1. Northern Hemisphere
The 500-hPa height pattern
during March featured positive anomalies over the high latitudes of the
central
North Pacific Ocean
, the
United States
, the eastern
North Atlantic Ocean
, and
Scandinavia
, and negative anomalies over
Alaska
and the western
Mediterranean Sea
(Figs. E9, E11).
The 200-hPa streamfunction anomaly pattern showed a strong inter-hemispheric
symmetry of anomalies across the subtropical
Pacific Ocean
, with anomalous ridges west of the date line, and anomalous troughs east of
the date line (Fig. T22). This pattern is
consistent with enhanced equatorial convection west of the date line and
suppressed convection east of the date line (Fig. T25).
The main surface temperature departures during March reflected
warmer-than-average conditions across the
United States
,
Europe
, and portion of
China
(Fig. E1). For central and eastern
China
, this marks the third straight month with temperatures in the upper 10th
percentile of occurrences (Fig. E1). The
main precipitation anomalies during March included above-average totals in
the Plains states and southwestern
Canada
, and below-average totals in the southeastern
U.S.
and
Scandinavia
(Fig. E3).
a. Pacific/
North America
In the subtropics, the
upper-level circulation during March strongly reflected the pattern of
anomalous convection across the equatorial Pacific, with enhanced
subtropical ridges flanking the region of enhanced convection and enhanced
mid-Pacific troughs flanking the region of suppressed convection (Figs. T22,
T25). These conditions are known to be
associated with an enhanced split flow in the exit region of the East Asian
jet stream (Fig. T21) and with
high-latitude blocking over the central North Pacific, as was observed
during March.
The 500-hPa circulation
pattern featured persistent positive height anomalies across
North America
and persistent negative height anomalies over
Alaska
in the region immediately downstream of the persistent blocking ridge (Figs.
E9, E11). This
pattern was associated with well above-average temperatures across the
United States
, with departures of 3°-4°C observed in many regions (Fig. E1).
In contrast, much of
Alaska
experienced well below-average temperatures in the region downstream of the
blocking ridge, where departures in many regions were in the lowest 10th
percentile of occurrences.
The circulation pattern over
the eastern
U.S.
was also associated with a marked eastward shift of the mean
Hudson Bay
low to the western
North Atlantic
. The resulting anomalous zonal flow and reduced storm activity contributed
to exceptionally dry conditions in portions of the
Ohio
Valley
,
Midwest
,
Gulf
Coast
, and mid-Atlantic regions, with totals in the latter three regions falling
into the lowest 10th percentile of occurrences (Figs. E3,
E5).
b.
North Atlantic
/
Europe
The 500-hPa circulation
pattern during March featured persistent positive height anomalies over the
eastern
North Atlantic
and
Scandinavia
(Fig. E9). These conditions were associated
with an anomalous flow of mild, marine air into the continent, which
contributed to well above-average temperatures during the month. The most
anomalous warmth was centered across northern
Europe
and
Scandinavia
, where temperatures at many locations were in the upper 90th
percentile of occurrences. These regions also experienced well below-average
precipitation, which coincided with the mean position of the upper-level
ridge.
2. Southern Hemisphere
The 500-hPa circulation
pattern during March featured positive height anomalies in the polar region
and generally negative height anomalies across the higher mid-latitudes (Fig.
E15). In the subtropics, the upper-level
circulation featured an anomalous ridge over
Australia
and the western South Pacific, and an enhanced mid-Pacific trough east of
the date line (Fig. T22). This anomaly
pattern is consistent with the anomalous equatorial convection across the
Pacific basin (Fig. T25).
In
Australia
, the persistent upper-level ridge contributed to extremely warm
temperatures in the east. In southern
Africa
, the rainy season extends from October to April. For the region as a whole,
precipitation was slightly below average during March, with the most
significant dryness occurring in northern
Mozambique
(Figs. E3, E4). So
far during the 2006-07 rainy season, area-averaged totals were below-normal
in October, February, and March, near-normal in November and December, and
above normal in January.
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