1. Northern Hemisphere
The 500-hPa height
field during October featured above-average heights in the polar region and
below average heights in the middle latitudes (Fig.
E9). This pattern reflected one of the strongest
negative phases of the Arctic Oscillation on record dating back to 1950.
Regional aspects of the circulation included ridges in the Gulf of Alaska and
the eastern North Atlantic, and amplified troughs across the central North
Pacific, western North America, and the central
. These anomalies projected onto several regional teleconnection patterns (Table E1, Fig. E7),
including exceptionally strong negative phases of the West Pacific (-2.4) and
Polar/ Eurasia (-2.6) patterns, a modest negative phase of the NAO (-1.0), and a
positive phase of the
temperature signals during October included above average temperatures in
, and below average temperatures in central
and the central
The main precipitation signals during October included above-average totals
across much of North America and eastern Europe, and below-average totals in
a. North Pacific/
circulation during October featured a ridge over the Gulf of Alaska and a strong
trough in western
This pattern was associated with exceptionally cool temperatures across central
and the central
, with departures in many regions (-2o to -4oC) falling
into the lowest 10th percentile of occurrences (Fig. E1). In contrast, much of
recorded above average temperatures during the month, with departures in many
regions exceeding +3oC.
recorded well above average precipitation during October (Fig.
E3). Precipitation in the central and northwestern
generally exceeded 175% of normal (Fig. E6),
with departures in the upper 90th percentile of occurrences. In the
, the Great Plains,
Valley, Mid-Atlantic, and Northeast have recorded above average precipitation
for much of the last five months (Fig. E5).
The strength of the
mean upper-level ridge was below average west of the Tibetan Plateau, which
allowed for the mean ridge axis to be centered over western
(Fig. T22). This pattern
contributed a continuation of drier than average conditions across eastern
, where totals were generally in the lowest 30th percentile of
occurrences. This marks the second straight month with significant precipitation
2. Southern Hemisphere
The 500-hPa circulation during October
reflected an anomalous zonal wave-3 pattern, with above average heights over the
central South Pacific and in the areas south of
and, and below average heights located south of South America and near
The main temperature anomalies during
October reflected cooler than average temperatures in extreme southern
For the entire South African monsoon region the start to the 2009-10 monsoon
season, which runs from October through April, featured near-average rainfall
totals (Fig. E4).
Below average precipitation was observed throughout eastern
during the month, with northeastern
recording deficits for the fourth straight month.
The Antarctic ozone hole typically reaches
its peak aerial extent during September and October. During 2009 the size of the
ozone hole was approximately equal to the 1999-2008 mean. The 2009 ozone hole
reached a peak extent of 24 million square kilometers in late September, and
still covered 13.5 million square kilometers in late October (Fig. S8).