1. Northern Hemisphere
500-hPa circulation during December exhibited two main features. The first was a
hemispheric-scale pattern of anomalies characterized by above average heights in
the polar region and below average heights in the middle latitudes (Fig.
E9). This pattern reflects a negative phase of the
Arctic Oscillation (AO), and also projects strongly (-1.8) onto the negative
phase of the North Atlantic Oscillation (NAO) (Table
E1, Fig. E7). A
negative NAO index has prevailed for the last 18 months (since June 2009).
second main circulation feature during December was a strong negative phase of
the Pacific/ North American (PNA) teleconnection pattern (-1.8). This pattern
reflects above-average heights over the high latitudes of the North Pacific, and
below average heights over the central subtropical North Pacific and western
North America. A negative PNA pattern is typical of La Niņa.
Niņa was also associated with enhanced mid-Pacific troughs in both hemispheres
over the tropical and subtropical Pacific basin (Fig.
T22). 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). These conditions
are consistent with the negative PNA pattern (Fig.
main surface temperature signals during December included warmer than average
conditions across eastern Canada and south-central Russia, and
cooler-than-average conditions in the southeastern U.S., Alaska, most of Europe,
and north-central Russia (Fig. E1). Monthly precipitation was
above-average in the western U.S. and southwestern Europe, and below average in
the southeastern U.S., southern Alaska, and extreme western Canada (Fig. E3).
a. North Pacific
and North America
mean 500-hPa circulation during December featured a strong ridge over the high
latitudes of the North Pacific and northeastern Canada, and troughs over the
Gulf of Alaska and the eastern U.S. (Fig.
E9). This pattern showed links to both La Niņa
and the negative phase of the AO/ NAO.
Niņa 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 retract westward the exit region of the east
Asian jet stream (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 December, this westward shift was indicated by a tendency
for strong ridging over the high latitudes of the North Pacific and by a broad
trough over the Gulf of Alaska. This trough resulted in well above average
precipitation in the western and southwestern U.S. (Fig. E3). Typically, that region sees
below average wintertime precipitation during La Niņa.
500-hPa circulation over eastern North America continued to reflect a negative
phase of the AO/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.
with this pattern, average surface temperatures in northeastern Canada exceeded
the 90th percentile of occurrences (Fig.
E1), while temperatures in the southeastern U.S.
were in the lowest 10th percentile of occurrences. The largest
precipitation anomalies during December included above average totals in the
western U.S., and continued below average totals in the southeastern and
Mid-Atlantic regions of the U.S. (Fig. E5).
The southeastern portion of the U.S. tends to record below average wintertime
precipitation during La Niņa.
the extratropical North Atlantic, the 500-hPa circulation during December
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 extending from eastern North America to southern Europe. The
negative NAO has prevailed in every month since June 2009, with the exception of
characteristic cool-season feature of the negative NAO is southward shift of the
mean North Atlantic jet stream (Fig. T21).
During December, the mean Atlantic jet stream entered the continent in northern
Africa (Fig. E10),
which is well south of its normal position near Great Britain. As a result, the
normal northward heat transport associated with this jet stream was absent.
Instead, much of Europe experienced an anomalous flow of cold arctic air during
the month, which resulted in well below average temperatures across much of the
continent (Fig. E1).
The subsequent eastward transport of this colder air also contributed to well
below average temperatures in north-central Russia.
2. Southern Hemisphere
the South Pacific Ocean, the 500-hPa circulation during December reflected above
average heights in the middle latitudes and below average heights at high
latitudes (Fig. E15).
In the subtropics, the upper-level (200-hPa) streamfunction pattern reflected an
anomalous trough across central South Pacific in association with La Niņa (Fig. T22).
The main temperature
anomalies during the month included well below average temperatures in eastern
Australia, where departures were in the lowest 10th percentile of
occurrences. The main precipitation signals reflected above average totals
throughout the eastern half of Australia, with extremely high totals and severe
flooding in the northeast (Fig. E3).
The South African
rainy season lasts from October to April. During December, rainfall was above
average in southeastern Africa, with departures generally in the upper 70th
percentile of occurrences. To date, precipitation was slightly below average in
October, near average in November, and above average in December (Fig.
E4). Seasonal rainfall is typically above average
in this region during La Niņa.