1. Northern Hemisphere

The 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).

Prominent temperature departures during April included warmer than average conditions across the western and north-central U.S. , Scandinavia , central Europe , and much of China , and a continuation of warmer than average SSTs over the high latitudes of the North Atlantic (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 90^th 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.

c. China

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 90^th 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 portions of Mozambique and Tanzania , and also in south-central South Africa (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.