Beginning with this
month, all anomalies reflect departures from the 1981-2010 base period.
1. Northern Hemisphere
The 500-hPa circulation during
January featured a hemispheric-scale pattern of anomalies characterized by above
average heights in the polar region and generally below average heights in the
middle latitudes (Fig. E9).
This pattern reflects a negative phase of the Arctic Oscillation (AO, Fig.
S2), and also projects strongly (-1.5) onto the
negative phase of the North Atlantic Oscillation (NAO) (Table
E1, Fig. E7). A
negative NAO index has prevailed for the last 19 months (since June 2009).
Also evident during January was a
continuation of enhanced mid-Pacific troughs in both hemispheres over the
tropical and subtropical central/eastern Pacific (Fig. T22).
In the NH, these conditions contributed to an enhanced East Asian jet
exit region, and to above average 500-hPa heights across Alaska and the Gulf of
Alaska. These features are consistent with the ongoing mature phase of La Niņa.
The main surface temperature
signals during January included warmer than average conditions across eastern
Canada, the southwestern U.S., and south-central Russia, and cooler-than-average
conditions in the eastern U.S., Alaska, most of Europe, and large parts of
northern Asia (Fig. E1).
Monthly precipitation was below average in the western and eastern U.S. and
Canada, western Canada, portions of central Europe, and central Russia (Fig.
E3). No significant regions of above average
precipitation were observed this month.
a. North Pacific and North America
The mean 500-hPa circulation
during January featured a strong ridge over the Gulf of Alaska and Alaska, and a
deep trough over the eastern U.S. (Fig. E9).
This pattern showed links to the mature La Niņa conditions and to the ongoing
strong negative phase of the AO/ NAO.
La 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 normally favors corresponding westward shifts in the
downstream ridge and trough axes normally located over western and eastern North
America, respectively. This latter feature was not present during January.
The 500-hPa circulation over
eastern North America continued to reflect a strong 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, while the negative anomalies extended from the eastern U.S. to
southern Europe. 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).
Consistent with this pattern,
monthly 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 January
included below average totals in the western and eastern U.S., with many areas
recording totals in the lowest 10th percentile of occurrences (Fig.
the U.S., many areas recorded a multi-month continuation of below average
precipitation. These areas included the Southwest, the Southeast, the Gulf
Coast, the Great Plains, and the Great Lakes region.
b. North Atlantic
Across the extratropical North
Atlantic, the 500-hPa circulation during January 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
A characteristic cool-season
feature of the negative NAO is southward shift of the mean North Atlantic jet
stream (Fig. T21).
During January, the mean Atlantic jet stream entered the continent in northern
Africa, which is well south of its normal position near Great Britain (Fig. E10). As
a result, the normal northward heat transport associated with this jet stream
was greatly diminished, and was replaced by an extensive northwesterly flow of
much colder air across Europe and western Russia. These conditions contributed
to well below average temperatures across northern Europe, Scandinavia, and
western Russia, with many locations recording totals in the lowest 10th
percentile of occurrences (Fig. E1).
The subsequent eastward transport of this colder air, combined with a strong
high pressure system and likely enhanced radiative cooling, also contributed to
significantly below average temperatures across central Russia.
2. Southern Hemisphere
Over the South Pacific Ocean, the
500-hPa circulation during January 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 enhanced ridge across Australia
and the western South Pacific, and an amplified trough across central South
Pacific. This overall anomaly pattern is consistent with La Niņa (Fig.
The main temperature anomalies during January included a
continuation of well below average temperatures in eastern Australia, where
departures were again in the lowest 10th percentile of occurrences.
The main precipitation signals reflected above average totals in northwestern
Australia, and across the entire South African monsoon region (Fig.
The South African rainy season lasts from October to April.
During January, rainfall for the region as a whole was the highest for any
January dating back to at least 1979 (Fig.
E4). For the 2010-11 rainy season, precipitation
was slightly below average in October, near average in November, and above
average in both December and January. Seasonal rainfall is typically above
average in this region during La Niņa.