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HOME > Expert Assessments > Climate Diagnostics Bulletin > Extratropical Highlights
 
Extratropical Highlights - August 2003
 

1. Northern Hemisphere

The 500-hPa circulation during August 2003 featured a persistent pattern of positive height anomalies over western North America, central Europe, north-central Russia (Figs. E10, E12), and negative height anomalies over the central North Atlantic, eastern Europe, and the polar region. The prominent temperature anomalies during the month included a continuation of significantly warmer-than-average conditions over the western United States and Europe, with anomalous warmth also observed across eastern Canada, north-central Russia, and southeastern China. (Fig. E1). Prominent precipitation anomalies during August included a continuation of above-average totals in the southeastern and eastern United States, and below-average totals in Europe (Fig. E3).

a. North America

The mean upper-level circulation featured an amplified upper-level ridge extending northward from Texas to northwestern Canada (Fig. E10), along with an anomalous northward shift of the mean jet stream into central Canada (Fig. E11). It also featured a persistent upper-level trough over eastern North America. These conditions were associated with reduced storminess over the western United States, and significantly increased storminess (Fig. E14, right) and above-average rainfall across the southeastern and eastern United States and eastern Canada (Figs. E3, E6).The Southeast and Ohio Valley regions have now recorded above-average precipitation in eleven of the last twelve months beginning September 2002 (Fig. E5).

North America was dominated by anomalous warmth during August (Fig. E1), with the largest temperature anomalies associated with the enhanced upper-level ridge in the west. Temperatures averaged 2C above normal over most of the western U.S. and central Canada, with the largest anomalies exceeding 3C over the northern Plains States and south-central Canada.

b. Europe

A strong upper-level ridge was again evident across Europe during August (Fig. E10), which contributed to significantly warmer and drier than average conditions throughout the continent. Monthly mean temperatures exceeded the 90th percentile throughout Europe (Fig. E1) and rainfall totals were generally below the 10th percentile (Figs. E3, E4). Area-averaged rainfall over southern Europe has been below average since March, with totals dropping below-the 10th percentile in three of the past six months.

2. Southern Hemisphere

In the Southern Hemisphere the 500-hPa circulation during August featured above-average heights across the high latitudes of the three ocean basins, and below-average heights across the central South Pacific and in the area south of Africa (Fig. E16). Over the South Pacific this anomaly pattern was associated with a pronounced eastward extension of the wintertime jet stream in the middle latitudes (Fig. T21, top), and with persistent high latitude blocking activity in the area northeast of New Zealand (Fig. E18).

Eastern Australia experienced a continuation of above-average temperatures during August (Fig. E1) in association with a persistent anomalous anticyclonic circulation at upper-levels (Fig. T21). The anomalous warmth also covered New Zealand, where monthly mean temperatures averaged above the 70th percentile.


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