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).
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).
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°-3°C
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°-3°C
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°-2°C
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.
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,
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.
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
2. Southern Hemisphere
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
, the central
, and the western
(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.
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).