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HOME > Expert Assessments > Climate Diagnostics Bulletin > Extratropical Highlights
 
Extratropical Highlights - June 2004
 

1. Northern Hemisphere

The 500-hPa circulation pattern during June featured above average heights from Mongolia eastward to Alaska, over Greenland, and southwestern Europe, and below-average heights across Canada, the central North Atlantic, Scandinavia, and eastern Siberia (Fig. E9). These circulation anomalies contributed to strong negative values of the West Pacific (-2.3) teleconnection pattern and the North Atlantic Oscillation (2.1, NAO), and to a strong positive value (+2.0) of the East Atlantic teleconnection pattern (Table E1, Fig. E7).

      Prominent temperature departures during June included warmer-than-average conditions across Alaska, western Canada, the high latitudes of the North Atlantic, and Western Europe, and cooler-than-average conditions across central and eastern Canada, and over the central United States (Fig. E1). The prominent precipitation anomalies during June reflected above-average rainfall across the Gulf Coast and southeastern regions of the United States, and below-average totals over Alaska and southwestern Europe (Figs. E3, E4, E5).

a. Pacific/North America

Over the North Pacific above-average 500-hPa heights extended across the western and central North Pacific, and below-average heights were observed over eastern Siberia. This anomaly pattern reflects the strong negative phase (-2.2) of the West Pacific teleconnection pattern (Table E1, Fig. E7), and was associated with a northward shift of the East Asian jet stream. Over North America the circulation during June reflected a persistent blocking ridge over Alaska and western Canada, and an amplified Hudson Bay trough that was also expanded westward (Fig. E9). These ridge and trough axes are approximately 30 west of their climatological positions over the Rocky Mountains and eastern Canada, respectively. These circulation anomalies were associated with a well-defined jet core across the north-central and northeastern United States (Fig. E10).

      The circulation across Alaska and Canada is similar to that observed during April and May. During June it contributed to near-record warmth in Alaska and to anomalously cold temperatures across central and eastern Canada (Fig. E1) for the third straight month. In Alaska temperatures were 2-3C above average during June and generally exceeded the 90th percentile of occurrences. Much of central and eastern Alaska also experienced near-record low precipitation during June, which along with the prolonged and excessive heat, led to many wildfires in that state. In contrast, mean June temperatures in Canada were 2-3C below average and generally within the lowest 30th percentile of occurrences. This anomalously cold air also extended southward across the Plains States, where temperatures were 1-2C below average during the month.

Also in the United States, significantly above-average precipitation covered the Gulf Coast and Southeast States during June, with totals in most areas exceeding the 70th percentile of occurrences. This enhanced rainfall is related to a series of cold frontal passages that penetrated well into the Deep South in association with the amplified upper-level trough. It is also related to increased storminess (Fig. E13) and enhanced convection within the right entrance region of the anomalous jet stream over the central United States (Fig. E10).

b. North Atlantic

The 500-hPa circulation pattern during June featured above-average heights over southern Greenland and below-average heights farther south.  This pattern reflects a strong negative phase (-2.1) of the NAO, and was associated with a nearly continuous, zonal jet stream extending from Nova Scotia to England (Fig. E10). A reverse north-south dipole pattern of height anomalies was evident over Western Europe. Southwestern Europe was situated in the southern part of this dipole pattern beneath a persistent upper-level ridge, and experienced significantly below-average precipitation (Figs. E3, E4).

North Atlantic SSTs have been considerably above average during the past year. During June they remained above the 90th percentile across the high latitudes of the North Atlantic, which contributed to ongoing warmth across Iceland and parts of Great Britain. This anomalous warmth is likely associated with the ongoing warm phase of the Atlantic multi-decadal mode that began in approximately 1995.

c. China

           Eastern China experienced below-average rainfall during June, with area-average totals in the lowest 10th percentile of occurrences (Fig. E4). This suppressed rainfall appears to be related to three factors. First, the Asian monsoon ridge was expanded eastward and the downstream upper-level trough was shifted to the east of China (Fig. T22). Therefore, Eastern China was situated in descending motion between the mean upper-level ridge and trough axes. Second, the jet stream winds across northern China were weaker than average (Fig. T21), which likely contributed to reduced storminess across the region. Third, eastern China was situated in the area of sinking motion along the right exit region of the anomalous jet stream extending across Asia.

2. Southern Hemisphere

           In the Southern Hemisphere the 500-hPa circulation pattern during June featured a zonal wave-3 pattern at high latitudes and above-average heights at lower latitudes, with main positive anomaly centers located over Australia , the central Indian Ocean , and the western South Atlantic (Fig. E15). Over Australia persistent above-average heights were associated with a poleward shift of the mean jet core to the extreme southern part of the continent (Fig. T21). This circulation separated anomalously dry conditions over Australia from exceptionally wet and stormy conditions just south of the continent (Fig. E3). Both Tasmania and portions of New Zealand experienced above-average precipitation in association with this anomalous jet stream and storm track.

Much of southern Australia also experienced warmer than average surface temperatures during June. This warmth is associated with anomalous 850-hPa westerlies across the entire southern part of the continent, and a nearly complete disappearance of inflow of cooler air from the Great Australian Bight into southern Australia.

Southern South America experienced above-average surface temperatures during June, and extreme southern Argentina experienced above-average precipitation. These conditions are related to a persistent and large amplitude upper-level trough over the high latitudes of the eastern South Pacific (Figs. T20, T21, E15).


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