1. Northern Hemisphere

The 500-hPa circulation during September featured above-normal heights over eastern North America and the high latitudes of both the North Pacific and North Atlantic Oceans, and below-normal heights across the east-central North Atlantic, southern Europe, and northern Russia (Figs. E10, E12). Prominent temperature (Fig. E1) and rainfall (Figs. E3, E4, E5) departures during the month included anomalously warm and wet conditions over the eastern half of North America and southern Europe, and cooler and drier than normal conditions over northwestern Europe.

a. North America

Above-normal 500-hPa heights covered eastern North America during September (Figs. E10, E12) for a fourth straight month, resulting in a continuation of well above-average surface temperatures in that region (Fig. E1). During September surface temperatures throughout the upper Midwestern and northeastern states, as well as across southeastern Canada, averaged more than 2°C above normal and exceeded the 90^th percentile (Fig. E1).

Precipitation in the Inter-Mountain and Southwest regions of the United States has been below normal since June 2001 (Fig. E3, E5). Below-average rainfall also persisted in the Pacific Northwest region during September for the eighth consecutive month (Fig. E5).

Above-average rainfall stretched from the central Gulf Coast states to Ohio during September, with totals generally exceeding the 90^th percentile throughout the region (Figs. E3, E6). Much of the excess rain resulted from Hurricane Isidore, which entered the United States late in the month after weakening to a tropical storm. In the Ohio Valley and the Southeast regions the hurricane produced the first month of well above-normal precipitation since August 2001 (Fig. E5). In the Midwest it produced the first month of above-normal precipitation since May 2002.

b. Europe and Asia

Over Europe the upper-level circulation during September featured a strong ridge across the north and a broad trough across the south. This circulation was associated with a disappearance of the normal westerly winds (Fig. E11) and associated westerly disturbances over northern Europe, which resulted in well below-average rains across the region (Figs. E3, E5). It was also associated with a well-defined jet core and above-average rainfall extending from the eastern North Atlantic to the Black Sea.

Over eastern Asia the upper-level circulation during September featured a ridge over China, a trough over the western North Pacific (Fig. T22), and a jet core over Mongolia (Fig. T21). This circulation was associated with below-average rainfall across southern and central China and above-average rainfall across northern China and Mongolia (Figs. E3, E5). These conditions contrast with the very persistent circulation and rainfall anomalies that prevailed across China and Mongolia during the preceding four months. During May-August 2002 the upper-level trough axis and storm track was situated over central and southeastern China, and an amplified ridge was centered over western Mongolia. This circulation produced excessive rains and large-scale flooding in the Yangtze River Valley (Fig. E4), and significantly below average precipitation in northern China and Mongolia during the climatological peak in their rainy season.

2. Southern Hemisphere

The upper-level circulation during September featured a persistent pattern of below-average heights across the high latitudes of the North Pacific, and above-average heights throughout the polar region that extended well into the upper stratosphere (Figs. E16, S1). This anomaly pattern is linked to a pronounced stratospheric warming between 300-hPa and10 hPa (Fig. S2 bottom), with the largest temperature anomalies (exceeding +14°C) found near the 70-hPa level (Fig. S2 bottom). At 50-hPa monthly mean temperatures averaged over the entire polar region (between 65°S-90°S) were more than 11°C above normal (Fig. S3, bottom right). This value exceeds the previous record anomaly of +9°C recorded in 1988.

The 2002 stratospheric warming was associated with a reduced strength of the Antarctic circumpolar vortex (Fig. E17), and with a decrease in size of the vortex to its lowest value since at least 1992 (Fig. S8, middle). The extreme warmth also resulted in a nearly complete disappearance of polar stratospheric clouds (Fig. S8, bottom) and their associated ozone hole (Fig. S8, top). By the end of September 2002 the ozone hole covered only 3 x 10⁶ square miles, which is far less than the 1992-2001 mean size of 21.5 x 10⁶ square miles.