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HOME > Expert Assessments > Climate Diagnostics Bulletin > Extratropical Highlights
Extratropical Highlights - August 2004

1. Northern Hemisphere

The 500-hPa circulation pattern in August featured above-average heights over Alaska, northwestern Canada, Greenland and western Russia, and below-average heights over the north-central U.S., much of Europe, eastern and central Siberia and over the Pacific from Japan eastward to the Gulf of Alaska (Fig. E9). This circulation pattern was associated with a continuation of above-average surface temperatures and below-average precipitation in Alaska, and with cooler-than-average conditions in central Canada and the central U.S. (Figs. E1, E3). It was also associated with above-average temperatures throughout northern and central Europe.

a. North America

Over North America the circulation during August featured a blocking upper-level ridge over Alaska, and an amplified trough over central Canada and the Great Lakes (Fig. E9). The blocking ridge over Alaska contributed to a shift in the mean jet stream and storm track to well south of the state (Fig. E10), which led to a continuation of exceptionally warm and dry conditions throughout the state. Farther east anomalous northwesterly flow upstream of the amplified upper-level trough contributed to a continuation of below-average surface temperatures across central Canada and over most of the United States east of the Rocky Mountains. This anomalous circulation is similar to that observed during April-July. The persistence of these circulation features has led to five consecutive months of extremely warm and dry conditions in Alaska, and to a prolonged period of anomalously cold temperatures across central and eastern Canada and large portions of the central and eastern U.S. (Fig. E1).

Also in the United States, significantly above-average precipitation was observed over the Pacific Northwest and along the eastern seaboard during August, with totals in these areas exceeding the 70th percentile of occurrences. Frequent cold frontal passages in the central United States contributed to increased storminess and above-average precipitation in the eastern United States.

b. Europe

The amplified ridge over high latitudes of the North Atlantic was associated with a southward shift in the jet stream over the eastern Atlantic and Europe, which contributed to above-average temperatures in northern Europe and wetter-than-average conditions in portions of central Europe (Figs. E1, E3).

2. Southern Hemisphere

The 500-hPa circulation pattern during August featured below-average heights at high latitudes and in the mid-latitudes Australia eastward across the South Pacific, and above-average heights over the southeastern Pacific centered around 50S (Fig. E15). Temperatures were generally near to above average over South America, southern Africa and southeastern Australia (Fig. E1). Below-average temperatures were observed over portions of western and northern Australia.

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