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

1. Northern Hemisphere

The 500-hPa circulation during December 2003 featured above-average heights across the central North Pacific, eastern Canada and the high latitudes of the North Atlantic, and Eurasia, and below-average heights over the Gulf of Alaska and the polar region (Figs. E10, E12). These anomalies were associated with an eastward extension of the East Asian jet axis to the western United States, and a strong meridional orientation to the North Atlantic jet stream (Fig. T21).

Above-average temperatures were observed across Canada, the western U.S., most of northern Russia, and Siberia, during December (Fig. E1), while below-average temperatures were observed in the southeastern United States. Prominent precipitation anomalies during the month included above-average totals over the northwestern and northeastern U.S., and the Mediterranean Sea (Fig. E3).

a. Pacific/North America

The mean upper-level circulation during December featured a pronounced extension of the East Asian jet stream, which brought increased storminess and above-average precipitation to the northwestern U.S. (Fig. E3). The circulation also featured a below-average strength of the mean Hudson Bay Low (Fig. E10). The result was a significant flow of marine air into Canada and the western U.S., and a reduced flow of cold Canadian air into the north-central United States (Fig. E11). Surface temperatures throughout these regions averaged +2o to +4oC above normal during the month (Fig. E1).

b. North Atlantic/Eurasia

Positive 500-hPa height anomalies extended from Hudsonís Bay to Great Britain, and then continued eastward across Siberia and most of Russia. These anomalies were associated with a disappearance of the mean Icelandic Low, and a pronounced southwest - northeast orientated North Atlantic jet stream. This resulting anomalous poleward heat transport contributed to above-average temperatures across southern Greenland, Iceland and Scandinavia (Fig. E1). Also, the anomalous eastward extension of this flow (Fig. E11) contributed to above-average temperatures (+3o to +6oC above average) across Siberia and northern Russia.

At lower latitudes the southern branch of the split flow pattern over the eastern North Atlantic contributed to above-average precipitation across the Mediterranean Sea and extreme northwestern Africa, with totals in many areas exceeding the 70th percentile (Fig. E3).

2. Southern Hemisphere

In the Southern Hemisphere the 500-hPa circulation during December featured above-average heights over Antarctica, and below-average heights south of the three continents (Fig. E16). Over southern South America a broad trough contributed to anomalously cool (Fig. E1) and wet (Fig. E3) conditions across the region, with mean temperatures in many locations in the lowest 10th percentile of occurrences. Farther north well above-average temperatures were observed across eastern Brazil, with values generally averaging above the 90th percentile. Northeastern South America also recorded well below-average rainfall during the month, with totals at most locations in the lowest 10th percentile of occurrences (Figs. E3, E4).

In Southern Africa the rainy season normally lasts from October to March. The 2003-2004 rainy season was well below-average during October-December 2003, with area-averaged precipitation totals in the lowest 10th percentile of occurrences during November and December (Fig. E4). During December the primary area of below-average rainfall spanned the heavy agricultural areas of southeastern Africa and Tanzania, and extended northward into Kenya (Fig. E3).


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