1. Northern Hemisphere
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.
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 90th percentile (Fig.
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 90th
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
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.