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Extratropical Highlights - July 2000

1. Northern Hemisphere
The anomalous circulation during July featured a reversal in the pattern that was observed in June for many areas in the middle and high latitudes of the Northern Hemisphere. This reversal was particularly impressive over North America and western Europe. For the eastern United States and most of Europe, this reversal was associated with below-average temperatures, which contributed to a drop in the average Northern Hemisphere and global temperatures to their lowest values since May 1997 and October 1997, respectively (Fig. E2).

a. North America

Above-average 500-hPa heights during July covered central and northeastern Canada, with an extension southward into the western United States, while below-average heights covered the entire eastern conterminous United States and Alaska (Fig. E9). This anomaly pattern was accompanied by enhanced northwesterly flow into the Great Lakes and eastern states, which contributed to abnormally cool temperatures throughout the region (Fig. E1). In contrast, temperatures in the Southwest and Rocky Mountain states were above-average (1-2C) during the month, in association with the positive 500-hPa height anomalies. Temperatures were also significantly above average over northern Canada.

The anomalous circulation during July also contributed to generally drier-than-normal conditions over many sections of the United States, except for northwestern Washington and portions of the mid-Atlantic and Northeast (Fig. E3). In contrast, enhanced precipitation occurred over western and central Canada, associated with anomalous cyclonic flow along the west coast of North America (Fig. E9). In many areas of the United States rainfall departures in July were opposite to those observed in June. Along the Gulf Coast and in the southeastern states long-term drier than average condition persisted (Fig. E5).

B. Europe/ Asia

Most of Europe experienced cooler-than-average temperatures during July (Fig. E1), in contrast to the very warm conditions experienced in June. These cooler conditions were associated with below-normal 500-hPa heights across the region (Fig. E9), a feature which also contributed to above-average rainfall throughout northern and central Europe during the month (Fig. E3).

Generally drier-than-average conditions prevailed over most of northern China for the second consecutive month, associated with higher-than-average 500-hPa heights over the region. Drier-than-normal conditions also affected southern and eastern sections of India, contrasting with the very wet conditions observed the previous month.

2. Southern Hemisphere

The Southern Hemisphere circulation during July (Fig. E15) featured above-average 500-hPa heights over the Antarctic Peninsula and the neighboring South Atlantic, and in mid-latitudes over the central and eastern South Pacific. Negative height anomalies were observed southwest of Australia, over the southwestern South Atlantic and neighboring South America, and at high latitudes over the eastern South Pacific. The pattern of height anomalies over the southwestern Atlantic and South America was associated with frequent blocking in the vicinity of the Weddell Sea. This contributed to an abnormal influx of cold air into northern Argentina, Paraguay, Uruguay, eastern Bolivia and southern Brazil, where monthly average temperatures ranged from 2C to 4C below normal, and temperatures ranked below the 10th percentile in several areas (Fig. E1).

In contrast to recent months, most of Australia, except for the northeastern portion, experienced above-average temperatures in July (Fig. E1), in association with an anomalous cyclonic circulation, surface and aloft, southwest of the continent and anomalous northwesterly low-level winds over southern portions of the continent (Figs. E14, E15, T20 bottom). Generally drier-than-average conditions persisted over eastern portions of the continent (Fig. E3).

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Page last modified: August 24, 2007
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