1. Northern Hemisphere

The 500-hPa circulation during April 2003 featured positive height anomalies over the central latitudes of the North Pacific, the Norwegian Sea, Scandinavia, and western Europe, and negative height anomalies over the western United States, eastern Canada, the central North Atlantic, and eastern Siberia (Figs. E10, E12). The prominent temperature anomalies during April included cooler-than-average conditions over the western United States, eastern Canada, and in both the Black Sea and Caspian Sea regions, and warmer-than-average temperatures over the central United States, western Europe, China, and portions of eastern Russia (Fig. E1). Prominent precipitation anomalies during the month (Fig. E3) included a continuation of above-average totals in the Pacific Northwest, Inter-Mountain, and mid-Atlantic regions of the United States (Fig. E5), and continued below-average precipitation over much of Europe (Fig. E4).

a. Pacific/North America

The upper-level circulation over the North Pacific featured a persistent pattern of above-average heights in the middle latitudes, and below-average heights over the high latitudes of the western North Pacific and eastern Siberia (Figs. E12, E13). These anomalies were associated with a strong positive phase (+1.9) of the West Pacific teleconnection pattern (Table E1, Figs. E7, E8). They were also associated with a below average strength of the East Asian jet stream across the western and central Pacific (Fig. T21).

Over the United States a persistent upper-level tough in the West (Fig. E12) contributed to a continuation of above-average precipitation in the Pacific Northwest and Inter-Mountain regions, and to well above-average precipitation over most of California (Figs E3, E5). These areas also experienced below average temperatures during the month.

The largest positive surface temperature anomalies during April were observed over Alaska (+2° to +4°C), and the largest negative anomalies (-1 to -3°C) were observed over eastern Canada in association with an amplification of the mean Hudson Bay trough. Most of the central U.S. recorded slightly above average surface temperatures during the month, while the eastern seaboard and northeast regions experienced slightly below-average temperatures.

b. Europe

The upper-level circulation during April featured a ridge across the high latitudes of the North Atlantic and western Europe, and a full-latitude trough extending from northwestern Russia southward to the eastern Mediterranean Sea (Fig. E10). This circulation contributed to a continuation of anomalously warm conditions over much of Europe and to anomalously cool conditions across southwestern Russia (Fig. E1).

The persistent upper level circulation also contributed to continued drier than average conditions in western Europe, with monthly precipitation totals falling below the 10^th percentile in that region (Fig. E3). For Europe as a whole, area-averaged precipitation totals in the Northern Europe region have been below the 10^th percentile since February, and area-averaged totals in the Southern Europe region have been in the 20-30^th percentile range since March (Fig. E4).

2. Southern Hemisphere

In the Southern Hemisphere the 500-hPa circulation during April featured an anomalous zonal wave-3 pattern, characterized by above-average heights south of each of the three continents, and below-average heights over the high latitudes of the three ocean basins (Fig. E16). Surface temperatures in southern Africa were 2°-3°C above average during April, which is above the 90^th percentile of occurrences (Fig. E1). Most of southeastern Africa experienced warmer and drier than average conditions during April, which marks the end of that region’s rainy season. In Australia temperatures were slightly above average (+1°C) over the eastern half of the continent during April, while rainfall was below average in the northeast and along the east coast.