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Extratropical Highlights - December 2001

1. Northern Hemisphere
The mean December circulation featured above-average 500-hPa heights across eastern Canada, the eastern United States, and the high latitudes of the North Atlantic, and below-average heights over the western North Pacific, Alaska, and the low latitudes of the eastern North Atlantic (Fig. E10). However, in the middle latitudes the circulation was also characterized by a major reversal in sign of the 500-hPa circulation anomalies during mid-month (Figs. E13, A2.2). Along the 40th parallel, for example, the first half of the month was characterized by below-average heights over the eastern hemisphere and above-average heights over the western hemisphere. An opposite sign of this anomaly pattern prevailed during the second half of the month (Fig. E13). This mid-month transition was accompanied by a phase reversal of several prominent teleconnection indices, including the Arctic Oscillation (Fig. A2.2), the North Atlantic Oscillation (NAO) (Fig. E8), the Pacific/ North American pattern (PNA), the East Atlantic/ West Russia pattern, and the Scandinavia pattern. It was also accompanied over North America by a reversal in the positions of the mean upper-level ridge and trough axes (Fig. A2.2).

North America

During the first half of December the upper-level atmospheric circulation featured a deep trough over western North America, and a ridge over the eastern United States and western North Atlantic (Fig. A2.2, top right). This pattern reflected a disappearance of the mean trough from its climatological position over eastern Canada and the eastern United States, and of the mean ridge from its climatological position over western North America. In contrast, the North American circulation during the second half of the month featured amplified ridges and troughs in approximately their climatological mean positions (A2.2, bottom left).

For the month as a whole large positive temperature anomalies were observed throughout North America for a third consecutive month, with December mean values exceeding the 90th percentile across the northeastern quadrant of the United States and most of eastern Canada (Fig. E1). The largest absolute temperature anomalies during December (exceeding 8C) were observed over southeastern Canada.

This anomalous December warmth was accompanied by a continuation of below-average precipitation over the eastern half of the United States (Fig. E3), with the Northeast and mid-Atlantic regions also experiencing significant precipitation deficits for a third consecutive month (Fig. E5). Elsewhere, below-average precipitation has been recorded in the Intermountain and Ohio Valley regions since August, in the Southwest since May, and in the Southeast for the second consecutive month.

2. Southern Hemisphere

The Southern Hemisphere circulation during December reflected a continuation since October of positive 500-hPa height anomalies across the middle latitudes, and negative anomalies over the high latitudes of the eastern South Pacific (Fig. E16, E18, E19). These conditions were also accompanied for a third consecutive month by an anomalous large-scale 500-hPa trough and below-average surface temperatures over central and southern Australia (Fig. E1).

In southern Africa area-averaged monthly rainfall totals reached the 90th percentile for the second consecutive month (Fig. E4), with the largest local rainfall anomalies again found in the heavy agricultural and climatologically heavy rainfall region of eastern South Africa (Fig. E3 top). The South African monsoon season typically lasts from November thru April.

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