Extratropical Highlights – August 2013
1. Northern Hemisphere
The 500-hPa circulation during August
featured above-average heights over western Canada, the east-central North
Atlantic, and northern Europe/ northwestern Russia, and below-average heights
over the Gulf of Alaska, Greenland, and Mongolia (Figs. E9, E11).
The main land-surface temperature signals during August included above-average
temperatures across western North America, Europe, northwestern Russia, and China,
and below-average temperatures in portions of the central and eastern U.S. (Fig. E1). The
main precipitation signals included above-average totals in the Southeast and
Pacific Northwest regions of the U.S., and drier-than average conditions in the
mid-western U.S., Europe, and northwestern Russia (Fig. E3).
In the U.S., although exceptional
drought conditions in the western half of the country have largely faded over the
past few months, severe or extreme drought persisted during August throughout
much of that region.
a. North America
The mean 500-hPa circulation during
August featured a strong ridge centered over the Rocky Mountains and a deep
trough over the eastern U.S. (Fig. E9). This pattern reflected a westward shift of the
mean summertime ridge axis and an amplification of the Hudson Bay trough. Consistent
with this pattern, western North America experienced exceptionally warm
temperatures, and portions of the central and eastern U.S. recorded
below-average temperatures (Fig. E1).
Much of the western half of the
U.S. continued to be impacted by severe or extreme drought. At the end of August,
the “U.S. Drought Monitor” indicated extreme or exceptional drought from central
New Mexico to western Nebraska. Severe drought was recorded across much of the
remaining region between southern Oregon and Nebraska and extending southward
to Mexico. Severe drought conditions also developed in Iowa and northern
Missouri, with moderate drought covering much of the remainder of the upper
The 500-hPa circulation featured
above-average heights over the eastern North Atlantic and northwestern Russia,
and below-average heights over Greenland (Fig.
E9). This pattern was associated with
exceptionally warm (Fig. E1) and dry (Fig.
E3) conditions across much of Europe,
Scandinavia, and northwestern Russia. Area-averaged precipitation totals in
northern Europe were among the lowest since 1979 (Fig. E4). This marks the second
consecutive month with well below-average totals in that region.
The Asian monsoon ridge remained stronger
than average during August, as indicated by positive streamfunction anomalies
at 200-hPa extending across northern China (Fig. T22). This pattern was
associated with a northward shift of the mean belt of westerlies
and with a continuation of above-average surface temperatures in central and
eastern China as large areas recorded departures in the upper 90th
percentile of occurrences (Fig. E1).
2. Southern Hemisphere
The mean 500-hPa circulation during
August featured above-average heights in the polar region, and over the high
latitudes of the central South Pacific and Indian Oceans (Fig. E15). It also featured
below-average heights in the areas poleward of the three continents.
Across Australia, the 200-hPa
circulation featured an amplified ridge centered over the southwest, and an
associated poleward shift of the mean jet axis (Fig. T22). These conditions
contributed to exceptionally warm surface temperature across the continent (Fig. E1), and to exceptionally dry
conditions in the east (Figs. E3, E4).
The Antarctic ozone hole typically develops during August
and reaches its peak aerial extent in September and October. By the end of
August 2013, the ozone hole (Fig. S6) spanned 10.2 million square kilometers, which is
less than the 2003-2012 mean of 18 million square kilometers (Fig. S8, top).
This size is consistent with colder than average polar stratospheric
temperatures during June-August at both 2 hPa and 10 hPa (Fig. S4). During August, the aerial coverage of polar
stratospheric cloud (Fig. S8, bottom) and the SH polar vortex (Fig. S8, middle) were near average.