1. Northern Hemisphere
500-hPa circulation during October featured above-average heights over western
North America, Greenland, and across the western and central North Pacific
Ocean, and below average heights over the Gulf of Alaska, eastern Canada and
southern Europe (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, anomalous troughs
prevailed across most of the sub-tropical western and central Pacific in both
signals are consistent with the combined influences of La Niña and an enhanced
west African monsoon system (Figs. T23,
T24), and with ongoing exceptionally
warm SSTs across the tropical and high latitudes of the North Atlantic (Fig.
T18). This combination of factors
contributed to a continuation of exceptionally strong Atlantic hurricane
main surface temperature signals during October included warmer than average
conditions across Canada and the western United States, southern Greenland,
central Russia, and portions of the Middle East (Fig.
E1). Below average temperatures were observed in
eastern Europe. Monthly precipitation totals (Fig.
E3) were above-average in the northeastern U.S.,
California, the eastern Mediterranean Sea region, tropical western Africa, and
southern China. Monthly precipitation was below average over Mexico and across
much of the southeastern and central United States (Fig. E6).
a. North America
mean 500-hPa circulation during October featured an amplified wave pattern
across North America, with a strong ridge over western North America flanked by
troughs over both the Gulf of Alaska and eastern North America (Fig.
E9). This pattern dictated the temperature (Fig.
E1) and precipitation (Fig.
E3) anomaly patterns across the continent. It was
associated with well above average temperatures over much of western North
America, with monthly departures in many regions exceeding the 90th
percentile of occurrences. It was also associated with increased storminess and
above average precipitation in California and in the northeastern U.S., and with
well below average precipitation across the southeastern and central U.S. Some
of the largest precipitation deficits were observed in Texas, Florida, and
Missouri, where monthly totals were generally less than 25% of normal (Fig.
the extratropical North Atlantic, the 500-hPa circulation featured an ongoing
negative phase (-0.9) of the North Atlantic Oscillation (NAO) (Fig.
E7, Table E1). This
phase is characterized by above average heights over Greenland, and below
average heights generally extending from eastern North America to southern
Europe. The negative NAO has prevailed in every month since June 2009, with the
exception of September 2009.
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 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 across the eastern North Pacific and tropical North
Atlantic (Fig. T20).
combination of conditions is consistent with an enhanced west African monsoon
circulation (Figs. T23,
T24) and has been a prominent feature
of the tropics-wide circulation since 1995. During October, the above conditions
contributed to reduced vertical wind shear over the tropical North Atlantic, and
to exceptionally conducive wind patterns that extended westward from Africa. As
a result, five Atlantic hurricanes developed during the month. Similar anomaly
patterns have prevailed throughout the 2010 Atlantic hurricane season, which was
extremely active during June-October with 19 named storms, of which 12 became
hurricanes and 5 became major hurricanes.
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.
2. Southern Hemisphere
500-hPa circulation during October reflected above average heights in the middle
latitudes and below average heights over the high latitudes of the eastern South
Pacific (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. Another characteristic of La Niña seen
during October was a marked westward retraction and weakening South Pacific jet
core, as indicated by easterly wind anomalies centered along the jet axis near
30°S (Fig. T21).
The Antarctic ozone
hole typically develops during August, reaches its peak aerial extent during
September, and then slowly dissipates during October and November. During
October, the size of the ozone hole was near the 2000-2009 mean, covering more
than 15 million square kilometers during most of the month (Fig.
The 2010 ozone hole
did not form until late August, making this the latest formation date since
2000. 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, the ozone hole covered 14
million square kilometers early in the month, and expanded to 20 million square
kilometers late in the month. The ozone hole spanned nearly 12.5 million square
kilometers at the end of October.