1. Northern Hemisphere
During August the 500-hPa
circulation featured a persistent pattern of positive height anomalies over
the central North Pacific, the high latitudes of the
North Atlantic
, and near the
Caspian Sea
, and negative height anomalies over
Alaska
, eastern
Canada
, northern
Europe
, and the polar region (Figs. E9, E11).
The main surface temperature departures during the month reflected
warmer-than-average conditions in the southeastern
United States
,
Scandinavia
, south-central
Russia
, and most of
China
(Fig. E1). Below-average temperatures were
observed in central
Europe
and central
Russia
.
Above-average precipitation
was recorded during August across large portions of the southwestern and
central
United States
, eastern Europe, and western
Russia
(Figs. E3, E6).
Below-average precipitation occurred in the northwestern
United States
and southeastern
Canada
,
Scandinavia
, and southwestern
China
(Figs. E3, E4).
a.
North America
The mean 500-hPa circulation
pattern featured a trough centered along the west coast of the
U.S.
, and a persistent ridge extending southward from central
Canada
to the southwestern
U.S.
(Fig. E9). The Inter-Mountain and Northern
Plains regions were situated downstream of the trough axis and recorded
above-average precipitation during the month (Fig. E5).
In the southwestern
U.S.
an enhanced monsoon brought above-average precipitation to
Arizona
and
New Mexico
, which contributed to area-averaged totals in both the Southwest and
Inter-Mountain regions above the 90th percentile of occurrences.
Below-average precipitation
was recorded in the
Pacific Northwest
region, which was situated immediately upstream of the mean trough axis.
Precipitation was also below average downstream of the mean ridge axis, with
area-averaged totals in the Great Lakes, mid-Atlantic, and Northeast regions
all recording totals in the lowest 30th percentile of
occurrences.
b.
North Atlantic
/
Eurasia
The 500-hPa circulation
pattern during August featured an anomalous ridge over the high latitudes of
the
North Atlantic
and
Scandinavia
and an anomalous trough over central and eastern Europe (Fig. E9).
This circulation contributed to significantly above-average precipitation
throughout
Europe
and western
Russia
, with totals in many areas exceeding the 90th percentile of
occurrences. It also contributed to exceptionally warm surface temperatures
in
Scandinavia
, with departures exceeding the 90th percentile of occurrences in
many areas.
c.
China
Exceptionally warm and dry
conditions were observed across southwestern and south-central
China
during August. These conditions were associated with an amplification of the
Asian monsoon ridge along its northeastern flank (Fig. T22),
and a northward shift in the main belt of westerly winds toward northern
China
(Fig. T21). As a result, much of southern
China
was situated directly beneath the mean upper-level ridge axis, well south of
the influence of mid-latitude weather systems and low-latitude frontal
passages.
2. Southern Hemisphere
The mean 500-hPa
circulation during August featured an anomalous zonal wave-3 pattern, with
positive height anomalies over the central ocean basins and
Antarctica
, and negative height anomalies centered over southern
Australia
and south of both
South America
and
Africa
(Fig. E15). In
Australia
, the persistent ridge led to anomalously warm and dry
conditions, with temperatures generally exceeding the 70th
percentile of occurrences and rainfall totals generally less than the 30th
percentile of occurrences. The most significant dryness occurred in the
southeast, where totals were below the 10th percentile of
occurrences.
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