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Extratropical Highlights - October 2001

1. Northern Hemisphere
The Northern Hemisphere circulation during October featured a persistent pattern of above-average 500-hPa heights over northeastern Asia, the eastern half of North Pacific, the southwestern U.S., the central and subtropical North Atlantic, and Europe, and below-average heights across Alaska, Canada, the eastern North Atlantic, and central Russia. (Figs. E10, E12, E13). It also featured a large-scale pattern of weak cyclonic 200-hPa streamfunction anomalies in many regions of the subtropics in both hemispheres (Fig. T22, bottom).

a. North Pacific and North America

Over the central and eastern North Pacific the north-south dipole pattern of circulation anomalies in the subtropics and mid-latitudes was associated with a pronounced split-flow configuration of the East Asian jet stream (Fig. T21, top). The northern branch of the jet extended well east of normal across the Gulf of Alaska and northwestern U.S. (Fig. E11, right, T21), where it was associated with increased precipitation (Figs. E3, E6) and below-average surface temperatures (Fig. E1). The southern branch reflected a well-defined subtropical jet stream extending from the date line eastward across northern Mexico and the southeastern U.S. (Fig. T21, top).

Above average surface temperatures covered the southwestern U.S. and eastern Canada during October, and below-average temperatures extended across the Gulf Coast states (Fig. E1). Precipitation totals were above average from the central Gulf Coast northeastward to the Great Lakes during the month (Figs. E3, E6). This marks the first time since May 2001, and only the second time since February 2001, that the Midwest and Great Lakes regions recorded above-average monthly precipitation (Fig. E5). In contrast, October precipitation totals in the northeastern and mid-Atlantic regions averaged well below the 10th percentile, in association with drier-than-normal conditions along the entire eastern seaboard of the U.S..

b. North Atlantic and Europe

Over the North Atlantic positive 500-hPa height anomalies across the central and subtropical latitudes during October reflected a strong positive phase of the East Atlantic teleconnection pattern (Figs. E7, E8 and Table E1). This pattern was accompanied by an amplification and poleward shift of the mean low-level anticyclonic circulation located over the central North Atlantic (Figs. T19, T20), with anomalously warm SSTs (Fig. E1, bottom) found along both the poleward and equatorward flanks of this anomalous anticyclonic circulation.

Over Europe strong southwesterly flow and above-average 500-hPa heights contributed to anomalously warm surface temperatures throughout the continent. Beneath the mean upper-level ridge axis, rainfall totals averaged below the 10th percentile over southeastern Europe and the central Mediterranean Sea.

2. Southern Hemisphere and ozone hole

The mean Southern Hemisphere circulation during October again featured persistent positive 500-hPa height anomalies throughout the middle latitudes, and negative anomalies over the high latitudes of the South Pacific (Fig. E16, E18, E19). Below-average heights also covered southern Australia during the month, in association with an anomalous equatorward shift of the mean jet stream position to the central portion of the continent (Fig. T21). This shift was accompanied by a pronounced northward penetration of anomalously cold air throughout the central and southern portions of the continent.

In the lower stratosphere below-average heights (Fig. S1) and temperatures [averaging more than 8C below normal (Fig. S2, bottom)] at 30-hPa and 50-hPa were observed throughout the polar region and the area extending toward southern Africa. At 10-hPa daily temperatures between 65S-90S averaged near the lowest values on record during the month (Fig. S4). These conditions were associated with an exceptionally large polar vortex (Fig. S8, middle), and with the third largest ozone hole (shaded region, Fig. S6, bottom) in the historical record with a coverage of 22 x 106 to 24 x 106 km2 (Fig. S8, top).

The Antarctic ozone hole is defined by total column ozone values less than 220 Dobson Units (DU). The 2001 ozone hole developed in mid-August, and reached peak strength during September and October. By the end of October the size of the ozone hole had decreased to approximately 15 x 106 km2, and by the first week in November it had decreased further to 12.5 x 106 km2.


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