1. Northern Hemisphere
500-hPa circulation during November featured above-average heights over the high
latitudes of the North Pacific, eastern Canada, Greenland, and south-central
Russia. The circulation featured below average heights over the central North
Atlantic, Europe, and central Siberia (Fig.
E9). Over the Atlantic basin, the anomaly pattern
reflected a continuation of the negative phase of the North Atlantic Oscillation
(NAO) (Table E1,
Fig. E7), which has been in place for the last 17
months (since June 2009).
the subtropical Atlantic basin, the 200-hPa streamfunction anomaly pattern
continued to show a pronounced inter-hemispheric symmetry, with anticyclonic
anomalies extending from the America’s to Africa in both hemispheres (Fig.
T22). This pattern has persisted
throughout the past several months in association with the combination of an
enhanced west African monsoon system, La Niña, and well above average (or
record) SSTs across the tropical Atlantic (Fig.
T18). These conditions contributed to
a very active 2010 Atlantic hurricane season, which produced 19 named storms, of
which 12 became hurricanes and 5 became major hurricanes.
the tropical and subtropical Pacific basin, enhanced mid-Pacific troughs were
evident in both hemispheres during November (Fig.
T22), in association with the ongoing
La Niña (Fig. T18).
In the NH, one consequence of this pattern was a pronounced westward retraction
of the east Asian jet stream and associated jet exit region (Fig.
T21), which favored above average
heights over the high latitudes of the central North Pacific typical of La Niña
main surface temperature signals during November included warmer than average
conditions across Canada, Alaska and nearly all of Eurasia (Fig.
E1). Below average temperatures were observed in
the northwestern U.S. and Scandinavia. Monthly precipitation was above-average
over large portions of Europe and northwestern Russia, and (Fig.
E3), below average along portions of the U.S. Gulf
Coast, nearly all of the U.S. eastern seaboard (Fig.
E6), Turkey, and eastern China.
a. North Pacific
and North America
mean 500-hPa circulation during November featured a broad ridge over the high
latitudes of the North Pacific and a broad trough over western and central North
America (Fig. E9). It also featured a
disappearance of the mean Hudson Bay trough. This pattern showed links to both
La Niña and the negative phase of the NAO.
Niña is associated with deep tropical convection focused over Indonesia and the
eastern Indian Ocean, along with a disappearance of tropical convection from the
western and 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
acts to amplify and shift westward the exit region of the east Asian jet stream
As a result, that jet stream also retracts westward, which favors corresponding
westward shifts in the downstream ridge and trough axes normally located over
western and eastern North America, respectively. During November this westward
shift was indicated by a tendency for broad ridging at 500-hPa over the high
latitudes of the North Pacific and by a broad trough that covered western and
central North America.
500-hPa circulation over eastern North America also showed consistency with the
ongoing negative phase of the NAO. Specifically, the pattern of positive height
anomalies over eastern Canada extended well eastward to Greenland and the high
latitudes of the central North Atlantic, and occurred in combination with
negative height anomalies across the central North Atlantic. This north-south
dipole pattern, along with its associated southward shift of the mean North
Atlantic jet stream, reflects the negative phase of the NAO (Fig. T21).
absence of the mean Hudson Bay trough during November contributed to well above
average surface temperatures in northeastern Canada, where temperature
departures exceeded the 90th percentile of occurrences (Fig.
E1). The largest precipitation anomalies during
November included below average totals in the Gulf Coast, Southeast, and
Mid-Atlantic regions of the U.S. (Fig. E5).
The southern portion of the U.S. tends to record below average precipitation
during La Niña.
the extratropical North Atlantic, the 500-hPa circulation featured an ongoing
negative phase of the North Atlantic Oscillation (NAO) (Fig.
E7, Table E1). This
phase is characterized by above average heights over Greenland, and below
average heights generally extending from eastern North America to southern
Europe. The negative NAO has prevailed in every month since June 2009, with the
exception of September 2009.
characteristic cool-season feature of the negative NAO is southward shift of the
mean North Atlantic jet stream (Fig. T21).
During November, the mean Atlantic jet stream entered Europe in the vicinity of
Portugal (Fig. E10),
which is well south of its normal position near Great Britain.
the subtropical North Atlantic, anticyclonic streamfunction anomalies at 200-hPa
extended from the America’s to Africa in both hemispheres (Fig.
T22). This inter-hemispheric symmetry
was associated with upper-level easterly wind anomalies that extended across
tropical northern Africa and the tropical North Atlantic (Fig.
T21). It was also associated with an
extensive area of low-level westerly wind anomalies that extended across the
eastern North Pacific and tropical North Atlantic (Fig. T20).
combination of conditions was evident throughout the Atlantic hurricane season,
which lasts from June through November. It has links to the enhanced west
African monsoon circulation that was seen well into October (Fig.
E4), La Niña, and well above average SSTs across
the tropical Atlantic (Fig. T18).
These conditions contributed to a very active 2010 Atlantic hurricane season,
which produced 19 named storms, of which 12 became hurricanes and 5 became major
The 500-hPa circulation
during November featured a large amplitude trough over Europe and a broad ridge
over south-central Russia (Fig. E9).
This pattern was associated with an extensive flow of marine air into the
continent, which subsequently extended across central Asia. As a result, much of
the Eurasia recorded well above average temperatures (3+°C above average)
during the month, with many areas recording departures in the upper 90th
percentile of occurrences (Fig. E1).
This same flow pattern contributed to above average precipitation from central
Europe to north-central Siberia, with most locations recording totals in the
upper 70th percentile of occurrences (Fig. E3).
2. Southern Hemisphere
The 500-hPa circulation during November reflected above average heights in the
middle latitudes and below average heights over the high latitudes of the
eastern South Pacific (Fig. E15).
A similar anomaly pattern was evident in October. In the subtropics, the
upper-level (200-hPa) streamfunction pattern reflected an anomalous trough
across central South Pacific in association with La Niña (Fig.