Beginning with this
month, all anomalies reflect departures from the 1981-2010 base period.
500-hPa circulation during February featured above average heights across the
high latitudes of the North Pacific and portions of the polar region, and below
average heights extending from western North America to central Asia (Fig.
E9). This pattern projected strongly onto the
negative phase (-2.4) of the Pacific/ North American teleconnection pattern (Table E1, Fig. E7).
It is notable that for the first month since September 2009, the monthly 500-hPa
circulation did not project strongly onto the negative phase of the North
Atlantic Oscillation (NAO).
200-hPa, the circulation during February featured enhanced troughs in both
hemispheres over the tropical and subtropical central/eastern Pacific (Fig.
In the NH, this feature contributed to a westward retraction of the East
Asian jet and to an enhanced jet exit region. Such a jet structure favors a
westward retraction of the downstream ridge and trough axes, as seen during
February by above average 500-hPa heights over the high latitudes of the central
North Pacific and a deep trough over the western United States. These features
are consistent with the ongoing mature phase of La Niña.
to near-average surface temperatures prevailed over the NH land masses during
February (Fig. E1).
The most significant negative departures were observed in the western U.S. and
western Russia. The main precipitation signals during February included
below-average totals along the U.S. Gulf Coast, southeastern Europe, and the
southern half of China (Fig. E3).
a. North Pacific
and North America
mean 500-hPa circulation during February featured a strong ridge over the high
latitudes of the central North Pacific, and a deep trough over much of the U.S.
(Fig. E9). This pattern showed links to La
Niña, and projected strongly onto the negative phase (-2.4) of the Pacific/
North American teleconnection pattern (Table
E1, Fig. E7).
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 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. All of these features were present during February.
the U.S., the main surface temperature signals during February included below
average temperatures in the West, and above average temperatures in the
mid-Atlantic region (Fig. E1). The main precipitation anomalies
included below average totals in the Pacific Northwest and along the U.S. Gulf
Coast (Fig. E5).
This marks the fifth consecutive
month with well below average precipitation along the U.S. Gulf Coast (Fig.
the North Atlantic, the 500-hPa circulation during February featured below
average heights in the middle and high latitudes, and above average heights at
lower latitudes. This pattern
projected weakly onto the positive phase of the North Atlantic Oscillation (NAO)
(Table E1, Fig. E7).
This represents a notable change from the strong negative phase of the NAO that
had prevailed from October 2009 – January 2011.
with this change, the main Atlantic jet stream entered Europe in its more normal
position over Great Britain, rather than over southern Europe (or even farther
south) as had been seen since October 2009. The associated enhanced northward
heat transport brought above average temperatures to much of Europe during
February. These conditions differ markedly from those observed in January, when
extensive northwesterly flow brought much colder air across Europe.
anomalies reflect departures from the 1981-2010 base period.
500-hPa circulation during February featured 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 amplified trough across the
central South Pacific (Fig. T22). Over the South
Pacific Ocean, the overall height anomaly pattern is consistent with La Niña.
precipitation signals during February reflected above average totals across
Australia, with departures exceeding the 90th percentile of
occurrences across large portions of the south and west (Fig.
E3). Much of central South America also recorded
above average precipitation.
The South African
rainy season lasts from October to April. During February, rainfall for the
region as a whole was below average, with the most significant deficits observed
in Mozambique and northeastern South Africa (Fig.
E4). For the 2010-11 rainy season, precipitation
was below average in October and February, near average in November, and above
average in both December and January. Seasonal rainfall is typically above
average in this region during La Niña.