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.
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.
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.
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.