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Extratropical Highlights - January 2001

1. Northern Hemisphere
The 500-hPa circulation during January was dominated by above-normal heights across central Canada, the central latitudes of the eastern Pacific, and from Scandinavia southward to the eastern Mediterranean Sea (Fig. E9). The circulation also featured a continuation of negative height anomalies over the high latitudes of the North Pacific, over the eastern North Atlantic and western Europe, and over central Russia and central Siberia.

Over North America, this anomalous circulation resulted in significantly above-average temperatures across Canada and the northern tier of the United States (Fig. E1). In Europe, it contributed to a continuation of 1) above-average temperatures throughout the continent, and 2) above-average precipitation over southern Europe (Figs. E3, E4). In contrast, exceptionally cold temperatures were again observed during the month over portions of north-central Russia and central Siberia.

a. North America

A strong split-flow pattern characterized the circulation over western North America, with the northern branch of the flow bringing milder, marine air into western and central Canada. Farther east, a weakening of the climatological mean "Hudson Bay Low" was accompanied by a reduced northwesterly flow from Canada into the northern United States (Figs. E9, E10). This circulation pattern resulted in well above-average temperatures across Canada and the northern half of the United States (Fig. E1), with the largest anomalies averaging 7-9C over western Canada. This anomalous warmth is in dramatic contrast to the extremely cold temperatures observed during December 2000, in association with a pronounced flow of arctic air from northern Siberia into Canada and the United States.

b. Europe

Above-average heights covered Scandinavia and the high latitudes of the North Atlantic during January, while below-average heights covered the east-central North Atlantic (Fig. E9). This overall anomaly pattern is consistent with a positive phase of the East Atlantic (EA) teleconnection pattern (Table E1, Figs. E6, E7). The positive phase of this EA pattern has persisted for four consecutive months, and has been particularly pronounced for the past two months. This pattern has contributed to an amplified flow of mild, marine air into Europe, which has resulted in significantly above-average temperatures across the continent. It has also contributed to a southern shift of the main storm track toward southern Europe, which has resulted in enhanced rainfall across southwestern and southern Europe during the period.

c. Asia

Exceptionally cold temperatures have covered central Siberia and north-central Russia since November 2000. During January, surface temperatures averaged 2-5C below normal throughout the region. This excessive cold resulted from a persistent pattern of below-average heights at 500-hPa throughout the region, which has produced a sustained flow of polar air into the region at lower levels (Fig. E8).

2. Southern Hemisphere

The circulation during January featured above-average heights in the lower- midlatitudes, and an expanded areal extent of the polar vortex (Fig. E15) in association with below-average heights over large portions of the higher midlatitudes. Over Antarctica, near-average heights were observed during January, which contrasts with the extremely large positive height anomalies that dominated the region last month.

The primary regions of above-average temperatures during January were observed along the 30S latitude band (Fig. E1, bottom). Particular areas of significantly above-average temperatures included southeastern Brazil and the southern half of Australia. This overall anomalous warmth was linked to a poleward extension of the subtropical ridges around much of the hemisphere (Fig. T22), and with a concurrent reduction in the strength of the upper-level westerly winds in the vicinity of 30S (Fig. T21).

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