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Extratropical Highlights - July 1999

1. Northern Hemisphere

A strong inter-hemispheric symmetry of upper-level circulation features was again evident during July (Fig. T22). This pattern featured positive streamfunction anomalies across much of the middle latitudes of the Northern Hemisphere, and across the subtropical latitudes of the Southern Hemisphere from Australia westward to South America. It also featured anomalous cyclonic circulation anomalies across the low latitudes of both hemispheres over the central Pacific. These conditions are consistent with the ongoing pattern of tropical rainfall and tropical convective activity, which featured an amplified Southeast Asian monsoon system (Fig. T25), and above-normal rainfall across the Sahel region of northern Africa and over large portions of the equatorial Indian Ocean.

a. North America

During July the western United States and Canada were situated downstream of an upper-level trough, while most of eastern North America remained under the influence of an amplified upper-level ridge (Fig. E9). This circulation contributed to below-normal temperatures over large portions of the western United States and Canada (Fig. E1) and to above-normal monsoonal rains in the southwestern United States (Figs. E3, E5).

Farther east, extremely warm temperatures again dominated eastern North America during the month, with values in some regions exceeding the 90th percentile. Extreme dryness accompanied this warmth over much of the eastern seaboard of the United States, with record low area-averaged totals observed in the Northeast. In fact, record low rainfall has been observed in the Northeast during five of the last nine months (Fig. E5). Record low accumulated rainfall totals for the April-July period have also been observed in portions of the mid-Atlantic region, where severe drought conditions prevailed during July.

b. North Atlantic, Eurasia

An anomalous anticyclonic circulation again prevailed across southern Europe and northern Africa during July, resulting in a continuation of extremely warm temperatures (exceeding the 90th percentile) over much of the region (Fig. E1). Abnormally warm temperatures (2-4C above normal) were also observed in eastern Europe and western Russia during the month, as well as over most of eastern Siberia. This warmth was consistent with the persistence of the anomalous anticyclonic circulation at upper levels extending eastward from the North Atlantic to the western North Pacific.

Over southern and eastern China and the adjacent ocean waters, significantly above-normal rainfall was observed during July. Since 1 June 1999, 550 mm to 1050 mm has fallen across much of the region, which is 140 mm to 690 mm above the long-term mean. This rainfall was aided by increased typhoon activity, and by strong intraseasonal (MJO) activity.

2. Southern Hemisphere

The Southern Hemisphere circulation during July featured an amplified upper-level ridge extending westward from Australia to South America, and an amplified low-latitude trough over the central South Pacific (Figs. T22, E15). This circulation has prevailed for the past several months, and remains consistent with the tropical rainfall patterns. During July these conditions were associated with a poleward shift of the wintertime jet stream (Fig. T21) and with increased storminess and above-normal rainfall across southern South America (Fig. E3). Over Australia, the persistence pattern of above-normal heights was associated with warmer-than-normal temperatures throughout the southern half of the continent, with portions of the southeast and west recording values above the 90th percentile (Fig. E1).

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