|1. Northern Hemisphere
|The Northern Hemisphere circulation
during September featured above-average 500-hPa heights over the central North Pacific,
Canada, the high latitudes of the North Atlantic, and Scandinavia, and below-average
heights over northeastern Siberia, the Gulf of Alaska, and across southern and
southeastern Europe (Fig. E10). Overall, the areas of
above-average heights were associated with well above-average surface temperatures, and
the areas of below-average heights were associated with below-average surface temperatures
In North America, the mean circulation during
September reflected a persistent upper-level ridge and above-average surface temperatures
across the western United States and Canada, and a persistent upper-level trough and
below-average surface temperatures over the eastern U.S. In the southwestern U.S.
below-average rainfall was again observed during September in association with anomalous
descending motion beneath the upper-level ridge axis (Figs. E3,
E6). This region experienced significant rainfall deficits
throughout its mid-June--September 2001 monsoon season (Fig. E5).
In contrast, above-average rains were observed in the area immediately downstream of the
mean upper-level trough axis during September, with the largest surpluses observed across
Florida and extending from the Carolinas northeastward toward Nova Scotia.
Over the North Atlantic the September mean pattern of above-average heights at high
latitudes and below-average heights over portions of the middle latitudes was consistent
with a strong negative phase (-1.6) of the North Atlantic Oscillation (NAO) (Table E1, Figs. E7, E8).
This anomaly pattern also extended eastward across Europe and Scandinavia, and contributed
to rainfall totals exceeding the 90th percentile across much of Eastern Europe
2. Southern Hemisphere and ozone hole
The mean Southern Hemisphere circulation during September featured above-average
500-hPa heights in the polar region and throughout the middle latitudes, and below-average
heights over the high latitudes of the South Pacific (Fig. E16).
This anomaly pattern reflected an equatorward extension of the polar vortex during the
month, especially over the South Pacific.
The Antarctic ozone hole region (Fig. S6, bottom) is
defined by total column ozone values less than 220 Dobson Units (DU). The 2001 ozone hole
developed in mid-August, and during September was the third largest in the historical
record with a coverage of 22 x 106 to 24 x 106 km2 (Fig.
S8, top). This large region of ozone depletion is consistent
with an expanded polar vortex (Fig. S8, middle) and with an
expanded region of lower stratospheric temperatures below 78°C. Temperatures below this threshold allow for the
formation of polar stratospheric clouds (Fig. S8, bottom),
which contribute to enhanced ozone destruction.