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Extratropical Highlights - August 2001

1. Northern Hemisphere
The Northern Hemisphere 500-hPa circulation during August featured above-average heights from the central North Atlantic to southwestern Russia, and from northeastern Russia to the high latitudes of the central North Pacific (Fig. E10). Consistent with these circulation anomalies surface temperatures averaged above the 90th percentile across central and eastern Europe, southwestern Russia, and the Middle East during the month, as well as over large portions of central Siberia and northwestern China (Fig. E1).

In North America the atmospheric circulation from July through mid-August reflected an amplified summertime ridge from Texas northward to northern Hudson Bay. During the second half of August the mean ridge position shifted westward to the western U.S. and western Canada (Fig. E13). These conditions contributed to above-average surface temperatures across the northern United States and southern Canada during the month. They also contributed to above-average rainfall across eastern Texas and the Gulf Coast States (Figs. E3, E6), and to below-average rainfall in the southwestern, Ohio Valley, and northeastern portions of the United States (Fig. E5).

2. Southern Hemisphere

The mean August circulation in the Southern Hemisphere reflected an anomalous wave-3 pattern at 500-hPa, characterized by above-average heights over the high latitudes of the east-central ocean basins and by below-average heights in the regions poleward of the three continents (Fig. E16). These anomalies were opposite in sign to the anomalous zonal wave-3 pattern observed in July.

During August, the largest and most persistent region of above-average 500-hPa heights extended from southeastern South America to the central South Atlantic (Fig. E18). Accompanying these conditions monthly mean surface temperatures exceeded the 90th percentile across southeastern Brazil, and most of central and southern Argentina (Fig. E1). A pronounced dipole pattern of precipitation anomalies was also evident over southeastern South America during the month, in association with a poleward shift of the mean storm track and jet stream toward southern Argentina in the area south of the mean upper-level ridge.

3. Southern Hemisphere Ozone Hole

The Antarctic ozone hole is defined by total column ozone values below 220 Dobson Units (DU). The ozone hole developed in mid-August and reached near-record size by the end of the month (Fig. S6, bottom). This evolution is consistent with near-record low temperatures between 65S-90S at 10-hPa (Fig. S4, top right), and with a near-record areal coverage of temperatures below –78C at 50-hPa. Temperatures below this threshold allow for the formation of polar stratospheric clouds, which contribute to enhanced ozone destruction.

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