Extratropical Highlights – July 2013
1. North Pacific/ Northern Hemisphere
The 500-hPa circulation during July
featured above-average heights across the high latitudes of the North Pacific,
eastern Canada, and western Europe, and below-average
heights across central Russia (Figs. E9, E11). At 200-hPa,
cyclonic streamfunction anomalies were present in both hemispheres over the
western and central subtropical Pacific Ocean (Fig. T22). This pattern was
consistent with the combination of enhanced convection over Indonesia and
suppressed convection over the central equatorial Pacific (Fig. T25).
The main land-surface temperature signals
during July included above-average temperatures in the western U.S., Alaska,
Europe and central/ eastern China, and below-average temperatures in the
southeastern U.S. (Fig. E1). The main precipitation signals included above-average
totals across the southern U.S., and drier-than average conditions in the
northwestern U.S., eastern Alaska, and eastern Europe (Fig. E3). In
the U.S., long-term precipitation deficits led to a continuation of extreme and
exceptional drought in large areas of the southwestern U.S. and the southern Great
Plains, and to a continuation of severe drought in much of the remaining area
of the West.
a. North America
The mean 500-hPa circulation during
July featured a strong ridge centered over the Rocky Mountains and a deep
trough over the midwestern U.S. (Fig. E9). This pattern reflected a
westward shift of the mean summertime ridge axis and an amplification of the
Hudson Bay trough. The circulation also featured above-average heights across
the high latitudes of the North Pacific and extending into the northwestern
U.S. This pattern was associated with a weakening of the mean trough over the Gulf
of Alaska, and with an overall northward shift of the mean westerly winds and
Consistent with these conditions,
the northwestern U.S. experienced exceptionally warm and dry conditions during
July, with surface temperatures well above average the 90th
percentile of occurrences (Fig. E1) and precipitation totals in the lowest 10th
percentile of occurrences (Figs. E3, E5). In contrast, the
southern tier of the U.S. recorded well above-average totals in association
with the anomalous upper-level trough, with amounts exceeding the 90th
percentile of occurrences in portions of the southwest and southeast.
Area-average totals exceeded the 90th percentile of occurrences in
the Ohio Valley, the Southeast, and the Gulf Coast region (Fig. E5).
Much of the western half of the
U.S. continued to be impacted by severe- extreme- or exceptional drought. At
the end of July, the “U.S. Drought Monitor” indicated exceptional drought from central
New Mexico to northwestern Kansas, and extreme drought from northern Texas
northward to western Nebraska and southern Wyoming. Severe drought was recorded
across much of the remaining region between southern Oregon and Nebraska and
extending southward to Mexico.
b. China and Japan
The Asian monsoon ridge remained stronger
than average during July, as indicated by positive streamfunction anomalies at
200-hPa extending from the Middle East to northeastern China (Fig. T22).
This pattern was associated with a northward shift of the mean belt of westerly
winds across central Asia (Fig. T21). These conditions contributed to a
continuation of above-average surface temperatures in central and eastern China,
with some areas recording departures in the upper 90th percentile of
occurrences (Fig. E1).
2. Southern Hemisphere
The mean 500-hPa circulation during
July featured above-average heights in the middle latitudes and below-average
heights over the high latitudes of the South Pacific (Fig. E15). This pattern may have had
links to the Tropics via strong
cyclonic streamfunction anomalies across the subtropical South Pacific (Fig. T22),
which occurred in association with enhanced convection over Indonesia and
suppressed convection near the date line (Fig.
The main surface temperature and
precipitation anomalies reflected exceptionally warm and dry conditions in
eastern Australia (Fig. E1). These conditions were associated with positive 500-hPa
height anomalies and an enhanced low-level ridge (Fig. T21), which extended
from eastern Australia to South America.