The anomalous circulation during July
featured a reversal in the pattern that was observed in June for many areas in the middle
and high latitudes of the Northern Hemisphere. This reversal was particularly impressive
over North America and western Europe. For the eastern United States and most of Europe,
this reversal was associated with below-average temperatures, which contributed to a drop
in the average Northern Hemisphere and global temperatures to their lowest values since
May 1997 and October 1997, respectively (Fig. E2).a.
North America
Above-average 500-hPa heights during July covered central and northeastern Canada, with
an extension southward into the western United States, while below-average heights covered
the entire eastern conterminous United States and Alaska (Fig. E9).
This anomaly pattern was accompanied by enhanced northwesterly flow into the Great Lakes
and eastern states, which contributed to abnormally cool temperatures throughout the
region (Fig. E1). In contrast, temperatures in the
Southwest and Rocky Mountain states were above-average (1°-2°C) during the month, in
association with the positive 500-hPa height anomalies. Temperatures were also
significantly above average over northern Canada.
The anomalous circulation during July also contributed to generally drier-than-normal
conditions over many sections of the United States, except for northwestern Washington and
portions of the mid-Atlantic and Northeast (Fig. E3). In
contrast, enhanced precipitation occurred over western and central Canada, associated with
anomalous cyclonic flow along the west coast of North America (Fig.
E9). In many areas of the United States rainfall departures in July were opposite
to those observed in June. Along the Gulf Coast and in the southeastern states long-term
drier than average condition persisted (Fig. E5).
B. Europe/ Asia
Most of Europe experienced cooler-than-average temperatures during July (Fig. E1), in contrast to the very warm conditions experienced
in June. These cooler conditions were associated with below-normal 500-hPa heights across
the region (Fig. E9), a feature which also contributed to
above-average rainfall throughout northern and central Europe during the month (Fig. E3).
Generally drier-than-average conditions prevailed over most of northern China for the
second consecutive month, associated with higher-than-average 500-hPa heights over the
region. Drier-than-normal conditions also affected southern and eastern sections of India,
contrasting with the very wet conditions observed the previous month.
2. Southern Hemisphere
The Southern Hemisphere circulation during July (Fig. E15)
featured above-average 500-hPa heights over the Antarctic Peninsula and the neighboring
South Atlantic, and in mid-latitudes over the central and eastern South Pacific. Negative
height anomalies were observed southwest of Australia, over the southwestern South
Atlantic and neighboring South America, and at high latitudes over the eastern South
Pacific. The pattern of height anomalies over the southwestern Atlantic and South America
was associated with frequent blocking in the vicinity of the Weddell Sea. This contributed
to an abnormal influx of cold air into northern Argentina, Paraguay, Uruguay, eastern
Bolivia and southern Brazil, where monthly average temperatures ranged from 2°C to 4°C
below normal, and temperatures ranked below the 10th percentile in several
areas (Fig. E1).
In contrast to recent months, most of Australia, except for the northeastern portion,
experienced above-average temperatures in July (Fig. E1),
in association with an anomalous cyclonic circulation, surface and aloft, southwest of the
continent and anomalous northwesterly low-level winds over southern portions of the
continent (Figs. E14, E15, T20 bottom). Generally drier-than-average conditions persisted
over eastern portions of the continent (Fig. E3).
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