Extratropical Highlights – September 2014

 

1. Northern Hemisphere

The 500-hPa circulation during September featured above-average heights across eastern Siberia and Alaska, and from the high latitudes of the North Atlantic to western Russia (Fig. E9). The circulation also featured below-average heights across the high latitudes of the North Pacific, Greenland, the east-central North Atlantic, and central Russia.

The main land-surface temperature signals included above-average temperatures in the western U.S., Europe, and China, and below-average temperatures in north-central Russia (Fig. E1). The main precipitation signals included above-average totals in the southern U.S., southeastern Europe, and northeastern China, and below-average totals across the northeastern U.S. and most of Europe (Fig. E3).

 

a. North Pacific/ North America

The mean 500-hPa circulation during September featured troughs over the eastern North Pacific and the eastern U.S. (Fig. E9). This pattern was associated with above-average precipitation across the southern U.S. and with below-average precipitation in the northeastern U.S. (Fig. E3). The trough in the west also aided the northward migration into the southwestern U.S.  of deep tropical moisture and rainfall associated with an active east Pacific hurricane season.

Area-averaged totals in the Inter-Mountain region, Southern California, and the Southwest reached the 80^th percentile of occurrences (Fig. E5), with more than 175% of normal precipitation recorded in many regions within the drought-stricken states of Arizona, New Mexico, Utah, and Nevada (Fig. E6).

Nonetheless, a massive area of exceptional drought continued in central/ southern California and western Nevada, and severe or extreme drought extended across the U.S. Southwest, California, southern Oregon, central Washington, and southern Idaho.

 

b. Europe

The 500-hPa circulation during September featured above-average heights over the northeastern North Atlantic and Europe, and below-average heights across the east-central North Atlantic (Fig. E9). This pattern was associated with anomalously warm and dry conditions across northern Europe, Scandinavia, and northwestern Russia (Figs. E1, E3).

 

c. Northern Africa

The West African monsoon, which lasts from June-September, has been well below average so far this year. Area-averaged totals during October were near average, following well below-average totals during June and July (Figs. E3, E4). Overall, the suppressed west African monsoon has likely contributed to exceptionally non-conducive conditions for hurricane formation across the tropical North Atlantic, including enhanced vertical wind shear, sinking motion, increased atmospheric stability, and a southward shift of the African Easterly Jet (AEJ).  Another contributing factor has been an amplified Tropical Upper-Tropospheric trough (TUTT) over the western North Atlantic and Caribbean Sea (Fig. T22).

 

2. Southern Hemisphere

The mean 500-hPa circulation during September featured above average heights south of Australia and Africa, and below-average heights over the central Indian and Pacific oceans (Fig. E15). In southeastern Australia downstream of the mean ridge axis, exceptionally dry conditions continued during the month, with regional precipitation totals in the lowest 30^th percentile of occurrences (Figs. E3, E4).

The Antarctic ozone hole typically develops during August and reaches its peak aerial extent in September and October. By the end of September 2014, the ozone hole (Fig. S6) spanned 20 million square kilometers, which is near the 2004-2013 mean (Fig. S8, top). The aerial coverage of polar stratospheric cloud (Fig. S8, bottom) and the SH polar vortex (Fig. S8, middle) were also near average, as were polar stratospheric temperatures (Fig. S4).