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HOME > Expert Assessments > Climate Diagnostics Bulletin > Extratropical Highlights
 
Extratropical Highlights - June 2007
 

1. Northern Hemisphere   

The 500-hPa height pattern during June featured positive anomalies over the high latitudes of the central North Pacific, Greenland , the polar region, and Mongolia , and negative anomalies in the Gulf of Alaska , across the central North Atlantic , and western Russia (Figs. E9, E11). At 200-hPa, cyclonic streamfunction anomalies were evident across the subtropical Pacific Ocean in both hemispheres. This pattern reflected a weakening of the mean subtropical ridges over the western Pacific and a strengthening of the mean mid-Pacific troughs in both hemispheres (Fig. T22). These conditions are consistent with suppressed convection across the equatorial Pacific during the month (Fig. T25).

The main surface temperature departures during June reflected above-average temperatures in the eastern United States , Europe , and most of eastern Asia , and below-average temperatures in northwestern and central Russia (Fig. E1). The main precipitation anomalies included above average totals in the central U.S. and northwestern Europe , and below average totals across the northeastern U.S. and eastern Mongolia (Fig. E3).

 

a. North America

The 500-hPa circulation pattern during June featured an enhanced ridge over the southwestern United States and a broad trough over the Gulf of Alaska . The anomalous ridge appears to partly reflect a downstream response to the very persistent and amplified mid-Pacific trough at 200-hPa centered near Hawaii (Fig. T22). This overall pattern was associated with enhanced precipitation in western Canada (Fig. E3), and with above-average temperatures across the southwestern United States (Fig. E1).

Farther downstream, the mean upper-level trough position was shifted eastward to the western North Atlantic (Figs. E9, E11), resulting in anomalous upper-level convergence and descending motion across the mid-Atlantic and northeastern United States (Fig. T23). In both regions, area-averaged totals were in the lowest 20th percentile of occurrences for the second straight month (Fig. E5). Although the southeastern U.S. recorded near-average totals during June, long-term precipitation deficits in that region meant a continuation of extreme drought from Mississippi to western Georgia and across the southern half of Tennessee .

In contrast the southern Plains states recorded well above-average precipitation for the second straight month. For the Great Plains region as a whole, area-averaged totals have been above the 90th percentile of occurrences in five of the last seven months.

 

b. Eurasia

The 500-hPa circulation pattern during June featured a north-south dipole of height anomalies across the North Atlantic , with positive anomalies at high latitudes and negative anomalies farther south (Fig. E9). This pattern reflected a strong high-latitude blocking ridge, and a southward shift of the mean upper-level westerlies to central and southern Europe . Farther downstream, a broad ridge spanned eastern Europe and an anomalous trough was centered over northwestern and central Russia . The surface temperature anomaly patterns reflected these circulation features, with temperatures in the upper 90th percentile of occurrences across eastern Europe and temperatures in the lowest 30th percentile of occurrences in portions of northwestern and central Russia (Fig. E1).

In Asia , a persistent pattern of positive height anomalies at 500-hPa was associated with well above-average temperatures across eastern Mongolia and northern China , as monthly departures exceeding the 90th percentile of occurrences in many regions. Much of this region also recorded above-average temperatures in May.

 

2. Southern Hemisphere

      The 500-hPa height anomaly pattern during June featured a pronounced zonal wave-3 pattern, with positive anomalies south of Australia , over the central South Pacific, and south of Africa , and negative anomalies across Australia , most of the central South Atlantic Ocean , and the central Indian Ocean (Fig. E15). The circulation across Australia featured a ridge in the southwest and a strong trough in the southeast. This pattern contributed to below-average precipitation in the southwest and above-average precipitation along the eastern seaboard (Fig. E3). This circulation, in combination with the strong ridge south of the continent, was also associated with an anomalous southerly flow of cool marine air at 850-hPa into southeastern and south-central Australia (Fig. T20). As a result, much of central and eastern Australia recorded well below-average temperatures(2-3C below average), with large interior portions of the continent recording values in the lowest 10th percentile of occurrences (Fig. E1).

 

 


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