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HOME > Expert Assessments > Climate Diagnostics Bulletin > Extratropical Highlights
Extratropical Highlights - February 2006

1. Northern Hemisphere

The 500-hPa circulation pattern during February featured persistent positive height anomalies across the central North Pacific, over Greenland , and in the area west of the Tibetan Plateau, and persistent negative anomalies over eastern North America , most of Europe , and eastern Siberia (Figs. E9, E11). In the subtropics, cyclonic streamfunction anomalies at 200-hPa extended from eastern Africa to the United States (Fig. T22). Over the North Pacific and North America these circulation anomalies were consistent suppressed equatorial convection near the date line, in association with a combination of the Pacific cold episode and descending motion associated with the MJO. 

The main surface temperature departures during the month reflected warmer than normal conditions across Canada and the area around the Tibetan Plateau, and cooler than normal conditions across large portions of the U.S. , Europe , and western Russia (Fig. E1). The main precipitation departures included above-average totals in eastern Canada and southeastern Europe , and below-average totals in the southwestern and central U.S. and Scandinavia (Figs. E3, E5, E6).


            a. Pacific/ North America

In the extratropics, the 500-hPa circulation pattern featured a pronounced north-south dipole of height anomalies across the western and central North Pacific, which reflected a strong positive phase (+2.5) of the West Pacific (WP) teleconnection pattern (Table E1, Fig. E7). In the lower latitudes, cyclonic streamfunction anomalies at 200-hPa reflected a below-average strength of the mean subtropical ridge across the western and central North Pacific, and an above average strength of the mean trough over the eastern Pacific (Fig. T22). These conditions were associated with an anomalously weak East Asian jet stream, and with a westward retraction of that jet core to well west of the date line (Fig. T21). Over the eastern Pacific, the associated split-flow configuration resulted in an anomalous flow of mild air into northern Canada , where temperatures were 2°-4°C above average during the month (Fig. E1).

In North America , the dominant circulation feature was an amplified trough over the eastern U.S. and Canada . The enhanced northwesterly flow upstream of this trough contributed to an anomalous southward displacement of the mean surface cold frontal boundary, which led to slightly below-average temperatures from the Pacific Northwest to the eastern United States . Also, anomalous descending motion upstream of the mean trough axis, combined with reduced storminess, led to below-average precipitation across the western and central states (Figs. E3).

The most significant precipitation anomalies occurred in the drought-stricken southwestern  and southern Plains states, where February totals were generally less than 25% of normal (Fig. E6). In the Southwest, area-average totals were below the 10th percentile of occurrences for the sixth consecutive month (Fig. E5). For the entire Plains region, area-average totals have also been below normal for the past six months, with the largest deficits occurring in the southern Plains. In contrast, the below average totals in the Pacific Northwest during February signaled a reprieve from exceptionally heavy totals that had affected the region since October 2005. 


b. Europe

      The 500-hPa circulation pattern during February featured a persistent pattern of above-average heights over Greenland , and generally below-average heights across the central North Atlantic (Fig. E9). A pronounced split flow was also evident over the eastern North Atlantic . The northern branch of the jet stream headed north of Scandinavia and then strongly southward into western Russia (Fig. E10). The southern branch of the jet stream was situated over northern Africa . This circulation represented a nearly complete disappearance of the normal flow of marine air into Europe , and resulted in below-average temperatures throughout Europe and western Russia .


2. Southern Hemisphere

The mean 500-hPa circulation pattern during February featured above-average heights south of South America , and an extensive area of below-average heights across the South Atlantic and southern Indian Ocean (Fig. E15). Over Australia , the mean upper-level circulation featured a ridge in the west and a trough just off the east coast (Fig. T22). Anomalously cool (Fig. E1) and wet (Fig. E3) conditions were observed in Western Australia in the area upstream of the ridge axis, and anomalously warm and dry conditions were observed across the eastern half of Australia in the area between the ridge and downstream trough axis.

In southern Africa the rainy season normally lasts from October to April. Area-averaged rainfall was above normal during February, marking five consecutive months of above-average rains (Fig. E4). An enhanced South African rainy season is consistent with ongoing La Niņa conditions.



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