The 500-hPa height pattern during September reflected amplified
meridional flow over much of the hemisphere with persistent positive height
anomalies covering large portions of the middle and high latitudes (Figs. E9,
E11). The positive height anomalies were
particularly prominent over the Gulf of Alaska, eastern North America, the high
latitudes of the North Atlantic, central
, while small areas of negative height anomalies occupied the Bering Strait,
, and southeastern
. The main surface temperature departures during September reflected warmer than
average conditions across the eastern half of the
(Fig. E1). The main precipitation anomalies included above average
totals in eastern
and eastern Europe, and below average totals from the
region of the
, and in portions of west-central
(Figs. E3, E6).
In the subtropics, an extensive
area of cyclonic streamfunction anomalies at 200-hPa spanned the
in both hemispheres, with the largest anomalies observed in the Southern
(winter) Hemisphere (Fig. T22).
This pattern is consistent with the La Niña-related suppression of convection
across the eastern half of the equatorial
(Fig. T25). Over the Atlantic
Ocean and Africa, anticyclonic streamfunction anomalies during September
reflected a continuation of conditions that have been in place since the current
active Atlantic hurricane era began in 1995 (Bell and Chelliah, 2006, J.
Climate. 19, 590-612), This anomaly pattern is partly related to the
combination of an enhanced West African monsoon system and suppressed convection
over the Amazon Basin. This combination was again evident during September 2007
The 500-hPa circulation pattern during September featured persistent
above-average heights over the eastern
and the Gulf of Alaska, with below-average heights centered over western North
America and just west of
(Fig. E9). The associated anomalous meridional
flow contributed to exceptionally warm and dry conditions over much of the
and to above-average temperatures in western
(Fig. E1). This flow pattern also contributed
to well below-average precipitation from eastern
(Figs. E5, E6).
Long-term precipitation deficits have resulted in drought conditions throughout
and the entire southeastern quadrant of the country. In the Southeast, extreme
to exceptional drought is now present in many states, including Mississippi,
western Georgia, Tennessee, Kentucky, western South Carolina, North Carolina,
and southern Virginia. Severe drought occupies the surrounding areas to the
north from southern
The 500-hPa circulation pattern during September featured an anomalous wave
pattern extending from the central North Atlantic Ocean to central
(Fig. E9). This pattern included strong ridges
over the central North Atlantic and central
, and a deep trough over eastern Europe. This pattern resulted in a continuation
from August of near-normal temperatures across
, following several months of anomalous warmth. The persistent ridge in western
Russia contributed to exceptionally warm temperatures throughout the region,
with the most significant departures (2°C-3°C) occurring near the Black Sea
and Caspian Sea. In western Mongolia, a strong upper-level ridge also
contributed to anomalously warm conditions, with temperatures ranging from 3°C-4°C
above average throughout the region.
The 500-hPa height anomalies during September exhibited a
pronounced zonal wave-1 pattern similar to that observed in August. This pattern
featured above-average heights from
to the western South Atlantic Ocean and below-average heights across the
southeastern South Atlantic and southern
(Fig. E15). Below-average heights were also
again observed over the high latitudes of the central South Pacific Ocean, and
. In the subtropics, positive (cyclonic) streamfunction anomalies at 200-hPa
to the eastern
South Pacific Ocean
(Fig. T22). These anomalies were
situated along the equatorward flank of the wintertime South Pacific jet stream,
and reflected a pronounced weakening and westward retraction of the jet core
consistent with La Niña (Fig. T21).
In South America, warmer-than-average
conditions prevailed from central
(Fig. E1). Significantly below-average
precipitation occurred in southeastern
, while above-average totals were recorded in southern
. For extratropical
, the anomalously mild and wet conditions were associated with a strong
upper-level trough-ridge couplet, with the mean upper-level trough situated just
west of the continent.
, rainfall totals were in the lowest 10th percentile of occurrences
during September, in association with broad descending motion upstream of the
mean trough axis (Figs. E3, E15).
This region also recorded substantial precipitation deficits during August, with
totals in both months in the lowest 10th percentile of occurrences.
temperatures were 2°C-3°C above average during September, with many areas
recording departures in the upper 90th percentile of occurrences.
This anomalous warmth was linked to an amplified upper-level ridge across the
southern part of the continent (Fig. T22).
September the size of the Antarctic ozone hole was near the 1997-2006 mean,
exceeding 23 million km2 early in the month before decreasing to 20
million km2 by the end of the month (Fig.
S8a). The 2007 ozone hole developed rapidly in mid-August, quickly
increasing to more than 22 million km2 by the end of the month. This
evolution is consistent with the large spatial extent of the polar stratospheric
clouds (PSCs), which have been near the 1997-2006 mean since late May, and
reached peak their peak spatial coverage in early August (Fig.