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Extratropical Highlights - January 2002

1. Northern Hemisphere
The Northern Hemisphere circulation during January featured above-average 500-hPa heights across the central North Pacific, the eastern United States, Europe, and most of Asia, and below-average heights over the high latitudes of the North Atlantic and northern Russia (Figs. E10, E12, E13). The mid-latitude circulation again featured a major mid-month reversal of the large-scale ridges and troughs over the central Pacific and the United States (Figs. Fig. E13, A2.2), along with a sharp transition to the negative phase of the Pacific/ North American (PNA) teleconnection index (Figs. E8, A2.2, top left). In contrast the circulation across the North Atlantic, Europe, and north-central Russia exhibited considerable persistence during January, which contributed to exceptionally warm temperatures across Europe and near-record monthly temperatures (exceeding 4C-6C) across central Asia.

North America

During the last two months large swings in the locations of the long-wave ridges and troughs have been observed over the North Pacific and the United States. During the second of half of December and first half of January the circulation featured amplified ridges and troughs at upper levels over western North America and the eastern U.S., respectively (Fig. A2.2, top right). This circulation was accompanied by a pronounced eastward extension and amplification of the Aleutian Low, and by an extension of the East Asian jet to the east-central North Pacific.

During the second half of the month the circulation featured a disappearance of the Aleutian Low and a retraction of the East Asian jet toward the western Pacific. Over the U.S. a deep trough covered western North America during the period and a ridge persisted over the southeastern United States (Fig. A2.2, bottom left). These conditions were accompanied by a confluent southwesterly flow at upper levels across the central U.S., which brought near-record warmth to across the eastern half of the country.

For the month as a whole large positive temperature anomalies were again observed over large portions of North America, with monthly mean surface temperatures exceeding the 70th percentile across the eastern two-thirds of the United States and most of southern Canada (Fig. E1). This warmth was accompanied by a continuation of below-average precipitation over the eastern and Gulf coastal regions of the United States (Fig. E3). The Northeast and mid-Atlantic regions of the United States have experienced significant precipitation deficits since October (Fig. E5). The Intermountain and Southwest regions of the United States have experienced below-average precipitation since May.

2. Southern Hemisphere

The Southern Hemisphere circulation during January featured a continuation since October of positive 500-hPa height anomalies across the middle latitudes, and negative height anomalies over the high latitudes of the eastern South Pacific (Fig. E16, E18, E19). These conditions have also been associated with below-average surface temperatures over large portions of southern Australia throughout the period (Fig. E1).

In southern Africa below average rainfall was observed during January, following well above-average totals in the heavy agricultural and climatologically heavy rainfall region of eastern South Africa during both November and December (Fig. E4). The South African monsoon season typically lasts from November-April.

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