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Extratropical Highlights - February 2002

1. Northern Hemisphere
The Northern Hemisphere circulation during February featured a persistent pattern of above-average 500-hPa heights in the middle latitudes, and below-average heights at high latitudes (Figs. E10, E12, E13). The notable zonal symmetry of this anomaly pattern is consistent with the positive phase of the Arctic Oscillation (Fig. A2.2, top left). This circulation was associated with well above-average surface temperatures over central North America, Europe, and Asia (Fig. E1), which resulted in hemispheric mean temperatures averaging near 1C above normal for a second consecutive month (Fig. E2). It was also associated with a north-south dipole of precipitation anomalies in the middle latitudes, which featured below-average rainfall over most of the latitude band between 30N-40N, and above-average precipitation and increased storminess over large portions of the hemisphere between 50N-60N (Fig. E3 bottom).

a. North America

The circulation over North America featured above-average heights in the western United States in association with an amplified upper-level ridge, and below-average heights over western Alaska in association with a very persistent and intense upper-level trough (Fig. E10). This anomaly pattern was associated with an enhanced westerly flow of marine air into western and central Canada (Figs. E11 right, T20), and resulted in exceptionally warm surface temperatures (3-4C above average and exceeding the 90th percentile) across south-central Canada and both the north-central and northeastern United States.

In the United States precipitation totals during February were below average in the east, the southwest, California, and the inter-mountain region of the west (Figs. E3, E5). Precipitation has been significantly below average in the Ohio Valley region since August 2001. Precipitation has also been below average in the Mid-Atlantic region since October 2001, in the Northeast since April 2001, and in the Southwest and Inter-Mountain regions since May 2001. In the East the combination of above-average temperatures and below-average precipitation led to reduced snow cover extent across Ohio and Pennsylvania. In the Northern Plains the excessive February warmth also led to a reduced snow cover and snow cover extent.

b. Europe and Asia

Over the North Atlantic the 500-hPa circulation during February featured below-average heights at high latitudes and above-average heights in the middle latitudes (Fig. E10). This anomaly pattern projected strongly onto the positive phases of both the North Atlantic Oscillation (NAO) and the East Atlantic teleconnection pattern (Figs. E7, E8, Table E1). This circulation was associated with enhanced jet stream winds and increased storm activity across northern Europe, southern Scandinavia, and western Russia (Fig. E11, right), which contributed to significantly above-average precipitation (Figs. E3, E4) and surface temperatures (Fig. E1). In each of these regions monthly mean precipitation totals and surface temperatures exceeded the 90th percentile.

To the south of the main Atlantic jet stream above-average 500-hPa heights and reduced storm activity led to significantly below-average precipitation across southern Europe and the Mediterranean Sea during the month, with totals in many locations dropping below the 10th percentile. The corresponding lack of any significant cold-air outbreaks also contributed to above-average surface temperatures in this area, with monthly mean values again exceeding the 90th percentile at many locations.

Above-average 500-hPa heights and record/ near-record surface temperatures also covered most of Asia during February, including almost all of the Middle East, Russia and China. The largest temperature anomalies averaged +6C to +8C and extended from west-central Russia to Mongolia. In southern Russia this anomalous warmth was accompanied by reduced snow cover extent and an overall northward retreat of the main winter snow pack. Accompanying these conditions significantly below-average precipitation was observed across the Middle East and most of southern and eastern China.

2. Southern Hemisphere

The Southern Hemisphere circulation during February featured 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). This overall anomaly pattern has persisted since October, and has 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 February, in association with anticyclonic circulation anomalies at 200-hPa throughout the region (Figs. T22, E17) and a poleward shift of the main belt of westerlies to well south of the continent (Fig. E17). The South African monsoon season typically lasts from November-April. To date the heavy agricultural region of eastern South Africa recorded above-average rainfall in both November and December, and below-average rainfall in January and February (Fig. E4).


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