A persistent pattern of above-normal
heights at 500-hPa covered large portions of the middle latitudes during July, including
the United States, the North Atlantic, western Russia, and eastern Asia (Figs. E10, E12). Prominent temperature
anomalies during the month (Fig. E1) included warmer than
normal temperatures over most of the United States, Europe, and eastern Asia (Fig. E1). Prominent precipitation anomalies (Fig. E3) included below-average rainfall across the
northeastern quadrant of the United States (Fig. E5),
Mexico, and northwestern Russia, and above-average rainfall across southern Europe and
southeastern China (Fig. E4).
a. North America
Above-normal 500-hPa heights covered most of the United States and southern
Canada during July for a second month in association with a strengthening of the mean
summertime ridge. During July this circulation was associated with exceptionally warm
surface temperatures across the region, with departures exceeding the 90th
percentile in the southwestern and midwestern U.S. (Fig. E1).
Below-average rainfall covered the northeastern portion of the U.S. during July, with
totals in the lowest 10th percentile of occurrences observed in the Great Lakes
region (Figs. E3, E5).
Below-average rainfall was again observed in the Inter-Mountain, Southwest, Great Plains,
Midwest, and Ohio Valley regions during July (Figs. E3, E5). Precipitation has been below normal in the Inter-Mountain
and Southwest regions since June 2001, in the Great Plains region since October 2001, and
in the Ohio Valley since August 2001 (Fig. E5).
b. Europe and Asia
Most of eastern Europe, western Russia, and eastern Asia also experienced well
above-average surface temperatures during July (Fig. E1) in
association with anomalously strong ridges at upper levels (Fig.
E10). Monthly mean temperatures averaged 2-4°C above
average in these areas and exceeded the 90th percentile.
In southeastern China above-average rainfall continued for a third consecutive month (Fig. E4), in association with a persistent upper-level trough
and well-defined jet exit region over the country throughout period (Figs. T21, T22).
2. Southern Hemisphere
The upper level circulation during July
featured an amplified subtropical ridge across Australia (Fig. T22),
which again contributed to warmer and drier than normal conditions in both the southwest
and southeast (Figs. E1, E3). The
circulation also featured cyclonic streamfunction anomalies at 200-hPa extending from
west-central South America eastward to southern Africa. This circulation contributed to
beneficial rainfall in southeastern Africa (Fig. E4) during
the heart of that regions dry season.