Extratropical Highlights – September 2012

 

1. Northern Hemisphere

The 500-hPa circulation during September featured above-average heights over western North America, the central North Atlantic, southeastern Europe, central Siberia, and the high latitudes of the western North Pacific (Fig. E9). It also featured below-average heights over Alaska, the central United States, and the high latitudes of the eastern North Atlantic.

The main land-surface temperature signals during September included above-average temperatures across the western United States, Canada, southeastern Europe, and central/ eastern Siberia (Fig. E1). The main precipitation signals included above-average totals in the central U.S., Scandinavia and northwestern Russia, and below-average totals across the northwestern and north-central U.S. and nearly all of Canada (Fig. E3).

 

a. North America

The mean 500-hPa circulation during September featured an amplified wave pattern, with a ridge centered over western Canada and troughs centered near the date line and also extending southward from Hudson Bay to the U.S. Gulf Coast (Fig. E9). This pattern was associated with exceptionally warm (Fig. E1) and dry (Fig. E3) conditions across Canada and the U.S. Pacific Northwest. Western and eastern Canada each recorded temperatures in the upper 90^th percentile of occurrences, and central Canada recorded precipitation totals in the lowest 10^th percentile of occurrences. In the U.S. Pacific Northwest, area-averaged precipitation totals were near the record lows (1979-present) for a second straight month (Fig. E5). Exceptionally dry conditions also covered the U.S. central and northern Plains states during September, which reflects a continuation of severe precipitation deficits that began in May.

In contrast, areas within and downstream of the mean Hudson Bay trough axis recorded above-average precipitation, with surpluses extending northeastward from the central U.S. Gulf Coast to New England. For the Great Lakes and Midwest regions of the United States, September marked the first month since April that these areas recorded near- to above-average precipitation (Fig. E5).

Given these ongoing precipitation deficits, large portions of the central U.S. continued to be impacted by extreme or exceptional drought. At the end of September, the “U.S. Drought Monitor” indicated exceptional drought from Oklahoma northward to southern South Dakota, and extreme drought in portions of Wyoming, Colorado, Utah, Texas, Iowa, Arkansas, and Illinois.

           

 

b. Europe/ central Asia

The mean 500-hPa circulation during September featured an amplified Icelandic Low that extended eastward into Scandinavia, along with anomalous ridges over the central North Atlantic and central Siberia (Fig. E9). This pattern was associated with an amplified jet stream across the high latitudes of the North Atlantic (Fig. E10) and with an enhanced onshore flow of relatively mild, marine air into northern Europe and Scandinavia. These conditions contributed to

above-average precipitation across northern Scandinavia and northwestern Russia, with many areas recording totals in the upper 70^th percentile of occurrences (Fig. E3). They were also associated with above-average surface temperatures in eastern Europe and across Siberia, with large portions of eastern Siberia recording departures in the upper 90^th percentile of occurrences (Fig. E1).

 

c. African Sahel

The west African monsoon typically peaks during July-September. This monsoon was again enhanced during September, with above-average precipitation seen across the African Sahel and Sudan regions (Fig. E1). For the west African monsoon region as a whole, monthly area-averaged precipitation totals have been above-average since May, exceeding the 70^th percentile of occurrences during each of the last five months (Fig. E4). Overall, the west African monsoon system has been enhanced since 1995, in association with the warm phase of the Atlantic Multi-Decadal Oscillation (AMO).

 

2. Southern Hemisphere

The mean 500-hPa circulation during September featured above-average heights across the central and eastern South Pacific Ocean, and below-average heights across the high latitudes of the three ocean basins (Fig. E15). In the lower atmosphere, the subtropical high pressure system normally centered east of South America was again enhanced along its westward flank (Fig. T20). The associated anomalous poleward flow over eastern South America led to an anomalous poleward position of the mean cold frontal boundary, and resulted in a continuation of well above-average surface temperatures across that region (Fig. E1). This marks the second straight month in which large portions of eastern South America have recorded surface temperature departures above the 90^th percentile of occurrences.

Much of Australia also recorded above-average temperatures in September, with the most significant departures observed in the northeast where they exceeded the 90^th percentile of occurrences. Much of southern Australia also recorded below-average precipitation in September.

The Antarctic ozone hole typically develops during August and reaches peak aerial extent in September and early October. By the end of September 2012, the ozone hole (Fig. S6) spanned 15 million square kilometers, which is smaller than the 2002-2011 mean of 19.8 million square kilometers (Fig. S8, top). Overall, the size of the 2012 ozone hole has been near the lowest seen in the 2002-2011 period. This reduced size coincides with a below-average aerial coverage of polar stratospheric clouds throughout July- September (Fig. S8, bottom).