that have prevailed during the last several months. Extremely wet conditions were also observed across western Europe during September, with totals exceeding the 90th percentile throughout the region. Farther east, anomalously dry conditions were observed over eastern Europe and western Russia.

These anomalous temperature and rainfall patterns were linked to a large-amplitude anomalous circulation at upper levels, which featured a trough just west of Great Britain and a strong ridge extending southward from Scandinavia to the area west of the Black Sea. Also, the continued excessive warmth across southern Europe (and northern Africa) was again linked to the persistence of an anomalous anticyclonic circulation (Fig. T22, bottom) throughout the region at upper levels.

2. Southern Hemisphere

The Southern Hemisphere circulation during September (Fig. E15) featured above-normal heights over southeastern Australia and New Zealand, across most of Antarctica, and over the central Indian Ocean, and below-normal heights over the high latitudes of the eastern South Pacific, the high latitudes of the South Atlantic, and southern Africa. In the Australia/ New Zealand sector, the above-normal heights were associated with abnormally warm surface temperatures (Fig. E1), with large portions of eastern Australia recording values above the 70th percentile and most of New Zealand recording values above the 90th percentile. These circulation and temperature patterns conditions were also prevalent during August across the Australia/ New Zealand sector.

In South America, most of cental Argentina and southeastern Brazil experienced a continuation of anomalously warm (Fig. E1) and dry conditions (Fig. E3) during the month, while southern Argentina again experienced extremely wet conditions. This overall pattern has been linked to increased upper-level westerly winds (Fig. T21, bottom) and enhanced storminess across the southeastern South Pacific and southern South America during both August and September, and with an overall poleward displacement of the main frontal boundary away from central Argentina and southeastern Brazil.

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