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Extratropical Highlights - October 1999

1. Northern Hemisphere

A strong inter-hemispheric symmetry of anomalous upper-level circulation features was again evident during October 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 the Middle East, and anomalous cyclonic circulation anomalies across the low latitudes of the western Pacific. Anticyclonic circulation anomalies also persisted across the middle latitudes of the Northern Hemisphere during the month. Each of these features have also prevailed during the June-September period, and are consistent with the ongoing pattern of anomalous tropical convective activity that has persisted throughout the period. This pattern of anomalous tropical rainfall included suppressed convection over the central equatorial Pacific (Fig. T25), an amplified southeast Asian monsoon system, above-normal rains over central America, and increased precipitation across interior northern Africa near 10°N.

a. North America

The circulation over North America was dominated by an amplified ridge over the western United States and by an amplified trough over eastern Canada (Fig. E9). This pattern contributed to above-normal temperatures across the western U.S. (Fig. E1, top) and to below-normal temperatures across eastern Canada. It also contributed to below-normal precipitation across the central United States (Fig. E3), which is located between the persistent upper-level ridge and trough axes. In portions of this anomalously dry region, precipitation totals were below the 10th percentile during the month. For the midwestern region as a whole, rainfall totals have been significantly below normal throughout the June-October 1999 period, with record or near-record dryness observed in August and September (Fig. E5).

b. Europe/ Asia

Most of Europe, northern Africa and the Middle East experienced significantly above-normal temperatures during October (Fig. E1), with temperatures above the 70th percentile observed throughout the region. In southern Europe, northern Africa and the Middle East, this warmth represents a continuation of extremely warm conditions that have prevailed during the last several months. Virtually all of northern Europe also experienced well above-normal temperatures during September.

This anomalous warmth has been linked to the persistence of a large-amplitude anomalous circulation at upper levels, which featured a trough just west of Portugal and an anomalous ridge across Scandinavia. In the south, the continuation of excessive warmth across southern Europe, northern Africa and the Middle East was also linked to the persistence of an anomalous upper-level anticyclonic circulation throughout the region (Fig. T22, bottom).

2. Southern Hemisphere

The Southern Hemisphere circulation during October (Fig. E15) featured above-normal heights over large portions of the middle latitudes, which is similar to the anomaly pattern observed in the Northern Hemisphere (Fig. T22, bottom). In both hemispheres, this anomaly pattern has persisted since March 1999. In the Southern Hemisphere, the anomalous height pattern extending from eastern Australia to the eastern Pacific has been particularly persistent during the last three months. This pattern has contributed to exceptionally warm conditions across the eastern half of Australia , with large portions of eastern Australia and New Zealand recording values above the 70th percentile throughout the period (Fig. E1, bottom).

In South America, most of cental Argentina and southeastern Brazil experienced a continuation of anomalously dry conditions during October (Fig. E3). Rainfall deficits have been observed in these regions in each of the past three months. This dryness has been linked to an overall poleward displacement of the main frontal boundary away from central Argentina and southeastern Brazil, in association with an overall poleward displacement of the mean storm track across the eastern Pacific and southern South America. This anomalous circulation is consistent with ongoing La Niņa conditions.

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