1. Northern Hemisphere
circulation during August featured above-average heights over Greenland, western
Russia, eastern Asia, and the Gulf of Alaska, and below average heights over
western Europe and central Siberia (Fig. E9).
Over the subtropical Atlantic basin, the 200-hPa streamfunction pattern
continued to show a pronounced inter-hemispheric symmetry during August, with
anticyclonic anomalies extending from the America’s to Africa in both
hemispheres (Fig. T22).
Conversely, an anomalous trough was evident across most of the tropical western
North Pacific, with a similar anomaly pattern evident in the Southern
Hemisphere. These signals are consistent with the combined influences of La Niña
and an enhanced west African monsoon system (Figs.
The main surface
temperature signals during August included warmer than average conditions across
the eastern half of the United States, Mexico, western Russia, and most of China
Monthly precipitation totals (Fig. E3)
were above-average in northern Europe, and in the Inter-Mountain West and Gulf
Coast regions of the U.S. (Figs. E5,
E6). Monthly precipitation was below average in
the mid-Atlantic, Northeast, and Pacific Northwest regions of the U.S.,
southwestern Europe and southwestern Russia.
a. North America
The mean 500-hPa
circulation during August featured a broad and persistent ridge across the
central and eastern United States (Fig. E9).
This pattern was associated with a continuation from July of above average
temperatures across the eastern half of the country, with August departures in
many locations exceeding the 90th percentile of occurrences (Fig. E1). The mid-Atlantic and
northeastern states also experienced a continuation of well below average
precipitation during August (Fig. E5).
For both regions precipitation has been below average since April. During
August, ongoing precipitation deficits in the mid-Atlantic region led to the
development of moderate- to-severe drought. Elsewhere, precipitation was above
average in the Midwest and Gulf Coast regions of the country, and in the
Inter-Mountain West where totals have been above average for nine straight
subtropical Atlantic, anticyclonic streamfunction anomalies at 200-hPa extended
from the America’s to Africa in both hemispheres (Fig.
T22). This pronounced
inter-hemispheric symmetry of the anomaly pattern was associated with an
expanded area of upper-level easterly wind anomalies that extended across
tropical northern Africa and the tropical North Atlantic (Fig.
T21). It was also associated with an
extensive area of low-level westerly wind anomalies across the tropical Atlantic
These conditions lead to reduced vertical wind shear across the tropical
Atlantic, and are typical of the high activity era for Atlantic hurricanes that
began in 1995. This combination of conditions is consistent with an enhanced
west African monsoon circulation, which was again present during August (Figs.
and has been a prominent feature of the circulation since 1995.
circulation during August featured a strong and persistent ridge centered over
western Russia, along with an amplified trough over northern Europe (Fig.
E9). This pattern affected temperature and
precipitation patterns from Europe to Pakistan (Figs.
E1, E3). The strong
ridge led to exceptionally warm and dry conditions in western Russia, with many
areas experiencing record heat. The amplified trough contributed to well above
average precipitation in northern Europe, with many areas recording totals in
the upper 90th percentile of occurrences.
During part of the
month the ridge in western Russia was exceptionally amplified and resembled a
typical Omega-block. This blocking pattern was associated with a very strong
downstream trough that extended southward from central Russia to Pakistan. This
amplified trough interacted with a deep monsoonal flow of moisture coming into
the Indian sub-continent, which led to extreme precipitation totals and record
flooding in Pakistan.
2. Southern Hemisphere
circulation during August featured a zonally-symmetric pattern of height
anomalies, with above average heights in the middle latitudes and generally
below average heights over Antarctica (Fig.
E15). In the subtropics, the upper-level
streamfunction pattern reflected an anomalous trough across the western and
central tropical South Pacific, and an anomalous ridge extending from the
eastern subtropical South Pacific to southern Africa (Fig.
T22). A similar anomaly pattern was
evident in the Northern Hemisphere. These conditions have been present since
June, and likely reflect the combined influences of La Niña and an enhanced
west African monsoon system. One characteristic feature of La Niña was a marked
westward retraction of the South Pacific jet core, as indicated by easterly wind
anomalies centered along the jet axis east of the date line near 30°S (Fig.