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

1. Northern Hemisphere

The 500-hPa circulation pattern during May featured positive height anomalies across the central North Pacific, northeastern Canada , southern Europe , and south-central Russia , and negative anomalies over the eastern U.S. , the eastern North Atlantic , and Scandinavia (Fig. E9). In the subtropics, cyclonic 200-hPa streamfunction anomalies over the central North Pacific reflected the ongoing pattern of suppressed convection near the date line (Figs. T22, T25). These conditions were again associated with a pronounced westward retraction of the East Asian jet core and a westward shift of the associated jet exit region to just east of Japan (Fig. T21).

The main surface temperature departures during May reflected warmer than normal conditions across the southwestern U.S. , northeastern Canada , southern Europe , and Scandinavia (Fig. E1). The main precipitation departures included above-average totals in central Canada , northern Europe , and eastern China , and below-average totals in the central and Mid-Atlantic regions of the U.S. , and southern Europe (Figs. E3, E5, E6).

a. Pacific/ North America

Some aspects of the circulation across the North Pacific again reflected the ongoing pattern of suppressed convection near the date line. In particular, the westward retraction and confinement of deep tropical convection to the western Pacific resulted in an amplified trough over the central and eastern subtropical Pacific (Fig. T22). Accompanying this pattern, the East Asian jet stream was retracted well west of the date line, and the associated jet exit region was shifted westward to just east of Japan (Fig. T21).

Over the United States an anomalous upper-level ridge-trough pattern, characterized by above-average heights in the west and below-average heights in the east, was evident during the month. This pattern contributed to exceptionally warm surface temperatures in the southwestern U.S. and to large areas of below-average temperatures in the Mid-Atlantic region (Fig, E1). A large area of below-average precipitation was evident in the area of anomalous descending motion between the ridge and trough axes (Fig. E3). Area-average totals in both the Inter-Mountain and Great Plains regions were in the lowest 10th percentile of occurrences (Fig. E5), with many locations recording less than 25% of normal precipitation during the month (Fig. E6). Other areas of the U.S. , including the Southwest, the Gulf Coast , and the Mid-Atlantic region, recorded ongoing precipitation deficits during May. Each of these regions has experienced well below-average precipitation since January 2006, and the Gulf Coast has recorded below-average precipitation in every month since September 2005. In contrast, New England experienced wetter-than-average conditions during the month. With some areas in central New England recorded the wettest May on record.

b. North Atlantic / Eurasia

The 500-hPa circulation pattern during May featured a persistent north-south dipole pattern of anomalies, with negative anomalies extending from the eastern North Atlantic across southern Scandinavia , and positive anomalies extending across southern Europe and the Mediterranean Sea (Fig. E9). This pattern was associated with enhanced westerly flow and above-average precipitation across northern Europe , and with well below-average precipitation and above-average temperatures across southern Europe . Precipitation totals in this region have been in the lowest 10th percentile of occurrences for the last two months.

c. Eastern China

            East-central China recorded well above-average precipitation during May, with totals in many areas exceeding the 70th percentile of occurrences. For the region as a whole, area-averaged totals were above the 90th percentile of occurrences. This enhanced precipitation occurred in the area of anomalous ascending motion and increased storminess downstream of the mean upper-level trough position and within the right entrance region of the East Asian jet stream.  

2. Southern Hemisphere

      The mean 500-hPa circulation pattern during May featured generally above-average heights in the middle latitudes between 40°S-60°S, and below-average heights over southern Africa, in the area east of both Australia and South America, and across Antarctica (Fig. E15). Regionally, southeastern South America was situated between the mean upper-level ridge and trough axes in an area of in large-scale descending motion, and recorded well below-average precipitation during the month (Fig. E3). Accompanying this anomalous circulation, a strong on-shore flow at low levels contributed to below-average temperatures over much of the region (Fig. T20). Eastern Australia was also situated between the mean upper-level ridge and trough axes, and experienced anomalous onshore flow from the Great Australian Bight . This combination of conditions led to anomalously cool and dry conditions, with rainfall totals and temperatures in many areas dropping below the lowest 10th percentile of occurrences.

      In southern Africa the rainy season, which normally lasts from October to April, continued to produce beneficial rains in May. For the 2005-06 rainy season precipitation was above average in every month since October. An enhanced South African rainy season is consistent with a La Niņa episode.



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Page last modified: November 17, 2005
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