1. Northern Hemisphere
During October the 500-hPa
circulation pattern featured positive height anomalies over eastern
Asia
, the eastern North Pacific,
Greenland
, and the polar region, and negative height anomalies from the north-central
U.S.
to
Spain
, and over central
Siberia
(Figs. E9, E11). For the third straight month, the anomaly pattern
from eastern
North America
to
Europe
has reflected a strong negative phase of the North Atlantic Oscillation (NAO)
(Table E1, Fig. E7). During October the NAO index was -2.2, and for
the August-October season the NAO index was -1.9.
In the subtropics,
anticyclonic streamfunction anomalies were evident at 200-hPa in both
hemispheres over central and east-central Pacific (Fig. T22), which
is consistent with strengthening El Niño conditions during the month.
Conversely streamfunction anomalies over south-central
Asia
were consistent with suppressed convection across
Indonesia
and the eastern half of the equatorial
Indian Ocean
(Fig. T25).
The main surface temperature
departures during October reflected warmer-than-average conditions across
northeastern
Canada
,
Greenland
,
Europe
, and
China
, and cooler-than-average conditions over the central and eastern
United States
(Fig. E1). The main precipitation anomalies included above-average
totals in the Inter-Mountain, Southwest,
Ohio
Valley
,
Great Lakes
and Northeastern regions of the
U.S.
(Figs. E3, E5, E6), and below-average totals in the Pacific Northwest
U.S. and eastern
China
(Figs. E3, E4).
a. Pacific/
North America
In the subtropics, the mean
200-hPa circulation pattern during October was consistent with an increasing
strength of El Niño, as indicated by anticyclonic streamfunction anomalies
flanking the regions of enhanced convection over the central equatorial
Pacific (Fig. T22). In the extratropics the 500-hPa circulation
pattern featured an anomalous ridge over the eastern North Pacific and an
enhanced trough over eastern
North America
(Fig. T9). This pattern contributed to above-average temperatures
over
Alaska
and to below-average temperatures across central
Canada
and much of the
United States
(Fig. E1). The most significant departures occurred in south-central
Canada
, where monthly temperatures were more than 3°C below average and in the
lowest 10th percentile of occurrences. Farther east,
above-average heights covered northeastern
Canada
and the polar region, which contributed to above average surface temperature
throughout the region.
The anomalous precipitation
patterns were also generally consistent with the upper-air circulation, with
above-average totals in the central and eastern
U.S.
coinciding with regions of increased storminess in the vicinity of the mean
trough axis. The most significant surpluses occurred in the Inter-Mountain,
Southwest,
Ohio
Valley
,
Great Lakes
, and Northeast regions, where totals generally exceeded the 80th
percentile of occurrences (Fig. E6). The Inter-Mountain region has
recorded well above-average totals in each of the last four months, and the
Southwest region has recorded above-average totals in each of the last three
months. In contrast, the
Pacific Northwest
region of the
U.S.
recorded well below-average precipitation during October. This area was
situated immediately downstream of the mean upper-level ridge axis, and
recorded its fourth straight month of below-average precipitation.
b.
North Atlantic
/
Europe
The 500-hPa circulation
pattern during October featured an anomalous ridge over
Greenland
and an anomalous trough across the central
North Atlantic
(Fig. E9). These anomalies reflected a continuation of the strong
negative phase of the NAO (-2.2) that began in August. During October
Europe
was situated immediately downstream of the mean upper-level trough axis, and
again recorded significantly above-average temperatures (Fig. E1).
Monthly temperatures across the continent generally exceeded the 90th
percentile of occurrences, with the largest departures of 2°-3°C observed
in central and eastern Europe.
c.
China
An anomalous upper-level
trough was centered over northern
India
and western
China
during October. This feature reflected a weakening of the mean subtropical
ridge, consistent with suppressed convection across
Indonesia
and the eastern half of the equatorial
Indian Ocean
(Fig. T25). The anomalous trough was associated with anomalous
southwesterly flow and exceptionally warm surface temperature across
China
. Temperatures across central and northern
China
generally averaged 2°-3C during the month, and were in the upper 90th
percentile of occurrences.
2. Southern Hemisphere
The mean 500-hPa circulation pattern during October
featured generally above-average heights in the middle latitudes, and
below-average heights over the high latitudes of the South Pacific (Fig.
E15). In the subtropics, cyclonic streamfunction anomalies were observed
across the central and eastern South Pacific, along with an eastward
extension of the South Pacific jet stream. These conditions, along with the
negative height anomalies farther poleward, are a consistent response to El
Niño conditions.
In
Australia
, an enhanced mid-and upper-level ridge contributed to a continuation of
anomalously warm and dry conditions. The departures were especially
significant in the southeast, where monthly temperatures were in the upper
90th percentile of occurrences and precipitation totals were in
the 10th percentile of occurrences for the third consecutive
month.
In central and southern South America anomalous
southwesterly flow at upper levels, and a poleward shit of the mean frontal
boundary at lower levels, were evident downstream of the mean upper-level
trough. These conditions contributed to well above-average temperatures and
precipitation during the month.
The Antarctic ozone hole
re-developed in late August and remained extensive through the end of
October (Fig. S8a). The ozone hole was near record size from late
September through October, exceeding 25 million km2 in September
and early October, and 20 million km2 in late October. This
extensive ozone hole is consistent with anomalously low stratospheric
temperatures (Figs. S3, S4), and with an exceptionally large SH
circumpolar vortex (Fig. S8b). Temperatures at both 2 hPa and 10 hPa
were among the lowest on record from August through mid-October (Fig. S4),
which led to a record large spatial extent of polar stratospheric clouds (PSC,
Fig. S8c). PSC’s are the main mechanism by which the ozone hole is
created.
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