Extratropical Highlights – January 2012
1. Northern Hemisphere
The 500-hPa circulation during January
featured a persistent pattern of above-average heights over the eastern North
Pacific, the central North Atlantic, and northern Russia, and below-average
heights over the western North Pacific, Alaska, western Canada, and the eastern
Mediterranean Sea (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 January included well above-average temperatures across central North
America and northwestern Russia, and below-average temperatures in Alaska and
central Asia (Fig. E1).
The main precipitation signals (Figs. E3, E6) included above-average
totals in the Northwest and Ohio Valley regions of the U.S., and below-average
totals along the U.S. eastern sea-board and in the central Plains states, and
also in western Europe.
a. North Pacific and North America
The circulation over the North
Pacific continued to reflect a strong 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 January, the east Asian jet core was focused
over Japan, 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 is
indicated anomalous southerly winds immediately south of the jet axis and
anomalous northerly winds north of the jet axis.
Over North America, the mean
500-hPa circulation during January featured a deep trough over Alaska and
western Canada, and an enhanced ridge over the southwestern U.S. (Fig. E9).
This pattern was associated with an enhanced zonal flow of mild marine air into
North America (Fig. T21, E10),
which resulted in well above-average surface temperatures across the center of
the continent (Fig. E1). In many
areas temperature departures exceeded +4oC, and were in the upper 90th
percentile of occurrences. In Alaska, the deep trough was associated with an
enhanced flow of polar air into the western and southern parts of the state,
resulting in well below-average surface temperatures with some areas recording
totals in the lowest 10th percentile of occurrences.
The circulation also featured a
deep trough extending from Hudson Bay to the south-central U.S. Flanking this
trough was below-average precipitation over the central Plains states in the
area upstream of the mean trough axis, and above-average precipitation
extending from eastern Texas to New York in the area downstream of the mean
trough axis (Fig. E6).
Also evident during January was
below-average precipitation along the U.S. Gulf Coast and extending well up the
eastern Seaboard. The combination of above-average precipitation in the Ohio
Valley and below-average precipitation along the Gulf coast is consistent with
Much of U.S. southern Plains and
Southwest continued to experience severe drought in January, with exceptional
drought persisting in Texas and western Oklahoma. The area from the western
Florida Panhandle to southeastern South Carolina recorded extreme drought in
January, with large portions of southern Georgia recording exceptional drought.
b. North Atlantic and Europe
The 500-hPa circulation during January
featured strong ridges over the eastern North Atlantic and also over western
Russia, and a deep trough extending southward from Scandinavia to the eastern
Mediterranean Sea (Figs. E9, E11). This overall
circulation was associated with an enhanced northward transport of mild air
into Europe, and with below-average precipitation across western
Europe (Fig. E3).
It was also associated with a two-celled
pattern of precipitation anomalies flanking the amplified ridge in western
Russia, with well above-average (below-average) precipitation observed in the
area upstream (downstream) of the ridge axis (Fig. E3).
2. Southern Hemisphere
The mean 500-hPa circulation during
January featured an anomalous zonal wave-2 pattern in the middle latitudes, along
with below-average heights throughout the polar region (Fig. E15). At 200-hPa, the subtropical
circulation featured an enhanced mid-Pacific trough in response to suppressed
convection over the central equatorial Pacific (Figs. T22, T25).
A similar 200-hPa 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 January, rainfall for the region as a whole was near average (Fig. E4). To
date, precipitation for the 2011-12 rainy season was
near-average during October through January. Seasonal rainfall is typically
above average in this region during La Niña.