Extratropical Highlights
MARCH 2009
Forecast Forum
1. Northern Hemisphere
The 500-hPa height field during
March 2009 featured a wave-1 pattern with generally positive anomalies in the
Western Hemisphere
and negative anomalies in the
Eastern Hemisphere
(Fig. E9).
Notable departures from this pattern included persistent negative anomalies over
western
Canada
and positive anomalies over south-central
Russia
. Across the
Pacific Ocean
and
North America
, the circulation reflected a pronounced westward retraction of the East Asian
jet stream (Fig. T21),
along with a westward shift of the downstream ridges and troughs. These
conditions are consistent with La Nina.
The main temperature signals
during March generally reflected the anomalous upper-level circulation, with
above average temperatures across the southern half of the
United States
and central
Asia
, and below average temperatures in western
Canada
(Fig. E1).
The main precipitation signals included above-average totals in southwestern
Canada
, portions of the U.S. Gulf Coast, and eastern Europe, and below-average totals
in northwestern
Europe
and
Scandinavia
(Fig. E3).
a. North Pacific/
North America
At 200-hPa, La Niña again
contributed to a pronounced inter-hemispheric symmetry of the circulation
anomalies across the
Pacific Ocean
. In the subtropics, aspects of this circulation included enhanced mid-Pacific
troughs in both hemispheres flanking the suppressed equatorial convection, and a
westward retraction of the subtropical ridges to
Australasia
(Fig. T22). Associated with this
pattern, the East Asian jet stream was retracted westward toward the western
Pacific (Fig. T21).
These conditions contributed to a westward shift to the central North Pacific of
the mean ridge normally located over western
North America
, and a westward shift of the mean
Hudson Bay
trough to western
North America
(Fig. E9). The associated storm track was
also highly anomalous, with increased storminess extending across the
Gulf of Alaska
into western
North America
, and significantly decreased storminess across the northeastern
United States
(Fig. E13).
These conditions were associated
with a dipole pattern of temperature anomalies, with below average temperatures
in western
Canada
and above average temperatures across the southern half of the
United States
(Fig. E1).
They were also associated with increased precipitation in the northwestern and
Gulf
Coast
regions of the
U.S.
(Figs. E3, E6).
c.
Eurasia
The circulation during March
featured a split-flow pattern over the eastern
North Atlantic
, a strong trough over eastern Europe/
Black Sea
, and an anomalous ridge east of the
Caspian Sea
(Fig. E9).
This pattern was associated with increased storminess and above average
precipitation in eastern Europe, and with well above average temperatures across
central
Asia
.
2. Southern Hemisphere
The circulation during March continued
to exhibit a strong connection to La Niña, with an enhanced ridge over the
central South Pacific and an anomalous trough over the high latitudes of the
eastern South Pacific (Figs. E15,
T22).
In southern
Africa
, the rainy season extends from October through April. Rainfall during March was
above-average for the region as a whole (Fig.
E4), with surpluses mainly in the central and
eastern portions of southern
Africa
(Fig. E3).
Overall, the 2008-09 rainy season has been above average, which is consistent
with La Niña.
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