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 Midwest.

 

b. Europe/ northwestern Russia

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.

 

 

c. China

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 (Fig. T21), and with a continuation of above-average surface temperatures in central and eastern China as large areas recorded departures in the upper 90^th 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.