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1. Northern Hemisphere
The
500-hPa circulation during April featured above-average heights from
Alaska
and eastward to
Scandinavia
, over the subtropical eastern North Pacific, and across northwestern
Africa
, and below-average heights over the west-central north Pacific, near
Great Britain
, and northwestern
Russia
(Fig. E9). The main surface temperature
departures during April included above-average temperatures across
Canada
and the north-central
United States
, across both the high latitudes and subtropical latitudes of the
North Atlantic
, and much of
Europe
(Fig. E1). The main precipitation anomalies
reflected above-average totals in the Inter-Mountain region of the western
U.S.
, the northeastern
U.S.
, and both northern and southern
Europe
(Figs. E3, E5, E6).
a.
North America
The main 500-hPa circulation anomalies affecting
North America
during April included above-average heights across
Alaska
and
Canada
, and in the subtropics over the extreme eastern North Pacific, and
below-average heights off the west coast of
California
(Fig. E9). This pattern led to well
above-average temperatures across
Canada
, with the largest departures exceeding 3°C observed in central
Canada
and exceeding 2°C across eastern
Canada
(Fig. E1).
Farther south anomalous southwesterly flow and a series of
upper-level trough passages affected
California
and the Inter-Mountain regions of the
United States
during the month (Fig. E12). These
conditions resulted in above-average precipitation in both regions, with
area-averaged totals exceeding the 90th percentile of occurrences
in the Inter-Mountain region, and approaching the 70th percentile
in southern
California
(Fig. E5). The Inter-Mountain region has
received well above-average precipitation each month since October 2004,
which has either dissipated or significantly alleviated long-term drought
conditions throughout the region.
b.
North Atlantic
and
Europe
The 500-hPa circulation also
featured positive height anomalies over the high latitudes of the
North Atlantic
and over northwestern
Africa
, and negative anomalies west of
Great Britain
(Fig. E9). This pattern was associated with
increased storminess and above-average precipitation across both northern
and southern
Europe
, with totals in portions of northern
Europe
exceeding the 90th percentile of occurrences (Fig. E3).
Exceptionally warm SSTs were
again observed throughout the high latitudes and subtropical latitudes of
the
North Atlantic
, with departures in both regions exceeding the 90th percentile (Figs.
T18, E1). This
anomaly pattern reflects the ongoing warm phase of the Atlantic
multi-decadal mode that began approximately in 1995 (Goldenberg et al. Science,
2001). The anomalous warmth, combined with the enhanced southwesterly flow
into northern
Europe
, resulted in a continuation of above-average temperatures across much the
continent (Fig. E1).
2. Southern Hemisphere
The mean 500-hPa circulation
during April featured generally above-average heights in the middle
latitudes, and below-average heights at high latitudes (Fig. E15).
The above-average heights across southern
Australia
were very persistent (Fig. E17), and
contributed to exceptionally warm and dry conditions throughout the
continent (Figs. E1, E3).
Much of
Australia
experienced temperatures in the highest 90th percentile of
occurrences (Fig. E1), and precipitation
departures in the lowest 30th percentile (Fig. E3).
The most significant precipitation deficits were observed in southeastern
Australia
, where monthly totals were below the 10th percentile of
occurrences.
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