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HOME > Expert Assessments > Climate Diagnostics Bulletin > Extratropical Highlights
 
Extratropical Highlights - April 2005
 

1. Northern Hemisphere

      The 500-hPa circulation during April featured above-average heights from Alaska and eastward to Scandinavia , over the subtropical eastern North Pacific, and across northwestern Africa , and below-average heights over the west-central north Pacific, near Great Britain , and northwestern Russia (Fig. E9). The main surface temperature departures during April included above-average temperatures across Canada and the north-central United States , across both the high latitudes and subtropical latitudes of the North Atlantic , and much of Europe (Fig. E1). The main precipitation anomalies reflected above-average totals in the Inter-Mountain region of the western U.S. , the northeastern U.S. , and both northern and southern Europe (Figs. E3, E5, E6).

a. North America

The main 500-hPa circulation anomalies affecting North America during April included above-average heights across Alaska and Canada , and in the subtropics over the extreme eastern North Pacific, and below-average heights off the west coast of California (Fig. E9). This pattern led to well above-average temperatures across Canada , with the largest departures exceeding 3C observed in central Canada and exceeding 2C across eastern Canada (Fig. E1).

Farther south anomalous southwesterly flow and a series of upper-level trough passages affected California and the Inter-Mountain regions of the United States during the month (Fig. E12). These conditions resulted in above-average precipitation in both regions, with area-averaged totals exceeding the 90th percentile of occurrences in the Inter-Mountain region, and approaching the 70th percentile in southern California (Fig. E5). The Inter-Mountain region has received well above-average precipitation each month since October 2004, which has either dissipated or significantly alleviated long-term drought conditions throughout the region.

 

b. North Atlantic and Europe

The 500-hPa circulation also featured positive height anomalies over the high latitudes of the North Atlantic and over northwestern Africa , and negative anomalies west of Great Britain (Fig. E9). This pattern was associated with increased storminess and above-average precipitation across both northern and southern Europe , with totals in portions of northern Europe exceeding the 90th percentile of occurrences (Fig. E3).

Exceptionally warm SSTs were again observed throughout the high latitudes and subtropical latitudes of the North Atlantic , with departures in both regions exceeding the 90th percentile (Figs. T18, E1). This anomaly pattern reflects the ongoing warm phase of the Atlantic multi-decadal mode that began approximately in 1995 (Goldenberg et al. Science, 2001). The anomalous warmth, combined with the enhanced southwesterly flow into northern Europe , resulted in a continuation of above-average temperatures across much the continent (Fig. E1).

 

2. Southern Hemisphere

     The mean 500-hPa circulation during April featured generally above-average heights in the middle latitudes, and below-average heights at high latitudes (Fig. E15). The above-average heights across southern Australia were very persistent (Fig. E17), and contributed to exceptionally warm and dry conditions throughout the continent (Figs. E1, E3). Much of Australia experienced temperatures in the highest 90th percentile of occurrences (Fig. E1), and precipitation departures in the lowest 30th percentile (Fig. E3). The most significant precipitation deficits were observed in southeastern Australia , where monthly totals were below the 10th percentile of occurrences.

 


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