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HOME > Expert Assessments > Climate Diagnostics Bulletin > Extratropical Highlights
Extratropical Highlights - August 2006

1. Northern Hemisphere

During August the 500-hPa circulation featured a persistent pattern of positive height anomalies over the central North Pacific, the high latitudes of the North Atlantic , and near the Caspian Sea , and negative height anomalies over Alaska , eastern Canada , northern Europe , and the polar region (Figs. E9, E11). The main surface temperature departures during the month reflected warmer-than-average conditions in the southeastern United States , Scandinavia , south-central Russia , and most of China (Fig. E1). Below-average temperatures were observed in central Europe and central Russia .

Above-average precipitation was recorded during August across large portions of the southwestern and central United States , eastern Europe, and western Russia (Figs. E3, E6). Below-average precipitation occurred in the northwestern United States and southeastern Canada , Scandinavia , and southwestern China (Figs. E3, E4).

      a. North America

The mean 500-hPa circulation pattern featured a trough centered along the west coast of the U.S. , and a persistent ridge extending southward from central Canada to the southwestern U.S. (Fig. E9). The Inter-Mountain and Northern Plains regions were situated downstream of the trough axis and recorded above-average precipitation during the month (Fig. E5). In the southwestern U.S. an enhanced monsoon brought above-average precipitation to Arizona and New Mexico , which contributed to area-averaged totals in both the Southwest and Inter-Mountain regions above the 90th percentile of occurrences.

Below-average precipitation was recorded in the Pacific Northwest region, which was situated immediately upstream of the mean trough axis. Precipitation was also below average downstream of the mean ridge axis, with area-averaged totals in the Great Lakes, mid-Atlantic, and Northeast regions all recording totals in the lowest 30th percentile of occurrences.

b. North Atlantic / Eurasia

The 500-hPa circulation pattern during August featured an anomalous ridge over the high latitudes of the North Atlantic and Scandinavia and an anomalous trough over central and eastern Europe (Fig. E9). This circulation contributed to significantly above-average precipitation throughout Europe and western Russia , with totals in many areas exceeding the 90th percentile of occurrences. It also contributed to exceptionally warm surface temperatures in Scandinavia , with departures exceeding the 90th percentile of occurrences in many areas.

c. China

Exceptionally warm and dry conditions were observed across southwestern and south-central China during August. These conditions were associated with an amplification of the Asian monsoon ridge along its northeastern flank (Fig. T22), and a northward shift in the main belt of westerly winds toward northern China (Fig. T21). As a result, much of southern China was situated directly beneath the mean upper-level ridge axis, well south of the influence of mid-latitude weather systems and low-latitude frontal passages.


2. Southern Hemisphere

      The mean 500-hPa circulation during August featured an anomalous zonal wave-3 pattern, with positive height anomalies over the central ocean basins and Antarctica , and negative height anomalies centered over southern Australia and south of both South America and Africa (Fig. E15). In Australia , the persistent ridge led to anomalously warm and dry conditions, with temperatures generally exceeding the 70th percentile of occurrences and rainfall totals generally less than the 30th percentile of occurrences. The most significant dryness occurred in the southeast, where totals were below the 10th percentile of occurrences.



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