The 500-hPa circulation during September featured above-average heights in the middle latitudes and below-average heights over western North America (Fig. E9). The main persistent positive anomalies were observed across the central North Pacific, the eastern half of Canada, and southwestern Europe (Fig. E11). This circulation was associated with below-average temperatures over western Canada, and above-average temperatures over eastern Canada and the northeastern United States (Fig. E1). Elsewhere, temperatures were above average throughout Europe, southeastern Asia and India, and the African Sahel and Sudan regions.

The main positive precipitation anomalies during September were observed over the eastern United States, where near-record totals were associated with three landfalling hurricanes. The main negative precipitation anomalies during the month were observed in the midwestern United States, and across the African Gulf of Guinea region. Elsewhere, through September 2004 the Atlantic hurricane season was well above normal and the East Pacific hurricane season was considerably below normal.

a. North America

Over North America the mean circulation during September featured a persistent upper-level trough over western Canada and Alaska, and a persistent ridge over the eastern half of Canada and the northeast quadrant of the United States. (Figs. E9, E11). The surface temperature anomalies were in phase with the upper-level height anomalies (Fig. E1). This anomaly pattern is opposite to that observed during April-August, when extremely warm temperatures covered Alaska, and anomalously cold temperatures prevailed across central and eastern North America.

During September the eastern U.S. experienced well above-average precipitation (Fig. E3) and increased storminess (Fig. E13) in response to three landfalling hurricanes. Hurricane Frances hit the east coast of Florida as a Category-2 storm on 5 September, Major Hurricane (MH) Ivan hit Alabama as a Category-4 storm on 16 September, and MH Jeanne hit the east coast of Florida as a Category-3 storm on 26 September.

b. Atlantic hurricane season

The 2004 Atlantic hurricane season is one of the most active since 1945, with twelve tropical storms (TS), eight hurricanes, and six major hurricanes by the end of September. Since 1995 eight of ten hurricane seasons have been above normal, compared to only three during the previous 25-years (1970-1994).

During September 2004 four hurricanes, three of which became major hurricanes, developed in the tropical Atlantic. Eight tropical storms formed during August, with five becoming hurricanes and an August record of three becoming major hurricanes.

Over the tropical Atlantic the above-normal Atlantic hurricane season was associated with 1) an amplified subtropical ridge at upper levels (Fig. T22), 2) reduced vertical wind shear in response to enhanced upper-level easterlies and reduced lower-level easterlies (Figs. T20, T21), 3) enhanced cyclonic shear at 850-hPa along the equatorward flank of the African easterly jet, which favors hurricane development from tropical disturbances moving westward from the African coast (Fig. T20), and 4) above average sea-surface temperatures (Fig. T18). These conditions have been associated with the increased hurricane activity since 1995 (Bell et al., Bull. Amer. Meteor. Soc., May 2004).

For August-September 2004 combined, six named storms made landfall in the United States. In addition to the three September landfalls (section 1a), TS Bonnie hit the Florida Panhandle on 12 August, major hurricane Charlie hit the west coast of Florida as a Category-3 storm on 13 August, and TS Gaston hit South Carolina on 30August. In the past three years seventeen named storms have made landfall in the U.S. (6 in 2002, 5 in 2003, and 6 in 2004), with thirteen hitting the Gulf Coast region (5 in 2002, 5 in 2003, and 3 in 2004).

c. East Pacific hurricane season

The East Pacific hurricane season again featured considerably below-normal activity during June-September, which reflects the generally suppressed activity observed in that region since 1995. By the end of September, the season had produced only ten named storms, with five becoming hurricanes and two (Darby and Javier) becoming major hurricanes.

2. Southern Hemisphere

The 500-hPa circulation during September featured above-average heights across the Indian Ocean, south of Australia, the high latitudes of the central South Pacific, and the polar region, and below-average heights south of South America (Fig. E15). At lower latitudes anticyclonic streamfunction anomalies prevailed from the central South Pacific eastward to southern Africa , and cyclonic anomalies were observed over the Amazon Basin and upstream of Australia (Fig. T22). In Australia, the anomalous upper-level trough brought increased cloud cover and below-average temperatures to the western part of the continent.

South America

In the Tropics the upper-level circulation reflected a weakening of the “monsoon” ridge, and was consistent below-average precipitation in the northern Amazon Basin and Central America. In both regions June-September precipitation totals were below the 10^th percentile of occurrences. The Amazon Basin also experienced warmer than normal conditions during the period, with mean temperatures ranging from 1°-2°C above average.

In the middle latitudes the anomalous anticyclonic circulation at upper levels was associated with reduced jet stream winds, along with a reduction in the number of synoptic-scale troughs and major cold frontal passages east of the Andes Mountains. These conditions contributed to warmer than normal conditions across extratropical South America.