The 500-hPa circulation
pattern during October featured above-average heights across the high
latitudes of the North Pacific and the North Atlantic, and below-average
heights over the western
United States
, the central North Atlantic, and eastern
Siberia
(Figs. E9, E11). This pattern was associated with strong negative
phases of both the North Atlantic Oscillation (NAO, -1.2) and the East
Pacific teleconnection pattern (-1.6) (Table E1,
Fig. E7).
The main temperature
departures during October included warmer than normal conditions over
Alaska
, the southeastern
U.S.
, and
Europe
(Fig. E1). The main precipitation departures included above-average
totals over the western and central
United States
and western Europe, and near record-low totals over eastern
China
(Figs. E3, E4).
a. Pacific/North America
The mean 500-hPa circulation pattern during October featured
above-average heights over the eastern North Pacific and eastern
Canada
, and below-average heights over the western
United States
. These conditions were associated with above-average temperatures over
Alaska
, the southeastern
U.S.
, and eastern
Canada
(Fig. E1). They were also associated with increased storminess and
above-average precipitation across the western and central
U.S.
(Fig. E3). Precipitation totals in the Pacific Northwest, Southern
California, Inter-Mountain, and
Great Plains
regions were significantly above-average during the month, with
area-average totals generally above the 80th percentile of
occurrences.
b. North Atlantic/Europe
During October above-average heights at 500-hPa covered the high
latitudes of the North Atlantic and below-average heights spanned the
central
North Atlantic
. This anomaly pattern reflected a strong negative phase of the NAO (Table
E1), and was associated with an anomalous southward shift of the mean
North Atlantic jet stream toward southern
Europe
(Fig. T21). This circulation was associated with a continuation of
well above-average surface temperatures across the high latitudes of the
North Atlantic and
Europe
, and with increased storminess and above-average precipitation in western
Europe.
c. Eastern China
Eastern China
experienced significantly below-average precipitation during October, with
area-average totals generally in the lowest 10th percentile of
occurrences (Fig. E3). This dryness represents a continuation of
below-average totals that have plagued the country for the past year (Fig.
E4). During October the below-average precipitation was related to
enhanced upper-level convergence (Fig. T23) occurring within an
anomalous left jet exit region upstream of the mean trough axis (Figs. T21,
T22).
2. Southern Hemisphere
The 500-hPa circulation pattern during October
featured above-average heights over the central South Atlantic, southern
Australia
, and the polar region, and below-average heights over the high latitudes
of the South Pacific (Fig. E15). At 200-hPa anticyclonic
streamfunction anomalies prevailed across the South Pacific from southern
Australia
to
South America
(Fig. T22).
In Australia
these conditions contributed to above-average surface temperatures
throughout the continent (Fig. E1). They also produced significantly
below-average precipitation in the southeast, with departures in some areas
dropping below the 10th percentile of occurrences (Fig. E3).
In southern
Africa
the rainy season normally lasts from October to April. The beginning of the
2004-2005 rainy season began with slightly below-average precipitation in
eastern
South Africa
, and slightly above-average totals in northern
Mozambique
and southern
Tanzania
.
The September-October 2004 Antarctic ozone hole was below the
1994-2003 mean, in response to a reduced polar stratospheric cloud (PSC)
cover compared to recent years (Fig. S8). These conditions were
related to above-average heights and temperatures over
Antarctica
in association with a below-average strength of the circumpolar vortex (Fig.
E15).