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Extratropical Highlights - September 2001

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 (Fig. E1).

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 Carolina’s 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 (Fig. E3).

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 –78C. Temperatures below this threshold allow for the formation of polar stratospheric clouds (Fig. S8, bottom), which contribute to enhanced ozone destruction.

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