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Extratropical Highlights - December 2000

1. Northern Hemisphere
The 500-hPa circulation during December was dominated by above-normal heights at high latitudes and below-normal heights in the middle latitudes (Figs. E9, E11). This anomaly pattern also extended well into the middle stratosphere, and remained prominent even at the 30-hPa level (Fig. S1). This overall anomalous circulation reflected a strong negative phase of the Arctic Oscillation (Fig. A2.2). It also projected regionally onto the positive phases of the Pacific/ North American (PNA) and the East Atlantic (EA) teleconnection patterns (Figs. E6, E7), and onto the negative phases of both the North Atlantic Oscillation (NAO) and the Polar/ Eurasia pattern.

Over North America, this anomalous circulation contributed to extremely cold temperatures across the United States and Canada (Fig. E1). It also contributed to exceptionally warm temperatures throughout Europe, and to above-average precipitation over southwestern Europe (Fig. E3).

December 2000 also featured a return to slightly below-average global temperatures for the first time since early 1997 (Fig. E2, top). This cooling was most noticeable in the Northern Hemisphere, where below-average hemispheric temperatures were also observed for the first time since early 1997 (Fig. E2, middle).

a. North America

The 500-hPa circulation over North America featured an amplified ridge extending northward from the extreme western United States to northern Siberia (Fig. E9). This feature was accompanied by a very strong surface pressure ridge which extended poleward from the central United States to northern Siberia. (Fig. E8). The areal extent of this pressure ridge was so expansive that it served as a duct for driving bitterly cold air from northern Siberia down into Canada and the central United States. This northerly flow, sometimes referred to as the "Siberian Express," brought significantly below-average temperatures to the central and eastern United States and Canada (Fig. E1), with monthly temperatures averaging below the lowest 10th percentile over the eastern half of the United States.

b. Europe

Above-average heights covered Greenland and the high latitudes of the North Atlantic during December, while below-average heights covered both the west-central and east-central North Atlantic (Fig. E9). This overall anomaly pattern is consistent with the negative phase of the North Atlantic Oscillation, and was also evident during November. In both months this circulation pattern was accompanied by a pronounced southward shift of the mean North Atlantic jet stream over much of the Atlantic basin (Fig. E10, left), with the mean jet core extending almost due eastward from the mid-Atlantic region of the United States to southern Spain. During December, this flow pattern contributed to a continuation of above-average temperatures across Europe and Scandinavia (Fig. E1), and to above-average rainfall in southwestern Europe (Fig. E3).

2. Southern Hemisphere

The circulation during December featured above-average heights throughout the Antarctic region, and below-average heights in the southern extratropics (Fig. E15). This overall pattern is referred to as the negative phase of the Antarctic Oscillation. It is very similar to the negative phase of the Arctic Oscillation, which dominated the Northern Hemisphere circulation during the month.

In Australia the northern section of the country experienced significantly cooler (Fig. E3) and wetter (Fig. E3) than normal conditions during December, while the southern sector experienced exceptionally warm and dry conditions. The above-average rainfall in the North and Northeast was consistent with a strengthening of Pacific cold episode (La Niņa) conditions (see Tropical Highlights), while the anomalously warm and dry conditions in the South were associated with a shift of the low-level westerlies to south of the continent (Fig. T20, top).

Elsewhere, near-average rains were observed over southern Africa during December, following well above-average rainfall in November (Fig. E4). To date the 2000-2001 southern Africa rainy season, which extends from October - April, has featured slightly above-average rainfall.

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