1. Northern Hemisphere

The 500-hPa circulation during May 2003 featured a persistent pattern of positive height anomalies over the central North Pacific, Mexico, and across the low latitudes of the North Atlantic extending eastward to Turkey, and negative height anomalies over the high latitudes of the eastern North Atlantic and the polar region (Figs. E10, E12). The prominent temperature anomalies during May included warmer-than-average temperatures across the southern United States, eastern Canada, Europe, and both western and central Russia, and cooler-than-average conditions across the northern United States (Fig. E1). Prominent precipitation anomalies during the month (Fig. E3) included above-average totals over the eastern United States and Scandinavia, and below-average totals over the Gulf Coast region of the United States, northeastern Canada, and portions of southern Europe (Fig. E4).

a. Pacific/North America

The upper-level circulation during May featured above-average heights across the western and central North Pacific, in association with a strong negative phase of the North Pacific teleconnection pattern (Table E1). These positive anomalies extended across the eastern Pacific during the first half of the month (Figs. E13, A2.1), followed downstream by an amplified trough over the western U.S. and above-normal heights across the southeast (Fig. A2.1).

These anomaly patterns were reversed during the second half of May, as indicated by a transition from the negative phase to the strong positive phase of the Pacific/ North American (PNA) teleconnection pattern (Fig. E8). During this latter period the mean upper-level ridge was located over the southwestern states, and an amplified upper-level trough was centered over the eastern states.

For the month as a whole these circulation patterns contributed to well above-average rainfall across the eastern United States, with totals exceeding the 90^th percentile in the Southeast, Mid-Atlantic, and Ohio Valley regions, and exceeding the 80^th percentile in the Great Lakes and Midwest regions (Figs. E3, E5). Significantly below-average rainfall was recorded in the Gulf coast region, with totals below the 10^th percentile observed over southeastern Texas and southern Louisiana. The largest positive surface temperature anomalies during May were observed across the South (+1° to +2°C), with and the largest negative anomalies (-1°C) were observed in the northeastern states.

b. Europe

The upper-level circulation during May featured a persistent trough at high latitudes of the eastern North Atlantic and an amplified ridge extending across the subtropical North Atlantic to the Caspian Sea (Fig. E10). This circulation contributed to a warmer than average conditions across Europe, with temperatures exceeding the 90^th percentile in southeastern Europe and exceeding the 70^th percentile elsewhere (Fig. E1).

2. Southern Hemisphere

In the Southern Hemisphere the 500-hPa circulation during May featured above-average heights across the central latitudes of the South Pacific, and in the areas south of the three continents, and below-average heights over the high latitudes of the South Pacific and the central Indian Ocean (Fig. E16). Surface temperatures were near normal across southern Africa and most of Australia during May, and generally above average over southern South America (Fig. E1).

Most of extratropical South America experienced well above-average precipitation during May, with totals exceeding the 90^th percentile over much of Argentina (Fig. E3). Eastern Australia experienced a continuation of below-average rainfall during May, with drier-than-average conditions also observed over most of the southern half of the continent.