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 80th
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 30th 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).