1. Northern Hemisphere
500-hPa circulation during February featured an anomalous zonal wave-1 pattern
with above-average heights at high latitudes and below average heights in the
middle latitudes (Fig. E9).
This overall pattern reflected the combination of El Niño and a record negative
phase of the Arctic Oscillation (-4.4) (Fig.
A2.1). Consistent with this AO signal, the
second strongest (after 1978) negative phase of the North Atlantic Oscillation (NAO)
for February was also recorded (-2.0) (Table
E1, Fig. E7).
similar combination of climate patterns has been in place since December 2009.
During December-February (DJF) 2009-10, the AO index averaged -3.4 and the NAO
index averaged -1.7, both of which are record values dating back to 1950.
February 2010, regional manifestations of the combined El Niño/AO/NAO signals
included a deep trough over the Gulf of Alaska, a significantly weaker than
average Hudson Bay trough, high-latitude blocking over the Atlantic sector, and
wintertime jet streams shifted well south of normal over both the Pacific and
Atlantic Oceans (Fig. T21). Consistent with
this anomalous circulation, the main temperature signals during February
included above average temperatures across Canada and Alaska, and below average
temperatures in the central and eastern U.S., Scandinavia, and much of northern
Russia (Fig. E1). The main precipitation signals
included above-average totals along the U.S. Gulf Coast region and southern
Europe, and below average totals in southwestern Alaska and western Canada (Fig.
a. North Pacific/
El Niño impacts during February included a 4-celled pattern of 500-hPa height
anomalies, with above average heights over the subtropical North Pacific Ocean
and eastern Canada, and below average heights over the eastern North Pacific and
southeastern U.S. (Fig. E9).
anomaly pattern is consistent with El Niño’s impacts on the structure and
location of the East Asian jet stream. Normally, the core of the East Asian jet
stream is located well west of the date line, and the jet exit region is
centered near the date line. During El Niño, convection is enhanced over the
central equatorial Pacific (Fig. T25),
which acts to strengthen and extend eastward the subtropical ridge across the
Pacific basin (Fig. T22).
As seen in February, these conditions lead to 1) an eastward extension of the
East Asian jet core, 2) a shift of the jet exit region toward the eastern
Pacific, and 3) a southward shift of the jet axis toward the southwestern U.S. (Fig. T21). As a result, the
Pacific storm track was shifted well south and east of normal, which contributed
to above average precipitation and increased storminess in the southern U.S. (Fig.
of major jet streams, one normally sees a split-flow pattern with a broad ridge
to the north and trough to the south. During February, the combination of
positive height anomalies over eastern Canada and negative height anomalies over
the southeastern U.S. is consistent with the El Niño-related changes in the
East Asian jet stream. These anomalies reflect a weaker than normal Hudson Bay
trough, and an amplified trough over the southeastern United States.
eastern North America the circulation anomalies during February were also
associated with a record negative phase of the Arctic Oscillation (-4.4) (Fig.
A2.1), and with a strong negative phase of the
North Atlantic Oscillation (-2.0) (Table E1,
Fig. E7). In combination with El Niño, these
conditions contributed to above average temperatures across Canada and to below
average temperatures across the eastern half of the U.S. They also contributed
to three major snow storms along the eastern seaboard of the U.S., and to above
average precipitation and increased precipitation across the southern part of
February, the AO/ NAO signal dominated the circulation across the North Atlantic
Ocean and Europe (Table E1,
Fig. E7). Specific aspects of this signal included
an blocking ridge at high latitudes and a deep trough extending from the
southeastern U.S. to southeastern Europe (Fig. E9). These conditions were associated
with a pronounced southward shift and zonal elongation of the North Atlantic jet
stream, with the main jet axis extending from the U.S. Gulf Coast to southern
Europe (Fig. T21).
February marks the third consecutive month with an NAO index below -1.0, and DJF
2009-10 marks the largest negative NAO (and AO) index dating back to 1950.
February 2010, these conditions were associated with north-south dipole patterns
of temperature (Fig. E1)
and precipitation (Fig. E3)
anomalies across the North Atlantic, Europe, northern Africa, and Russia.
Overall, the higher latitudes received below average temperatures and
precipitation, while the lower latitudes recorded above average temperatures and
2. Southern Hemisphere
circulation during February featured above average heights over Antarctica, New
Zealand, and the eastern South Pacific, and below average heights from South
America to the eastern South Atlantic (Fig.
E15). Regionally, an amplified trough over the
eastern South Pacific contributed to cooler (Fig.
E1) and wetter (Fig.
E3) than average conditions across extratropical
South America, with portions of southern Argentina recording temperature
departures in the lowest 10th percentile of occurrences and
precipitation totals in the upper 90th percentile of occurrences. In
Australia, precipitation was generally well below average in the west and well
above average in the southeast. This pattern is associated with an amplified
trough centered in the western part of the continent.
In southern Africa,
the rainy season lasts from October to April. During February 2010, totals were
near average with slightly below average precipitation recorded in portions of
Mozambique and above average precipitation recorded in extreme South Africa. The
South African rainy season tends to below average during El Niño.