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

1. Northern Hemisphere

      

The 500-hPa circulation pattern during September featured persistent positive height anomalies across the central North Pacific, eastern North America , and northern Europe , and negative height anomalies over western Canada and southern Europe (Figs. E9, E11). The main surface temperature departures reflected above-average temperatures throughout eastern North America , the high latitudes of the North Atlantic , and most of Eurasia (Fig. E1). The main precipitation anomalies during September included well below-average precipitation in the eastern United States , and in both southwestern and northeastern Europe (Figs. E3, E5, E6).

 

a. North America

The prominent 500-hPa circulation anomalies during September included a persistent pattern of above-average heights across the central North Pacific and eastern North America , and below-average heights over western Canada (Figs. E9, E11). This pattern was associated with anomalous southwesterly flow in the region between the mean upper-level trough and ridge axes from the southwestern to the central United States . The persistent ridge over eastern North America contributed to exceptionally warm and dry conditions in the eastern United States . Monthly mean temperatures were generally 2-3C above average in that region, with most areas recording anomalies in the upper 90th percentile of occurrences (Fig. E1). Also, monthly rainfall totals were generally in the lowest percentile of occurrences in the East (Fig. E3), with the Ohio Valley , Mid-Atlantic and Northeast regions all recording area-averaged totals in the lowest 10th percentile of occurrences (Fig. E5).

 

b. North Atlantic / Eurasia

The 500-hPa circulation pattern during September featured a north-south tri-pole pattern of height anomalies, with above-average heights from northern Europe to central Russia , and below-average heights at high latitudes and across southern Europe (Fig. E11). This pattern, combined with anomalous upper-level easterlies across Europe and western Russia , contributed to well above-average temperatures in these regions. It also contributed to below-average precipitation in both southwestern and northeastern Europe , with both areas located immediately downstream of the mean upper-level ridge axes.

 

2. Southern Hemisphere

The mean 500-hPa circulation pattern during September featured an anomalous zonal wave-3 pattern in the middle and high latitudes, along with above-average heights across the central South Pacific. The wave-3 pattern was characterized by above-average heights southeast of Australia , over the western South Atlantic , and over the central Indian Ocean , and below-average heights southwest of Australia , over the high latitudes of the eastern South Pacific, and in the area south of Africa (Fig. E15).

The main positive surface temperature anomalies were observed over southern Africa , consistent with the broad pattern of positive height anomalies extending across the South Atlantic into the Indian Ocean . The main area of below-average temperatures occurred over central South America , immediately downstream of the mean upper-level trough axis (Fig. E1).

The Antarctic ozone hole and polar stratospheric cloud (PSC) cover during September 2005 were near the 1994-2003 mean (Fig. S8). The main area of the ozone hole extended from the high latitudes of the central South Pacific to the central Indian Ocean (Fig. S6), and coincided with the area of below-average heights in the middle stratosphere (Fig. S1).

 

 


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