1. Northern Hemisphere
The 500-hPa circulation
pattern during April featured positive height anomalies across the central
North Pacific, the high latitudes of the
North Atlantic
, and the polar region, and negative anomalies over the eastern Pacific,
across
Greenland
to
Scandinavia
, and over north- central
Russia
(Fig. E9). In the subtropics, cyclonic
200-hPa streamfunction anomalies over the central North Pacific and
anticyclonic anomalies across the western North Pacific and southeastern
Asia
reflected the ongoing weak Pacific cold episode (La Niņa) (Figs. T22,
T25). These conditions were associated with a
pronounced westward retraction of the East Asian jet core, and a westward
shift of the associated jet exit region to well west of
Hawaii
. Farther downstream a pronounced westward shift of the mean upper-level
ridge and trough axes was also evident.
The main surface temperature
departures during April reflected warmer than normal conditions across
North America
,
Europe
, and central
Asia
, and cooler than normal conditions across
Alaska
and eastern
Siberia
(Fig. E1). The main precipitation departures
included above-average totals in the northwestern
U.S.
and portions of south-central
Canada
, and below-average totals in the
Gulf
Coast
region (Figs. E3, E5, E6).
a. Pacific/
North America
The circulation across the
North Pacific strongly reflected the ongoing Pacific cold episode. In
particular, the westward retraction and confinement of deep tropical
convection to the western Pacific resulted in the combination of an
amplified subtropical ridge over the western Pacific and an amplified trough
over the central and eastern subtropical Pacific (Fig. T22).
Accompanying this pattern, the East Asian jet stream was retracted well west
of the date line, and the associated jet exit region was shifted to well
west of
Hawaii
(Fig. T21). Also in the extratropics a
corresponding westward retraction of the mean downstream ridge and trough
positions at 500-hPa was also evident, with the mean ridge shifted from
western North America to the central North Pacific, and the mean trough
shifted from Hudson Bay to the eastern Pacific (Fig. E9).
This pattern reflected a complete disappearance of the mean
Hudson Bay
trough.
The main surface temperature
and precipitation departures reflected this anomalous circulation, with
above-average temperatures across
North America
(Fig. E1), and above-average precipitation
near
Hawaii
and over much of the northwestern
U.S.
(Fig. E3). In contrast, the
Gulf
Coast
region recorded below-average precipitation during the month. The
Gulf
Coast
has recorded below-average precipitation in every month since September
2005. During both March and April 2006, area-averaged totals in this region
were below the 10th percentile of occurrences.
b.
North Atlantic
/
Eurasia
The 500-hPa circulation
pattern during April featured above-average heights across southern
Europe
extending eastward to the
Caspian Sea
, and below-average heights over central
Russia
(Fig. E9). This pattern was associated with
an enhanced westerly flow into southern
Europe
, and an anomalous northerly flow into eastern
Siberia
. These conditions contributed to above-average temperatures from southern
Europe
to central
Asia
, and to below-average temperatures across eastern
Siberia
and portions of
Mongolia
.
2. Southern Hemisphere
In the subtropics, the mean 200-hPa circulation pattern
during April was consistent with ongoing weak La Niņa conditions, as
indicated by an enhanced ridge over
Australia
and an amplified trough over the central South Pacific (Fig. T22).
This pattern was associated with a large area of positive extratropical
height anomalies at 500-hPa across the central South Pacific (Fig. E15)
similar to that observed in the Northern Hemisphere. The 500-hPa circulation
pattern also featured positive height anomalies over the central
South Atlantic
and negative anomalies south of
Africa
,
Australia
and
South America
.
In southern
Africa
the rainy season normally lasts from October to April. Area-averaged
rainfall was above normal during April, marking six consecutive months of
above-average rains (Fig. E4). An enhanced
South African rainy season is consistent with a La Niņa episode.
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