500-hPa circulation during August featured above average heights over the polar
region and below average heights in the northern mid-latitudes (Fig.
E9). Regional aspects of this pattern included
below average heights over Alaska, Canada, northeastern Europe, and central
Siberia. Over the Atlantic Ocean, the circulation reflected a strong negative
phase (-1.85) of the North Atlantic Oscillation (NAO) (Fig.
E7, Table E1).
main surface temperature signals during August included warmer-than-average
conditions over most of the U.S., eastern Canada, Europe, and large portions of
eastern Asia (Fig. E1). The main precipitation signals
included above-average totals in the northeastern U.S. and northwestern Europe,
and below-average totals in the southern and mid-western U.S., and in southern
Europe (Fig. E3).
a. North Pacific
and North America
mean 500-hPa circulation during August featured a persistent ridge over the
central U.S., an anomalous trough extending from Alaska to eastern Canada, and
mean troughs over both the west and east coasts of North America (Fig.
E9). At 200-hPa, the most prominent circulation
feature was an enhanced summer-time ridge east of the Rocky Mountains (Figs. T21, T22),
which controlled the surface temperature (Fig.
E1) and precipitation (Fig.
E3) anomalies across much of the U.S.
warm surface temperatures covered much of the U.S. during August, with the
southern half of the country recording departures in the upper 90th
percentile of occurrences (Fig. E1).
The largest actual temperature anomalies were observed in the south-central
states, where monthly departures exceeded 5oC.
precipitation deficits also accompanied the amplified summer-time ridge, with
below average totals observed in the southern Plains states, the Gulf Coast
states, and large portions of the Midwest (Fig.
E3). For the state of Texas monthly totals were
generally less than 25% of normal (Fig. E6),
while temperatures were generally 3oC-5oC above average.
According to NOAA’s U.S. Drought Monitor, exceptional drought continued during
August across Texas, Oklahoma, southern Kansas, eastern New Mexico, and portions
of western Louisiana. Extreme drought was evident in southeastern Colorado and
mid-Atlantic and Northeast regions of the U.S. recorded above-average
precipitation during August (Fig. E5),
in association with the mean upper-level trough over the eastern U.S. Another
contributing factor to this enhanced rainfall was Hurricane Irene, which made
landfall late in the month in both North Carolina and central New Jersey. Irene
produced heavy rains exceeding 175 mm (7 inches) and wide-spread flooding from
eastern North Carolina northward to southern New York. The largest totals
exceeded 250 mm (10 inches) across eastern North Carolina, portions of the
Delmarva Peninsula, and southern New York. In Vermont, storm rainfall totals of
125mm – 175 mm (5-7 inches) also caused wide-spread flooding.
North Atlantic and Europe
500-hPa circulation during August featured above average heights over Greenland
and below average heights over northwestern Europe and eastern North America (Fig. E9). This persistent pattern (Fig. E11) projected strongly onto the
negative phase (-1.85) of the North Atlantic Oscillation (NAO) (Fig.
E7, Table E1).
Consistent with this pattern, northwestern Europe recorded increased
precipitation in the area immediately downstream of the mean upper-level trough,
and southern Europe recorded decreased precipitation (Fig.
The 500-hPa circulation during August
featured above average heights in the area south of Australia and Africa, and
also across Antarctica, and below average heights over the central South Pacific
Ocean and South Atlantic Ocean (Fig. E15).
The most significant surface
temperature signals were observed in southern and central Australia, where
departures ranged from +2.0oC to +3.0oC, and were in the
upper 90th percentile of occurrences (Fig.
The Antarctic ozone hole typically
develops during August and reaches peak aerial extent in September and October.
By the end of August, the ozone hole (Fig.
S6) spanned approximately 20 million square
kilometers, which is roughly equal to the 2001-1010 mean (Fig.