Extratropical Highlights – September 2011

 

1. Northern Hemisphere

The 500-hPa circulation during September featured a zonal wave-4 anomaly pattern, with above average heights over the central North Pacific, western Canada, Europe, and western Siberia. Below average heights were observed over the Gulf of Alaska, the eastern U.S., the high latitudes of the North Atlantic, and southwestern Russia (Fig. E9). Over the Atlantic Ocean, the circulation reflected a strong positive phase (+1.77) of the East Atlantic (EA) teleconnection pattern (Fig. E7, Table E1).

The main surface temperature signals during September included warmer-than-average conditions across Canada, the western U.S., Europe, and central Russia (Fig. E1). The main precipitation signals included above-average totals in the eastern U.S. and Scandinavia, and below-average totals in the central U.S. and central Europe (Fig. E3).

 

a. North Pacific and North America

The mean 500-hPa circulation during September featured an amplified wave pattern across the North Pacific and North America. Regional features of this pattern included an amplified ridge over western North America, and strong troughs over the Gulf of Alaska and the eastern U.S. (Fig. E9). The extensive ridge contributed to anomalously warm conditions across the western U.S. and Canada, with large portions of northern Canada recording departures in the upper 90^th percentile of occurrences (Fig. E1). It also contributed to anomalously dry conditions across much of Canada, and across the U.S. Great Plains in the region between the mean ridge and trough axes (Figs. E3, E6). Conversely, above-average precipitation was recorded from the central U.S. Gulf Coast to New England in the area downstream of the mean trough axis.

In the Great Plains, area-average precipitation totals during September were in the lowest 10^th percentile of occurrences (Fig. E5), with the most significant deficits reflecting ongoing exceptional drought conditions (according to NOAA’s U.S. Drought Monitor) in Texas, Oklahoma, southern Kansas, and southeastern New Mexico.

 

b. North Atlantic and Europe

The 500-hPa circulation during September featured a deep trough over Greenland and the high latitudes of the North Atlantic, and a broad ridge over Europe (Fig. E9). These persistent anomalies (Fig. E11) projected strongly onto the positive phase (+1.77) of the East Atlantic teleconnection pattern (Fig. E7, Table E1). This pattern was associated with above average temperatures across Europe and Scandinavia (Fig. E1). It was also associated with a north-south dipole pattern of precipitation anomalies, characterized by above average totals in Scandinavia and below average totals across central Europe (Fig. E3).

 

2. Southern Hemisphere

 

The 500-hPa circulation during September featured above average heights over Antarctica, and below average heights over the central South Pacific Ocean and the southeastern Indian Ocean (Fig. E15).  In the stratosphere, a ring of negative height anomalies extended around the hemisphere near 60°S (Figs. S1, S2). This pattern reflected an expanded polar vortex (Fig. S8, middle), and was associated with record low temperatures at both 2-hPa and 10 hPa between 65°S-90°S (Fig. S4). These conditions contributed to a record large amount of polar stratospheric cloud (Fig. S8, bottom)

The Antarctic ozone hole typically develops during August and reaches peak aerial extent in September and October. By the end of September 2011, the above conditions contributed to an extensive ozone hole (Fig. S6) spanning approximately 24 million square kilometers (Fig. S8, top). This size is approximately two million square kilometers larger than the 2001-2010 mean.