Extratropical Highlights – November 2013
1. Northern Hemisphere
The 500-hPa circulation during November
featured below-average heights throughout the polar region, and a zonal wave-3
pattern of height anomalies in the middle latitudes (Figs. E9, E11).
This wave-3 pattern reflected above-average heights over the high latitudes of
the North Pacific, the eastern North Atlantic and central Russia, and
below-average heights over eastern North America, Europe and Japan.
The main land-surface temperature signals
during November included well above-average temperatures across eastern Europe and central Asia, and below-average
temperatures in the eastern U.S. (Fig. E1). The main precipitation signals included above-average
totals in the southern U.S. and Mexico, and below-average totals in the northwestern
U.S., Europe, Scandinavia and southwestern Russia (Fig. E3).
a. North Pacific/ North America
The mean 500-hPa circulation during
November featured above-average heights over the high latitudes of the North
Pacific, and below-average heights over eastern Canada and the north-central
U.S. (Fig. E9).
The circulation also featured positive streamfunction anomalies at 200-hPa across
the southeastern U.S. and Gulf of Mexico (Fig.
Over the central and eastern North
Pacific, this overall circulation pattern reflected three main features. First,
it reflected a complete disappearance of the mean Aleutian Low and a corresponding
northward shift of the mean jet core, with the mean jet axis entering North America
in the vicinity of west-central Canada (Fig.
E10). Second, it reflected an amplification of
the mean Hudson Bay trough, along with enhanced northwesterly flow across
Canada and the north-central U.S. Third, it reflected
an enhanced ridge and strong confluent flow across the southeastern U.S., which
put that region in the right-entrance region of the jet stream.
This overall circulation pattern
contributed to below-average temperatures across the eastern one-third of the
U.S., with much of the region recording departures in the lowest 30th
percentile of occurrences (Fig. E1). It also contributed to above-average
precipitation in the southern U.S., Mexico, and Gulf of Mexico (Fig. E3).
In the southwestern U.S., well
above-average precipitation during the past three months (Fig. E5) has helped to considerably
lessen the severity of ongoing drought conditions. However, much of that region
still recorded moderate or severe drought at the end of November. Also,
moderate or severe drought persisted across large portions of the upper Midwest,
including Iowa, Minnesota, Illinois, western Wisconsin, and northern Missouri.
b. North Atlantic/ Eurasia
The mean 500-hPa circulation
featured above-average heights over the eastern North Atlantic and central
Russia, and below-average heights over Greenland/ Scandinavia and the
Mediterranean Sea (Fig. E9). This pattern was associated with a pronounced
spilt-flow configuration north of Great Britain. The northern branch of this split
flow brought milder Atlantic air across northern Europe and well into northern
Russia (Fig. E10),
while the southern branch brought a southwesterly flow of Mediterranean air
into south-central Russia. This combination contributed to well above-average
temperatures across central Asia, with many areas recording departures
(exceeding +5oC) in the upper 90th percentile of
occurrences (Fig. E1).
The southern branch of flow also
brought sinking motion and below-average precipitation to large portions of
Europe (Fig. E1).
November marks the fifth consecutive month of below-average precipitation for
both northern and southern Europe (Fig. E4).
2. Southern Hemisphere
The mean 500-hPa circulation during
November featured a zonal wave-1 pattern of height anomalies over Antarctica, and
above-average heights in the middle latitudes extending from the south of
Australia to South America (Fig. E15).
Over eastern Australia, the 200-hPa
circulation featured cyclonic streamfunction anomalies and an amplified trough (Fig. T22).
This pattern was associated with well below-average precipitation in
southeastern Australia, which was situated upstream of the mean trough axis (Fig. E3). In
the north and northeast, above-average precipitation during November followed
six consecutive months of below-average totals and three straight months with
totals in the lowest 20th percentile of occurrences (Fig. E4).
The South African rainy season
lasts from October to April. During November 2013, rainfall for the region was
below average (Fig. E4),
and many interior areas recorded totals in the lowest 30th percentile of
occurrences (Fig. E3).
Last month, the region recorded slightly above-average totals.
The Antarctic ozone hole typically develops during August,
reaches its peak aerial extent in September and October, and dissipates rapidly
during November. The 2013 ozone hole dissipated in mid-November, after being
smaller than the 2003-2012 average during August-October (Fig. S8). This marks the second straight
year with a below-average size of the ozone hole.