Extratropical Highlights – November 2011
1. Northern Hemisphere
The 500-hPa circulation during November
featured a persistent zonal wave-4 anomaly pattern (Fig. E11), with above average heights over the central North Pacific, eastern
Canada, Europe, and eastern Asia (Fig. E9). Below
average heights were observed over northwestern North America, the central
North Atlantic, central Russia, and eastern Siberia. This overall pattern
projected onto three prominent teleconnections,
including the positive phases of the North Atlantic Oscillation (NAO, +1.3) and
East Atlantic/ Western Russia pattern (+2.1), and the negative phase (-1.3) of
the East Pacific-North Pacific (EP) pattern (Table E1, Fig. E7).
The circulation over the Pacific/
North American sector showed links to La Niña. This link is most evident at
200-hPa, where a 3-celled pattern of extratropical height anomalies spanning
the central Pacific to eastern north America is seen to emanate from a large
area of cyclonic streamfunction anomalies over the central subtropical Pacific (Fig. T22).
These cyclonic anomalies reflect an enhanced mid-Pacific trough and a marked
westward retraction of the East Asian jet core (Fig. T21), both of which are
a well-known response to the La Niña- related suppression of tropical
convection across the central equatorial Pacific (Fig. T25).
The main surface temperature signals
during November included warmer-than-average conditions across eastern Canada, central
Europe, Scandinavia, and China, and below average temperatures in Alaska and
the Middle East (Fig. E1). The main precipitation signals included above-average
totals in the mid-western U.S., southern Europe, and eastern China, and below-average
totals along the U.S. Gulf Coast and much of Europe (Fig. E3).
a. North Pacific and North America
The mean 500-hPa circulation during
November featured above average heights over the central North Pacific and
eastern North America, and below average heights over
Alaska and western Canada (Fig. E9). This pattern projected onto the negative phase
(-1.3) of the East Pacific-North Pacific (EP) teleconnection pattern (Table E1, Fig. E7). When viewed in combination with the cyclonic
streamfunction anomalies over the central subtropical North Pacific, this
overall pattern can be interpreted as an anomalous wave train emanating from
the tropical Pacific in association with La Niña (Fig. T22).
La Niña is associated with deep
tropical convection focused over Indonesia and the eastern Indian Ocean, along
with a disappearance of tropical convection from the central equatorial Pacific
(Fig. T25).
This westward retraction in the area of deep convection acts to amplify the
mean mid-Pacific troughs at 200-hPa in both hemispheres (Fig. T22), which in the NH results
in a westward retraction the east Asian jet stream and
its associated jet exit region (Fig. T21). This jet structure favors
corresponding westward shifts in the downstream ridge and trough axes normally
located over western and eastern North America, respectively. During November, for
example, these features were located over the central North Pacific and western
North America, respectively.
These overall conditions were
associated with above average surface temperatures across eastern Canada and
below average temperatures in Alaska (Fig.
E1). Large portions of eastern Canada have
recorded positive temperature departures above the 90th percentile
of occurrences for the last two months. Also during November, above-average
precipitation was recorded in the Tennessee and Ohio Valleys, along with
below-average precipitation along the U.S. Gulf Coast (Fig. E3). These precipitation signals are
typical of the wintertime response to La Niña.
b. North Atlantic and Europe
The 500-hPa circulation during November
featured a large-amplitude wave pattern extending from eastern North America to
Mongolia (Fig. E9).
Prominent features of this pattern included a strong ridge over Europe and deep
troughs over both the central North Atlantic and western Russia. These
anomalies projected onto the positive
phases of the North Atlantic Oscillation (NAO, +1.3) and the East Atlantic/
Western Russia pattern (+2.1) (Table E1, Fig. E7).
This overall circulation was
associated with an enhanced northward transport of mild air into Scandinavia,
resulting in surface temperature departures above the 90th
percentile of occurrences (Fig. E1). It was also associated with an enhanced
southward transport of colder air and below average temperatures across
southwestern Russia and the Middle East. The mean ridge and trough positions
also strongly controlled the precipitation patterns, with above-average totals
observed over southern Europe and south-central Russia in the areas downstream
of the mean trough axes, and well below-average totals across the remainder of
Europe and western Russia in the vicinity of the amplified ridge axis (Fig. E3).
2. Southern Hemisphere
The 500-hPa circulation during November
featured above average heights over Antarctica, eastern Australia, the central
South Pacific Ocean, and the central South Atlantic Ocean (Fig. E15). It also featured below
average heights extending from southern Africa to South America. At 200-hPa the
subtropical circulation featured an extensive area of cyclonic streamfunction
anomalies across the central South Pacific Ocean in association with La Niña (Fig. T22).
In Australia, an east-west dipole
pattern of surface temperature anomalies was present during November, with
above average temperatures in the east and below average temperatures in the
west (Fig. E1).
These conditions were associated with a broad upper-level trough-ridge couplet,
which spanned the continent from west to east. Most of central and eastern
Australia also recorded above-average precipitation, with many areas recording
totals in the upper 90th percentile of occurrences (Fig. E3).
The SH
ozone hole was at record high size during the first half of the month (Fig. S8, top),
covering approximately 15 million square kilometers in mid-November. This size
is approximately double the average for the time of the year, and occurred in association
with a record large SH polar vortex (Fig.
S8, middle). The ozone hole then rapidly weakened
during the 3rd week of November is response to a sharp reduction in
size of the polar vortex. By the end of November, the ozone hole was near the
average size of 5 million square kilometers.
Overall, the 2011 ozone hole
developed rapidly in mid-August, which is slightly later than its normal onset
in early August. It then reached peak extent from mid-September to early October,
spanning approximately 24 million square kilometers. The ozone hole remained
persistent and large during mid-October through mid-November, with a record
areal extent of 20 million square kilometers throughout the period. The rapid
decay of the ozone hole in late November reflected the normal late-spring
weakening of the polar vortex.