1. Northern Hemisphere
The 500-hPa circulation during August 2003 featured a persistent pattern
of positive height anomalies over western North America, central Europe,
north-central Russia (Figs. E10, E12),
and negative height anomalies over the central North Atlantic, eastern
Europe, and the polar region. The prominent temperature anomalies during
the month included a continuation of significantly warmer-than-average
conditions over the western United States and Europe, with anomalous warmth
also observed across eastern Canada, north-central Russia, and southeastern
China. (Fig. E1). Prominent precipitation
anomalies during August included a continuation of above-average totals in
the southeastern and eastern United States, and below-average totals in
Europe (Fig. E3).
a. North America
The mean upper-level circulation featured an amplified upper-level ridge
extending northward from Texas to northwestern Canada (Fig.
E10), along with an anomalous northward shift of the mean jet
stream into central Canada (Fig. E11). It
also featured a persistent upper-level trough over eastern North America.
These conditions were associated with reduced storminess over the western
United States, and significantly increased storminess (Fig.
E14, right) and above-average rainfall across the southeastern and
eastern United States and eastern Canada (Figs. E3,
E6).The Southeast and Ohio Valley regions have
now recorded above-average precipitation in eleven of the last twelve
months beginning September 2002 (Fig. E5).
North America was dominated by anomalous warmth during August (Fig.
E1), with the largest temperature anomalies associated with the
enhanced upper-level ridge in the west. Temperatures averaged 2°C
above normal over most of the western U.S. and central Canada, with the
largest anomalies exceeding 3°C
over the northern Plains States and south-central Canada.
A strong upper-level ridge was again evident across Europe during August
(Fig. E10), which contributed to
significantly warmer and drier than average conditions throughout the
continent. Monthly mean temperatures exceeded the 90th
percentile throughout Europe (Fig. E1) and
rainfall totals were generally below the 10th percentile (Figs.
E3, E4). Area-averaged
rainfall over southern Europe has been below average since March, with
totals dropping below-the 10th percentile in three of the past
2. Southern Hemisphere
In the Southern Hemisphere the 500-hPa circulation during August
featured above-average heights across the high latitudes of the three ocean
basins, and below-average heights across the central South Pacific and in
the area south of Africa (Fig. E16). Over
the South Pacific this anomaly pattern was associated with a pronounced
eastward extension of the wintertime jet stream in the middle latitudes (Fig.
T21, top), and with persistent high latitude blocking activity in
the area northeast of New Zealand (Fig. E18).
Eastern Australia experienced a continuation of above-average
temperatures during August (Fig. E1) in
association with a persistent anomalous anticyclonic circulation at
upper-levels (Fig. T21). The anomalous
warmth also covered New Zealand, where monthly mean temperatures averaged
above the 70th percentile.