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HOME > Expert Assessments > Climate Diagnostics Bulletin > Extratropical Highlights
Extratropical Highlights - May 2007

1. Northern Hemisphere   

The 500-hPa height pattern during May featured positive anomalies over the high latitudes of the North Pacific, eastern North America , the eastern North Atlantic , and south-central Russia , and negative anomalies across the high latitudes of the North Atlantic to western Siberia (Figs. E9, E11).

The main surface temperature departures during May reflected above-average temperatures across much of the United States , Europe , and most of southern and eastern Asia (Fig. E1). The main precipitation anomalies included above average totals in the central U.S. , northwestern Europe , and north-central Russia , and below average totals across the eastern U.S. and portions of the western U.S. (Fig. E3).


a. North America

The 500-hPa circulation pattern featured a strong ridge over eastern North America and a broad trough over the western United States (Figs. E9, E11). This anomalous wave pattern strongly controlled the monthly temperature and precipitation patterns. Above-average precipitation in the Plains States coincided with the area of large-scale ascending motion downstream of the mean trough axis, while anomalously warm and dry conditions across the eastern half of the U.S. coincided with the mean upper-level ridge. The most significant precipitation deficits occurred in the Southeast, Ohio Valley , Great Lakes , and Mid-Atlantic, regions, where area-averaged totals were in the lowest 10th percentile of occurrences (Fig. E5). By the end of the month, ongoing precipitation deficits had resulted in moderate to severe drought across the southeastern U.S. , with the extreme drought conditions observed in Mississippi , Alabama , Georgia , Tennessee , and Florida .


b. Eurasia

The 500-hPa circulation pattern during May featured a north-south dipole of height anomalies, with negative anomalies across the high latitudes of the North Atlantic and positive anomalies farther south (Fig. E9). This pattern was associated with an enhanced westerly flow of marine air into northern Europe and southern Scandinavia , which contributed to above-average surface temperatures and increased precipitation throughout the region.

A similar north-south dipole pattern of height anomalies was also observed over central Asia . The persistent positive height anomalies across the southern latitudes contributed to above-average surface temperatures extending from the Middle East to China . Farther north, enhanced zonal flow led to fewer cold-air outbreaks and warmer surface temperatures in northeastern Asia and eastern Siberia .


2. Southern Hemisphere

The 500-hPa height anomaly pattern during May featured a pronounced zonal symmetry, with positive height anomalies in the middle latitudes (especially extending from eastern Australia to South America ) and over Antarctica , and negative height anomalies in the southern extratropics. The circulation across southern Australia featured above-average heights at 500-hPa centered just west and east of the continent, and below-average heights centered south of the continent.  At 850-hPa, this wave pattern was associated with anomalous southwesterly flow across eastern Australia, along with a complete disappearance of the normal onshore flow of cooler air from the Great Australian Bight. This combination of conditions contributed to suppressed rainfall along both the west and east coasts, and to exceptionally warm surface temperatures (exceeding the 90th percentile of occurrences) across the eastern half of Australia .



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