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 90th 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 10th
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.