500-hPa circulation during April featured above average heights over
western North America, the central North Atlantic, and the polar region,
and below-average heights over the high latitudes of the western North
Pacific, eastern Canada, and central Russia (Fig. E9). The
circulation also featured anomalous anticyclonic streamfunction anomalies
over the subtropical western North Pacific (Fig. T22).
This overall circulation is associated with strong positive phases
of both the West Pacific teleconnection pattern (+1.5) and the North
Atlantic Oscillation (NAO) (+0.9) (Table E1, Fig. E7).
temperature departures during April included warmer than average conditions
across the western and north-central
, and much of
, and a continuation of warmer than average SSTs
over the high latitudes of the
(Fig. E1). Prominent precipitation
anomalies included above-average rainfall across the south-central U.S.,
and below-average precipitation in the Pacific Northwest U.S., across the
Atlantic Ocean, Scandinavia, and eastern China (Fig. E3).
a. North America
The circulation over North
America featured a split-flow pattern with an amplified upper-level ridge
extending from the northwestern U.S. to the polar region and an upper-level
trough over the southwestern U.S. in the southern branch of westerlies (Fig.
E9). This circulation contributed to anomalously warm conditions over
the western and north-central U.S. It also contributed to below-average
precipitation over extreme western
Canada and the Pacific Northwest, and to above-average precipitation from Texas to Wyoming and Utah
(Fig. E6). The Pacific Northwest has
experienced below-average precipitation in the last two months, while the
Inter-Mountain region has experienced above-average precipitation in four
of the last six months (Fig. E5).
b. North Atlantic
North Atlantic SSTs have been considerably
above average during the past year. During April they remained well above
the 90th percentile across the high latitudes, and also across
large portions of the subtropics. This anomalous warmth is likely
associated with the ongoing warm phase of the Atlantic multi-decadal mode
that began approximately in 1995.
The upper-level circulation during April
featured an anomalous split-flow configuration over eastern Asia and an
enhanced confluent flow over the western North Pacific (Fig. T22).
This circulation is associated with a below-average strength of the East
Asian jet across China, and an anomalous eastward shift of that jet
entrance region to Japan (Fig. T21). These conditions contributed to
exceptionally warm conditions over China, with temperatures in many areas
exceeding the 90th percentile of occurrences.
Much of China also experienced well above-average surface
temperatures during February and March.
2. Southern Hemisphere
In the Southern Hemisphere the 500-hPa
circulation during April featured above-average heights south of Australia,
across the middle latitudes of the central South Pacific, and over the
extratropical Indian Ocean, and below-average heights south of Africa (Fig.
E15). This circulation was associated with anomalously warm and dry
conditions across eastern Australia (Figs. E1, E3), and enhanced
rainfall over southern South America and southeastern Africa.
In southern Africa the rainy season normally
lasts from October to March. During 2004, above average rains continued in
, and also in south-central
(Fig. E3). This enhanced rainfall
occurred in the vicinity of the mean upper-level trough axis (Fig. T22).
Overall, the 2003-2004 rainy season started off slow with below-average
rains during November and December, but ended strong with above-average
rains during January-April.