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 65°S-90°S at 10-hPa (Fig.
S4, top right), and with a near-record areal coverage of temperatures below
78°C at 50-hPa. Temperatures below this
threshold allow for the formation of polar stratospheric clouds, which contribute to
enhanced ozone destruction.