1. Northern Hemisphere

      The 500-hPa circulation pattern during August featured above-average heights over the western U.S. and Alaska, over the eastern North Atlantic, and over western Russia, and below-average heights over the high latitudes of the North Atlantic, southern Europe, and east of the Caspian Sea (Figs. E9, E11). The main surface temperature departures featured above-average temperatures across Alaska, the eastern half of the U.S., and eastern Europe/ western Russia (Fig. E1). The main precipitation anomalies during August included above-average totals from over the southeastern quadrant of the U.S. and southern Europe (Figs. E3, E5, E6).

a. North America

      The prominent 500-hPa circulation anomalies during August included above-average heights over the western U.S. and Alaska (Fig. E9). These anomalies contributed to exceptionally warm and dry conditions across Alaska, with monthly temperature departures exceeding the 90th percentile of occurrences (Fig. E1) and monthly precipitation departures generally in the lowest 10th percentile of occurrences (Fig. E3). They also contributed to well above-average temperatures in the extreme western U.S., with portions of California and the Pacific Northwest also recording temperatures in the 90th percentile of occurrences.

      The southeastern quadrant of the United States experienced above-average precipitation during August. Much of the excess rainfall during the month was related to Major Hurricane Katrina, which made landfall east of New Orleans on 29 August. Katrina produced extensive flooding and severe damage in coastal Louisiana, Mississippi, and western Alabama. The storm produced totals in excess of 400 mm in these areas, before tracking northwestward to western New York. Along its track, Katrina produced 200-275 mm of rain in northeastern Mississippi, western Tennessee, and central Kentucky, and 75-100 mm of rain across the Ohio Valley and western New York.

b. Europe/ western Russia

      The 500-hPa circulation pattern during August featured a persistent wave pattern characterized by above-average heights over the eastern North Atlantic and western Russia, and below-average heights over southern Europe and in the area east of the Caspian Sea (Fig. E11). This pattern contributed to above-average temperatures across eastern Europe and western Russia, and to above-average precipitation across southern Europe.

2. Southern Hemisphere

The mean 500-hPa circulation pattern during August featured above-average heights from Australia to the central South Pacific, over the subtropical eastern South Pacific, and over the western South Atlantic, and below-average heights over the east-central South Pacific and in the area south of Africa (Fig. E15). Over the eastern Pacific, the north-south dipole of anomalies was associated with a pronounced eastward extension of the South Pacific jet stream, with the main jet core extending from Australia to just upstream of South America (Fig. T21). This pattern contributed to anomalously warm and dry conditions over southeastern Australia, and to exceptionally warm conditions in southeastern South America (Fig. E1). It also contributed to well above-average precipitation over southern Argentina, with totals in many areas exceeding the 90th percentile of occurrences.