1. Northern Hemisphere
500-hPa height field during July featured above-average heights over the eastern
United States, the Gulf of Alaska, northern Europe/ north-western Russia, and
eastern Siberia, and below average heights over north-central Russia and the
polar region (Fig. E9). Over the North Atlantic, the
circulation again projected onto the negative phase of the North Atlantic
Oscillation (NAO) and the positive phase of the East Atlantic teleconnection
pattern (Table E1,
Fig. E7). Both of these patterns have persisted
since July 2009 (Fig. E7).
the subtropical Atlantic basin, the 200-hPa streamfunction pattern showed a
pronounced inter-hemispheric symmetry during July, with anticyclonic anomalies
extending from the America’s to Africa in both hemispheres (Fig.
T22). Conversely, an anomalous trough
was evident across most of the tropical western North Pacific. A similar anomaly
pattern was evident in the Southern Hemisphere. These signals are consistent
with the combined influences of La Niña and an enhanced west African monsoon
system (Figs. T23, T24).
main surface temperature signals during July included above average temperatures
across the eastern half of the U.S., Europe, northwestern Russia, and most of
China (Fig. E1).
The main precipitation signals included above-average totals in the
south-central and Midwestern U.S., and in northern China, and below average
totals in the mid-Atlantic, Northeast, and Pacific Northwest regions of the
U.S., and northwestern Russia (Fig. E3).
a. North America
mean 500-hPa circulation during July featured a broad ridge across the United
States (Fig. E9). This pattern was associated with
above average temperatures across the eastern half of the country, with
departures in many locations exceeding the 70th percentile of
occurrences (Fig. E1).
The most significant surface temperature departures were observed in the
mid-Atlantic and northeastern states, where values exceeded the 90th
percentile of occurrences. These regions were also drier than average during
July (Fig. E3), with the mid-Atlantic recording
its fourth consecutive month of below average precipitation (Fig.
E5). Elsewhere, precipitation was above average
from the central Gulf Coast to the Great Lakes.
500-hPa circulation during July featured above average heights across the
central North Atlantic, and below average heights over the northeastern North
Atlantic (Fig. E9). This
pattern projected onto the negative phase of the North Atlantic Oscillation (NAO)
and the positive phase of the East Atlantic teleconnection pattern (Table
E1, Fig. E7), both of
which have persisted since July 2009 (Fig.
the subtropical Atlantic, anticyclonic streamfunction anomalies at 200-hPa
extended from the America’s to Africa in both hemispheres (Fig.
T22). This pronounced
inter-hemispheric symmetry to the anomaly pattern was associated with an
expanded area of upper-level easterly wind anomalies that extended across
tropical northern Africa and the tropical North Atlantic (Fig. T21). It was also
associated with an extensive area of low-level westerly wind anomalies across
the tropical Atlantic (Fig. T20).
These conditions lead to reduced vertical wind shear across the tropical
Atlantic, and are typical of the high activity era for Atlantic hurricanes that
began in 1995. This combination of conditions is consistent with an enhanced
west African monsoon circulation, which was also present during July (Figs. T23, T24)
and has been a prominent feature of the circulation since 1995.
500-hPa circulation during July featured a strong and persistent ridge centered
over northwestern Russia and Scandinavia (Fig.
E9). This feature led to exceptionally warm and dry conditions. Much of
the region experienced record warm surface temperature departures exceeding +5°C
and monthly precipitation totals in the lowest 10th percentile of
occurrences (Fig. E3).
An anomalous upper-level ridge across northern China also contributed to above
average surface temperatures, with monthly departures in the upper 90th
percentile of occurrences over large portions of the country.
2. Southern Hemisphere
500-hPa circulation during July featured a zonally-symmetric pattern of height
anomalies, with above average heights in the middle latitudes and below average
heights over Antarctica (Fig. E15).
In the subtropics, the upper-level streamfunction pattern reflected an anomalous
trough across the western and central tropical South Pacific, and an anomalous
ridge extending from the eastern subtropical South Pacific to southern Africa (Fig.
T22). A similar anomaly pattern was
evident in the Northern Hemisphere. These conditions likely reflect the
influences of La Niña and an enhanced west African monsoon system.
main temperature signals during July reflected well above average temperatures
in eastern Australia, where monthly departures were generally in the upper 90th
percentile of occurrences (Fig. E1).