1. Northern Hemisphere
The 500-hPa circulation
pattern during August featured above-average heights over the western U.S.
and Alaska, over the eastern North Atlantic, and over western Russia, and
below-average heights over the high latitudes of the North Atlantic,
southern Europe, and east of the Caspian Sea (Figs. E9,
E11). The main surface temperature departures
featured above-average temperatures across Alaska, the eastern half of the
U.S., and eastern Europe/ western Russia (Fig. E1).
The main precipitation anomalies during August included above-average totals
from over the southeastern quadrant of the U.S. and southern Europe (Figs.
E3, E5, E6).
a. North America
The prominent 500-hPa
circulation anomalies during August included above-average heights over the
western U.S. and Alaska (Fig. E9). These
anomalies contributed to exceptionally warm and dry conditions across
Alaska, with monthly temperature departures exceeding the 90th
percentile of occurrences (Fig. E1) and
monthly precipitation departures generally in the lowest 10th
percentile of occurrences (Fig. E3). They
also contributed to well above-average temperatures in the extreme western
U.S., with portions of California and the Pacific Northwest also recording
temperatures in the 90th percentile of occurrences.
The southeastern quadrant
of the United States experienced above-average precipitation during August.
Much of the excess rainfall during the month was related to Major Hurricane
Katrina, which made landfall east of New Orleans on 29 August. Katrina
produced extensive flooding and severe damage in coastal Louisiana,
Mississippi, and western Alabama. The storm produced totals in excess of 400
mm in these areas, before tracking northwestward to western New York. Along
its track, Katrina produced 200-275 mm of rain in northeastern Mississippi,
western Tennessee, and central Kentucky, and 75-100 mm of rain across the
Ohio Valley and western New York.
b. Europe/ western Russia
The 500-hPa circulation pattern during
August featured a persistent wave pattern characterized by above-average
heights over the eastern North Atlantic and western Russia, and
below-average heights over southern Europe and in the area east of the
Caspian Sea (Fig. E11). This pattern
contributed to above-average temperatures across eastern Europe and western
Russia, and to above-average precipitation across southern Europe.
2. Southern Hemisphere
mean 500-hPa circulation pattern during August featured above-average
heights from Australia to the central South Pacific, over the subtropical
eastern South Pacific, and over the western South Atlantic, and
below-average heights over the east-central South Pacific and in the area
south of Africa (Fig. E15). Over the
eastern Pacific, the north-south dipole of anomalies was associated with a
pronounced eastward extension of the South Pacific jet stream, with the main
jet core extending from Australia to just upstream of South America (Fig.
T21). This pattern contributed to anomalously
warm and dry conditions over southeastern Australia, and to exceptionally
warm conditions in southeastern South America (Fig. E1).
It also contributed to well above-average precipitation over southern
Argentina, with totals in many areas exceeding the 90th
percentile of occurrences.