Extratropical Highlights – November 2012

 

1. Northern Hemisphere

The 500-hPa circulation during November featured a persistent anomalous zonal wave-4 pattern (Figs. E9, E11). Aspects of this pattern included above-average heights across the high latitudes of the North Pacific Ocean, the central U.S., and southwestern Russia, and below-average heights over the Gulf of Alaska, the southeastern U.S., the eastern North Atlantic, central Asia, and the western North Pacific (Fig. E9). The pronounced north-south dipole of height anomalies over the western Pacific reflected a strong negative phase (-2.0) of the West Pacific teleconnection pattern (Fig. E7, Table E1).

The main land-surface temperature signals during November included above-average temperatures in the western U.S., across Europe, and western Russia, and below-average temperatures in Alaska, the eastern U.S., and central Siberia (Fig. E1). The main precipitation signals included above-average totals in eastern China, and below-average totals over the eastern half of the U.S. and the Plains states (Fig. E3) where ongoing precipitation deficits (Fig. E5) have led to a continuation of exceptional drought conditions.

 

a. North America

The mean 500-hPa circulation during November featured a persistent anomalous wave pattern characterized by above-average heights over the high latitudes of the North Pacific and the western U.S., and below-average heights over the Gulf of Alaska and the southeastern U.S. (Figs. E9, E11). This pattern strongly influenced the temperature and rainfall patterns across North America, with anomalously cold and dry conditions observed downstream of the mean ridge axes (across southern Alaska and the eastern U.S.), and well above-average temperatures observed across the western U.S. (Figs. E1, E3).

Precipitation totals across the eastern U.S. were generally in the lowest 10^th percentile of occurrences, with many areas recording the lowest monthly totals for November in the record dating back to 1979 (Fig. E5). Similar deficits were observed in the central U.S., with the Great Plains region recording well below-average totals for the seventh straight month.  As a result, large portions of the central U.S. continued to be impacted by extreme or exceptional drought. At the end of November, the “U.S. Drought Monitor” indicated exceptional drought from Texas northward to central South Dakota, including portions of eastern Wyoming and eastern Colorado. Extreme drought persisted in portions of Arkansas, Iowa, and southern Minnesota.

 

b. Europe/ Asia

The mean 500-hPa circulation during November featured an anomalous wave pattern with above-average heights over southwestern Russia and below-average heights over the eastern North Atlantic and central Asia (Fig. E9). This pattern contributed to above-average temperatures across Europe, and to below-average temperatures over central Siberia (Fig. E1). It also contributed to below-average precipitation over southeastern Europe and southwestern Russia (Fig. E3).

A strong negative phase of the West Pacific teleconnection pattern (Fig. E7, Table E1) was also present during November, as indicated by a large area of above-average heights at 500-hPa over the high latitudes of the central North Pacific and eastern Siberia, and below-average heights extending from central Asia eastward to the date line. This pattern was associated with enhanced westerly jet stream winds across southern China and the low latitudes of the western North Pacific (Figs. T21, E10), and with a southward shift of the entrance region for the East Asian jet. This shift included an anomalous southerly flow of moist air at 850-hPa into eastern China (Fig. T20), and anomalous ascending motion across that region (Fig. T24). These conditions were associated with well above-average precipitation in eastern China, where monthly totals exceeded the 90^th percentile of occurrences (Figs. E3, E4).

 

2. Southern Hemisphere

The mean 500-hPa circulation during November featured above-average heights over Antarctica and below-average heights centered over the three ocean basins (Fig. E15). Much of Australia again recorded above-average temperatures during November, while exceptionally dry conditions were present in the northeast (Fig. E3).

The South African rainy season lasts from October to April. During November 2012, rainfall for the region as a whole was below average, with much of southeastern Africa recording totals in the lowest 30th percentile of occurrences (Figs. E3, E4). Previously, the region recorded above-average precipitation during both September and October.

      The Antarctic ozone hole typically develops during August, reaches peak aerial extent in September and early October, and dissipates rapidly during November. The 2012 ozone hole had dissipated by early November, after being smaller than average during August-October (Fig. S8)

     Overall, the size of the 2012 ozone hole was comparable to the smallest seen during the 2002-2011 period. This reduced size coincided with a below-average aerial coverage of polar stratospheric clouds throughout July- October, along with a below-average size of the SH polar vortex during October and November (Fig. S8).