1. Northern Hemisphere
The 500-hPa circulation
pattern during September featured persistent positive height anomalies
across the central North Pacific, eastern
North America
, and northern
Europe
, and negative height anomalies over western
Canada
and southern
Europe
(Figs. E9, E11).
The main surface temperature departures reflected above-average temperatures
throughout eastern
North America
, the high latitudes of the
North Atlantic
, and most of
Eurasia
(Fig. E1). The main precipitation anomalies
during September included well below-average precipitation in the eastern
United States
, and in both southwestern and northeastern
Europe
(Figs. E3, E5, E6).
a.
North America
The prominent 500-hPa
circulation anomalies during September included a persistent pattern of
above-average heights across the central North Pacific and eastern
North America
, and below-average heights over western
Canada
(Figs. E9, E11).
This pattern was associated with anomalous southwesterly flow in the region
between the mean upper-level trough and ridge axes from the southwestern to
the central
United States
. The persistent ridge over eastern
North America
contributed to exceptionally warm and dry conditions in the eastern
United States
. Monthly mean temperatures were generally 2°-3°C above average in that
region, with most areas recording anomalies in the upper 90th
percentile of occurrences (Fig. E1). Also,
monthly rainfall totals were generally in the lowest percentile of
occurrences in the East (Fig. E3), with the
Ohio
Valley
, Mid-Atlantic and Northeast regions all recording area-averaged totals in
the lowest 10th percentile of occurrences (Fig. E5).
b.
North Atlantic
/
Eurasia
The 500-hPa circulation
pattern during September featured a north-south tri-pole pattern of height
anomalies, with above-average heights from northern
Europe
to central
Russia
, and below-average heights at high latitudes and across southern
Europe
(Fig. E11). This pattern, combined with
anomalous upper-level easterlies across
Europe
and western
Russia
, contributed to well above-average temperatures in these regions. It also
contributed to below-average precipitation in both southwestern and
northeastern
Europe
, with both areas located immediately downstream of the mean upper-level
ridge axes.
2. Southern Hemisphere
The mean 500-hPa circulation pattern during September
featured an anomalous zonal wave-3 pattern in the middle and high latitudes,
along with above-average heights across the central South Pacific. The
wave-3 pattern was characterized by above-average heights southeast of
Australia
, over the western
South Atlantic
, and over the central
Indian Ocean
, and below-average heights southwest of
Australia
, over the high latitudes of the eastern South Pacific, and in the area
south of
Africa
(Fig. E15).
The main positive surface temperature anomalies were
observed over southern
Africa
, consistent with the broad pattern of positive height anomalies extending
across the
South Atlantic
into the
Indian Ocean
. The main area of below-average temperatures occurred over central
South America
, immediately downstream of the mean upper-level trough axis (Fig. E1).
The Antarctic ozone hole and
polar stratospheric cloud (PSC) cover during September 2005 were near the
1994-2003 mean (Fig. S8). The main area of
the ozone hole extended from the high latitudes of the central South Pacific
to the central
Indian Ocean
(Fig. S6), and coincided with the area of
below-average heights in the middle stratosphere (Fig. S1).
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