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

1. Northern Hemisphere

The 500-hPa circulation pattern during January featured persistent positive height anomalies across the central North Pacific, and from eastern North America to central Siberia , and persistent negative anomalies over Alaska , the Mediterranean Sea , and south-central Russia (Figs. E9, E11). The main surface temperature departures reflected well above-average temperatures across North America and below-average temperatures across much of Eurasia (Fig. E1). The main precipitation departures included above-average totals in the northwestern U.S. and Mediterranean Sea , and below-average totals in the southwestern U.S. and northern Europe (Figs. E3, E5, E6).


a. Pacific/ North America

The 500-hPa circulation featured a combination of above-average heights over the east-central North Pacific and eastern Canada , and below-average heights over Alaska and western Canada . This anomaly pattern reflected a nearly zonal flow across North America in association with a disappearance of the mean upper-level ridge normally located over western North America and a marked weakening of the climatological mean Hudson Bay trough (Fig. E9). This zonal flow produced near-record warmth throughout the continent, with much of the U.S. and Canada recording departures of +5C to +8C. For nearly the entire continent east of the Rocky Mountains , monthly temperatures were above the 90th percentile of occurrences.

The circulation also featured a pronounced eastward extension of the East Asian jet stream and an eastward shift of the jet exit region to just upstream of the northwestern United States (Fig. T22). This jet stream pattern resulted in a very focused storm track  and exceptionally heavy precipitation in the northwestern U.S. Monthly precipitation totals in the Pacific Northwest have exceeded the 90th percentile in three of the last four months. In contrast, a lack of storminess farther south resulted in well below-average precipitation in the southwestern United States . Area-average precipitation totals in this region have been below the 10th percentile for the last five months. These ongoing deficits have resulted in severe drought conditions across most of southern Arizona and western New Mexico .


b. Europe

    The 500-hPa circulation pattern during January featured above-average heights across Scandinavia and the adjacent polar region, and below-average heights across the Mediterranean Sea and south-central Russia (Fig. E11). 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 a significant flow of polar air into Russia . This combination resulted in below-average temperatures across much of Eurasia , with the largest negative departures (-5 to -8C) centered in south-central Russia and Kazakhstan where temperatures were below the 10th percentile of occurrences.


2. Southern Hemisphere

    The mean 500-hPa circulation pattern during January featured above-average heights from the central Indian Ocean to the central South Pacific, and below-average heights across the high latitudes of the South Pacific (Fig. E15). In Australia , the mean upper-level ridge axis was located over the middle of the continent. Below-average rainfall and temperatures occurred upstream of the ridge axis and above-average rainfall and temperatures occurred downstream over eastern Australia (Fig. E3). Temperatures in eastern Australia were again significantly above-average (3+ C above average) during the month, and generally exceeded the 90th percentile of occurrences (Fig. E1). This warmth is consistent with a disappearance of low-level flow from the Great Australian Bight , and with below-average precipitation throughout the region (Fig. T20).

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



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