Extratropical Highlights
– March 2013
1. Northern Hemisphere
The 500-hPa circulation during March
featured above-average heights in the polar region and Greenland, along with several
extensive zonally-elongated patterns of height anomalies in the middle
latitudes (Figs. E9,
E11). Features of these extensive patterns
included above-average heights across the North Pacific, and extending from
northern Africa to northern China, and below-average heights extending from the
eastern U.S. to eastern Siberia.
These conditions reflected a north-south
oriented dipole pattern over the North Atlantic Ocean,
and a north-south oriented tri-pole pattern across Eurasia. They projected
strongly onto two teleconnection patterns: the negative phase of the North
Atlantic Oscillation (-2.1, NAO) and the positive phase of the East Atlantic/
West Russia pattern (+2.3) (Table E1, Fig. E7).
The main land-surface temperature signals
during March included above-average temperatures over eastern Canada and most
of southern Eurasia, and below-average temperatures across central North
America and much of northern Eurasia (Fig.
E1).
The main precipitation signals included
above-average totals across southern Europe, and below-average totals in
northern Europe and in the Northwest, Gulf Coast and East Coast regions of the
U.S (Fig. E3).
Also in the U.S., long-term precipitation deficits led to a continuation of extreme
and exceptional drought in the Great Plains states, with drought expanding and
intensifying in the West.
a. North America
The mean 500-hPa circulation during
March featured above-average heights over western North America and eastern
Canada, and below-average heights over the eastern U.S. (Fig. E9). Over eastern North America, the
north-south dipole pattern of anomalies was part of a larger-scale pattern that
reflected a strong negative phase of the NAO (Table E1, Fig. E7).
The surface temperature anomalies
reflected this negative NAO pattern, with above-average temperatures in eastern
Canada and below-average temperatures across central North America and much of
the eastern U.S. (Fig. E1). The main precipitation anomalies over North
America reflected below-average totals across the U.S. Gulf Coast and east
coast, and along much of the west coast (Figs.
E3, E5). The
precipitation deficits in the eastern U.S. were also consistent with the
negative NAO pattern.
Large portions of the U.S. Plains
region continued to be impacted by extreme or exceptional drought. At the end
of March, the “U.S. Drought Monitor” indicated exceptional drought from portions
of Texas northward to central South Dakota, including eastern Wyoming and
southeastern Colorado. Extreme drought
persisted in much of Wyoming, northwestern Iowa, and southwestern Minnesota. In
the southwestern quadrant of the U.S., many regions recorded moderate or severe
Drought, with extreme drought developing across central and eastern New Mexico
and northwestern Nevada (Fig. E3).
b. North Atlantic and Eurasia
The mean 500-hPa circulation during
March featured above-average heights over the polar region and Greenland, and
from northern Africa eastward to northeastern China. It also featured below-average
heights from the eastern U.S. to eastern Siberia (Figs. E9, E11).
Over the North Atlantic, the north-south dipole pattern of height anomalies
reflected a strong negative phase of the NAO (-2.1) (Table E1, Fig. E7).
Typical negative NAO features included a southward shift and zonal elongation
of the North Atlantic jet stream, with the jet core entering Europe over the
Mediterranean Sea instead of in its normal location over Great Britain (Figs. T21,
E10). They also included an anomalous flow of
polar air into northwestern Europe and Scandinavia, and an anomalous flow of
cold Canadian air into the north-central U.S. (Fig. T21).
These conditions produced a
4-celled pattern of surface temperature anomalies typical of the negative NAO (http://www.cpc.ncep.noaa.gov/data/teledoc/nao_tmap.shtml),
which includes above-average temperatures over eastern Canada and southern
Europe/ northern Africa, and below-average temperatures over the eastern half
of the U.S. and northern Europe (Fig. E1). They also produced the typical north-south
dipole pattern of precipitation anomalies (http://www.cpc.ncep.noaa.gov/data/teledoc/nao_pmap.shtml ) over the eastern North Atlantic and Europe/
western Russia (dry in the north, and wetter/ stormier in the south), along
with below-average precipitation in the eastern U.S. (Fig. E3). In all three regions, the
observed precipitation anomalies were quite significant. Totals exceeded the 90th
percentile of occurrences across southern Europe and western Russia, while
deficits were in the lowest 10th percentile of occurrences across
the U.S. Gulf Coast and Scandinavia.
Over Eurasia, the 500-ha
circulation during March also projected onto a strong positive phase of the
East Atlantic/ West Russia pattern (-2.3), which includes above-average heights
over south-central Asia and below-average heights over western Russia (http://www.cpc.ncep.noaa.gov/data/teledoc/eawruss_map.shtml).
This pattern contributed to the enhanced
polar flow of cold air into Scandinavia and northwestern Russia, and also to
enhanced jet stream winds across central Russia which transported the
anomalously cold air across northern Eurasia. It also contributed to the
exceptionally low precipitation over Scandinavia, which was located in an area
of anomalous descending motion upstream of the mean trough axis (Fig. E9).
Additionally, the persistence of the mean ridge over south-central Asia
contributed to well above-average surface temperatures across that region, with
departures generally exceeding the 90th percentile of occurrences (Fig. E1).
2. Southern Hemisphere
The mean 500-hPa circulation during
March featured below-average heights across Antarctica, and an anomalous zonal
wave-3 pattern in the middle latitudes (Fig.
E15). At 200-hPa, the circulation featured an
anomalous trough across southern South America (Fig. T21). This feature was
associated with well below average surface temperatures east of the Andes
Mountains, with departures along southeastern Argentina falling into the lowest
10th percentile of occurrences (Fig.
E1).
The South African rainy season
lasts from October to April. March rainfall for the region as a whole was below
average (Fig. E4), with deficits mostly confined
to northern Mozambique (Fig. E3). To date
for the 2012-13 rainy season, rainfall for the entire region was above-average
during October and January, near-average in December, and below-average in
November, February and March.