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

1. Northern Hemisphere

The 500-hPa circulation pattern during June featured above-average heights over the eastern North Pacific, northeastern Canada , Europe and throughout the polar region, and below-average heights over the western United States and the central North Atlantic (Figs. E9, E11). Surface temperatures were above-average temperatures across Alaska , the northeastern quadrant of the U.S. , Europe , and both the high latitudes and subtropical latitudes of the North Atlantic , and below-average in western North America (Fig. E1). The main precipitation anomalies during June reflected above-average totals in the north-central U.S. , the Gulf of Mexico , across the tropical Atlantic / Caribbean Sea , and throughout southeastern Asia , and below-average totals in the south-central U.S. and northern Europe (Figs. E3, E5, E6).


a. Pacific/ North America

The main 500-hPa circulation anomalies affecting the Pacific/ North American sector during June were above-average heights over Alaska and eastern Canada , and below-average heights over the western United States (Fig. E9). Over eastern North America , this anomaly pattern reflected a complete disappearance of the climatological mean Hudson Bay Low. The monthly surface temperature departures were consistent with this anomaly pattern, with above-average temperatures across Alaska and the north-central U.S. , and below-average temperatures over the western United States .

The mean upper-level ridge axis during June was centered over the east-central United States . Upstream of the ridge axis , broad southwesterly flow and anomalous ascending motion contributed to well above-average precipitation in the northern Plains states, with totals in many areas exceeding the 90th percentile of occurrences. Conversely, anomalously dry conditions were observed along and downstream of the mean ridge axis, with portions of the Great Lakes region recording totals in the lowest 30th percentile of occurrences. This area has recorded below-average precipitation for the last four consecutive months (Fig. E5).

Above-average precipitation was also observed along the Gulf Coast region of the United States and throughout the Gulf of Mexico . Two tropical storms contributed to these large rainfall totals. Tropical storm Arlene formed in the northern Caribbean Sea on 9 June. It subsequently tracked through the Gulf of Mexico and made landfall along the central Gulf Coast . The second tropical storm of the season (Bret) formed over the western Gulf of Mexico and made landfall in eastern Mexico .


b. North Atlantic , Eurasia

The 500-hPa circulation pattern during June featured above-average heights over central and southern Europe (Fig. E11), which led to anomalously warm and dry conditions over much of the continent. Area-average precipitation totals in both northern and southern Europe were in the lowest 20th percentile of occurrences, while monthly mean temperatures were above the 70th percentile of occurrences.

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).


2. Southern Hemisphere

In the middle latitudes, the mean 500-hPa circulation pattern during June featured an anomalous zonal wave 3-4 pattern, with above-average heights over the three ocean basins and below-average heights in the vicinity of the three continents (Fig. E15). The circulation pattern at high latitudes featured above-average heights throughout the polar region and the central South Pacific, which extended well into the middle stratosphere (Fig. S1).  At lower latitudes, the 200-ha subtropical ridges were stronger than average throughout the Southern Hemisphere, with the largest anticyclonic anomalies occurring over tropical Australia and the eastern South Pacific (Fig. T22). These anomalies were related to a regional strengthening of the local Hadley circulations in association with amplified monsoon systems over southeastern Asia and Central America , respectively (Fig. T23).

This anomaly pattern was associated with enhanced upper-level westerlies and a nearly uniform South Pacific jet stream from Australia to southern South America (Fig. T21). Much of southern South America was situated in the diffluent exit region of this jet. This region experienced increased storminess and above-average precipitation, with monthly totals generally exceeding the 70th percentile of occurrences. In east-central South America , located equatorward of the jet exit region, monthly mean temperatures ranged from 2-3C above average and exceeded the 90th percentile of occurrences.

In Australia , a strong upper-level ridge covered the northern part of the continent while an amplified trough prevailed across the southwest (Fig. T22). These conditions were associated with enhanced jet stream winds across the continent, and an anomalous jet exit region in the east (Fig. T21). Eastern Australia was situated downstream of the mean upper-level trough axis and within the cyclonic shear side of the anomalous jet exit region, and therefore experienced increased storminess and enhanced precipitation during the month. Also downstream of the mean trough axis, anomalous low-level poleward flow from the interior of the continent resulted in above-average temperatures to much of southern Australia (Fig. T20). This anomalous flow reflected a nearly complete disappearance of the normal inland flow of cooler marine air from the Great Australian Bight .


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