Extratropical Highlights – August 2012

 

1. Northern Hemisphere

The 500-hPa circulation during August featured above-average heights over western North America, Greenland, southern Europe, and the high latitudes of the central North Pacific, and below-average heights over the eastern North Atlantic and eastern Siberia (Figs. E9, E11). Over the North Atlantic, the circulation continued to reflect a strong negative phase (-1.4) of the North Atlantic Oscillation (NAO) (Fig. E7, Table E1).

The main land-surface temperature signals during August included above-average temperatures in the western United States, eastern Canada, southern Europe, and southwestern Russia (Fig. E1). The main precipitation signals included above-average totals in the U.S. Gulf Coast and mid-Atlantic regions (Figs. E5, E6), and below-average totals across the northern tier and central regions of the U.S., western Canada, and southern Europe (Fig. E3).

 

a. North America

The mean 500-hPa circulation during August featured a continental-scale wave pattern with a ridge over western North America and a trough in the east (Fig. E9). This pattern represented a marked westward shift of the mean summertime ridge axis, which is normally located over central North America. During August, this anomalous circulation was associated with exceptionally warm (Fig. E1) and dry (Fig. E3) conditions within and downstream of the mean ridge axis, and with above-average precipitation within the base of the mean trough. 

Regionally, the western U.S. recorded exceptionally warm conditions, with surface temperatures generally above the 90^th percentile of occurrences (Fig. E1). Precipitation was well below average in the U.S. Pacific Northwest, and from the Great Plains and Great Lakes regions to northwestern Canada (Fig. E3). Area-averaged precipitation totals in Great Plains have been in the lowest 20^th percentile of occurrences for four straight months, while totals in the Great Lakes and Midwest regions have been below average for five straight months (Fig. E5). By the end of August, these ongoing rainfall deficits had resulted in exceptional drought from north-central Texas to Kansas, and in extreme drought extending northward to central South Dakota and from Missouri to northern Illinois.

            In contrast, above-average precipitation was recorded during August along the U.S. Gulf Coast and extending into the Mid-Atlantic region. Precipitation along the Gulf Coast was aided by Hurricane Isaac, which made landfall in eastern Louisiana and also produced significant rainfall in both Mississippi and Alabama. Isaac then moved northward into Illinois, bringing much-needed rainfall to that region.

 

b. North Atlantic and Europe

The 500-hPa circulation during August featured a continued negative phase (-1.4) of the NAO (Fig. E7, Table E1), which mainly reflected persistent positive height anomalies over Greenland and negative height anomalies over the eastern North Atlantic (Figs. E9, E11). This marks the fourth consecutive month with a negative NAO pattern, and three straight months with an NAO index below -1.3.

During August, this overall circulation was associated with a split-flow pattern across western Europe. These conditions contributed to above-average precipitation in the northern branch of westerly’s over Great Britain (Fig. E3), and to below-average precipitation and well above-average temperatures (Fig. E1) across southern Europe, with the anomalous warmth also extending eastward into southwestern Russia. For much of the region between France and the Caspian Sea, monthly surface temperature departures exceeded +2˚C and were above the 90^th percentile of occurrences. For southern Europe, area-average precipitation totals have been in the lowest 15^th percentile of occurrences for the last three months (Fig. E4).

 

c. African Sahel

The west African monsoon typically peaks during July-September. The monsoon was again enhanced during August 2012 (Fig. T24), producing well above-average precipitation across the African Sahel and Sudan regions (Fig. E1). For the west African monsoon region as a whole, area-averaged precipitation totals have been above-average since May, and have exceeded the 90^th percentile of occurrences during the last three 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 August featured an anomalous zonal wave-3 pattern, with above-average heights over the central ocean basins including eastern South America, and below-average heights poleward of the three continents (Fig. E15). The most prominent temperature and precipitation anomalies were found over eastern South America, in association with an anomalously strong subtropical high pressure system along its westward and southwestward flanks (Figs. E14, T20). This enhanced ridge contributed to a north-south dipole pattern of precipitation anomalies, with well below-average precipitation in southern and southeastern Brazil and well above-average precipitation farther south (Fig. E3). It also contributed to exceptionally warm conditions in eastern South America, where monthly surface temperatures were in the highest 90^th percentile of occurrences (Fig. E1).

Much of Australia also recorded above-average temperatures in August, with the most significant departures observed in the central and northeast regions where departures exceeded the 90^th percentile of occurrences. Much of southern Australia also recorded below-average precipitation in August, with the largest deficits observed in the southeast.

The Antarctic ozone hole typically develops during August and reaches peak aerial extent in September and October. By the end of August 2012, the ozone hole (Fig. S6) spanned only 4 million square kilometers, which is far smaller than the 2002-2011 mean of 18 million square kilometers (Fig. S8, top). This comparatively slow development of the 2012 ozone hole partly reflected a below-average aerial coverage of polar stratospheric cloud during July and August (Fig. S8, bottom). It also reflected a weaker than average polar vortex in the eastern hemisphere during August (Fig. S1), as indicated by above-average stratospheric heights and temperatures that extended well upward past the 10 hPa level (Fig. S2).