– February 2012
1. Northern Hemisphere
The 500-hPa circulation during February
featured a persistent pattern of above-average heights over the eastern North
Pacific, the eastern North Atlantic, and north-central Russia, and
below-average heights over the high latitudes of the North Pacific, northern
Africa, southern Russia, and eastern Siberia (Figs. E9, E11).
Over the subtropical Pacific Ocean, the 200-hPa circulation featured cyclonic streamfunction anomalies in both hemispheres east of the
Date Line (Fig. T22). This pattern is linked to La Niña,
and reflects enhanced mid-Pacific troughs in both hemispheres flanking the
suppressed convection over the central equatorial Pacific (Fig. T25).
The main land-surface temperature signals
during February included well above-average temperatures in Alaska, Canada, the
eastern U.S., and north-central Russia, and below-average temperatures across
Europe and western Russia (Fig. E1). The main precipitation signals (Figs. E3, E6) included above-average totals along the U.S.
Gulf Coast and southeastern Europe, and below-average totals along the U.S.
eastern seaboard, western Europe, and northwestern Russia.
a. North Pacific and North America
The circulation over the North
Pacific continued to exhibit a La Niña influence. La Niña is associated with deep tropical
convection focused over Indonesia and the eastern Indian Ocean, along with a
disappearance of tropical convection from the 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
results in a westward retraction the east Asian jet
stream, along with a westward-shift and amplification of the jet exit region (Fig. T21).
During February, the east Asian jet core was focused
over the western Pacific, and the jet exit region began well west of the date
line (Fig. T21). The jet exit region
was also enhanced between 150oE-180, as indicated
by anomalous southeasterly winds immediately south of the jet axis.
Over the eastern North Pacific and
western North America, the mean 500-hPa circulation during February featured a
4-celled anomaly pattern, with amplified ridges over the eastern North Pacific
and northwestern Canada, and troughs over Alaska and the southwestern U.S. (Fig. E9).
This pattern was associated with an enhanced flow of mild marine air into North
America (Fig. T21, E10),
which contributed to well above-average surface temperatures (departures
exceeding +5oC) across eastern Alaska and Canada (Fig.
The circulation also featured an
eastward shift of the mean Hudson Bay tough (HBL) to the western North Atlantic,
which resulted in a reduced southward transport of cold air from Canada and to
well above-average temperatures across eastern North America. This eastward shift
of the HBL also affected in two ways the precipitation patterns across the
eastern half of the U.S. First, it meant that the eastern seaboard was located
upstream (rather than downstream) of the trough axis in an area of
below-average precipitation and anomalous descending motion. Second, it allowed for a southerly flow of
marine air and ascending motion into the south-central U.S. and along the Gulf
Coast. These conditions contributed to well above-average precipitation along
the Gulf Coast (Fig. E5), with southeastern Texas and Louisiana recording
totals in excess of 175% of normal (Fig.
Despite this enhanced precipitation
much the southern U.S., including Gulf Coast region, continued to experience moderate
to exceptional drought during February. Exceptional drought conditions
persisted in Texas and western Oklahoma, and extreme drought spanned the area
from northern Florida to southeastern South Carolina.
b. North Atlantic and Eurasia
At high latitudes, the 500-hPa
circulation during February featured an amplified wave pattern that extended
from the eastern North Atlantic to eastern Siberia (Figs. E9, E11).
Features of this pattern included blocking ridges over the eastern North
Atlantic and north-central Russia, and deep troughs over western Russia and
eastern Siberia. At lower latitudes, the circulation featured an extensive trough
from northwestern Africa to the Caspian Sea.
These overall conditions resulted
in a southward and eastward transport of cold air across Europe and western
Russia, which resulted in well below-average surface temperatures across the
region (Fig. E1).
In many areas, monthly temperature departures were in the lowest 10th
percentile of occurrences.
The above circulation anomalies
also strongly affected the precipitation patterns, with exceptionally low
totals observed over western Europe and north-western
Russia downstream of the mean ridge axes, and above-average totals observed
over southeastern Europe and southern Russia in association with the mean
trough axis (Fig. E3).
2. Southern Hemisphere
The mean 500-hPa circulation during
February featured generally below-average heights in the middle latitudes and
above-average heights over the high latitudes of the central North Pacific (Fig. E15). At 200-hPa, the subtropical
circulation featured an enhanced mid-Pacific trough in response to the suppressed
convection over the central equatorial Pacific (Figs. T22, T25).
A similar anomaly pattern was also evident in the Northern Hemisphere,
and is consistent with La Niña.
The South African rainy season lasts from October to
April. During February, rainfall for the region as a whole was below average (Fig. E3), and
area-averaged totals were in the lowest 30th percentile of
occurrences (Fig. E4).
The most significant deficits were observed in Mozambique, where totals were in
the lowest 10th percentile of occurrences. To date, precipitation
for the 2011-12 rainy season has been near average,
with near-average totals recorded during October through January and
below-average totals observed in February. Seasonal rainfall in this region is often
above average during La Niña.