1. Northern Hemisphere
500-hPa circulation during September featured above-average heights over
Greenland, northern Russia, eastern Asia, and Alaska, and below average heights
in the Gulf of Alaska, south-central Europe, and central Siberia (Fig.
E9). Over the subtropical Atlantic basin, the
200-hPa streamfunction pattern continued to show a pronounced inter-hemispheric
symmetry, 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, with a similar anomaly pattern evident in the Southern
Hemisphere. These signals are consistent with the combined influences of La Niña,
an enhanced west African monsoon system (Figs.
and ongoing exceptionally warm SSTs across the tropical and high latitudes of
the North Atlantic (Fig. T18).
This combination of factors contributed to exceptionally strong hurricane
activity in the Atlantic Basin, and to well below average in eastern North
Pacific hurricane region.
main surface temperature signals during September included warmer than average
conditions across the southern half of the United States, Greenland, and western
Russia (Fig. E1). Monthly precipitation totals (Fig. E3) were above-average from Mexico
northward to the U.S. Great Lakes, across southern Canada, in eastern Europe,
and in eastern China (Figs. E5,
E6). Monthly precipitation was below average along
the U.S. Gulf Coast and East Coast.
a. North America
mean 500-hPa circulation during September featured troughs in the western and
eastern U.S., and a ridge across the southwestern U.S. (Fig.
E9). This pattern was associated with above
average temperatures across the southern half of the country, with September
departures in many locations exceeding the 70th percentile of
occurrences (Fig. E1).
Precipitation was above average in the Great Plains and Midwest regions of the
country, and throughout eastern Texas (Figs.
E3, E5), with departures
in many regions exceeding 175% of normal (Fig.
E6). Conversely, monthly precipitation totals were
below average in the southwest, and along an axis extending from the central
Gulf Coast northeastward to New England. The most significant deficits were
observed in portions of Mississippi, Tennessee, and Indiana, where monthly
totals were less than 25% of normal.
the subtropical North Atlantic, anticyclonic streamfunction anomalies at 200-hPa
extended from the America’s to Africa in both hemispheres (Fig.
T22). This pronounced
inter-hemispheric symmetry of the anomaly pattern was associated with
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 that
extended from the eastern North Pacific across the tropical Atlantic (Fig.
combination of conditions is consistent with an enhanced west African monsoon
circulation, which was again present during September (Figs.
and has been a prominent feature of the circulation since 1995. These conditions
contributed to reduced vertical wind shear and to exceptionally conducive wind
patterns that extended westward from Africa, resulting in considerable Atlantic
hurricane activity throughout the month.
above conditions are typical of the high activity era for Atlantic hurricanes
that began in 1995. They are also consistent with a continuation of
exceptionally warm SSTs in the Atlantic basin, which have also prevailed since
1995 in association with the warm phase of the Atlantic Multi-decadal
Oscillation (AMO). Since March 2010, record warm SSTs have persisted across the
tropical North Atlantic.
the eastern tropical North Pacific, atmospheric anomalies associated with La Niña
contributed to increased vertical wind shear, and to a confinement of the
eastern Pacific ITCZ to the southwest coast of Mexico. These conditions
contributed to well below average hurricane activity in the eastern Pacific,
with only one named storm (minimal Tropical Storm Georgette) forming during the
2. Southern Hemisphere
The 500-hPa circulation during September reflected a zonal wave-2 pattern of
height anomalies. Main aspects of this pattern included above average heights
southeast of Australia and over the South Atlantic Ocean, and below average
heights south of New Zealand and over the eastern Indian Ocean (Fig.
E15). In the subtropics, the upper-level
(200-hPa) streamfunction pattern reflected an anomalous trough across the
western and central South Pacific, and an anomalous ridge extending from the
eastern South Pacific to southern Africa (Fig.
T22). A similar anomaly pattern was
evident in the Northern Hemisphere. These conditions have been present since
June, and likely reflect the combined influences of La Niña and an enhanced
west African monsoon system. One characteristic feature of La Niña was a marked
westward retraction of the South Pacific jet core, as indicated by easterly wind
anomalies centered along the jet axis east of the date line near 30°S (Fig.
The Antarctic ozone hole typically develops during August, and reaches peak
aerial extent during September. During 2010, the ozone hole did not form until
late August, making this the latest formation date since 2000 (Fig.
S8). This delay of onset reflected a significant
decrease in polar stratospheric clouds that occurred during July and August in
association with a sharp increase in polar stratospheric temperatures at both
10-hPa and 2-hPa (Fig. S4).
During September 2010, the ozone hole covered 14 million square kilometers early
in the month, and expanded to 20 million square kilometers late in the month.
This latter size was approximately equal to the 2000-2009 mean (Fig.