Beginning with this
month, all anomalies reflect departures from the 1981-2010 base period.
500-hPa circulation during April featured above average heights across the
southern United States, Europe, and eastern Siberia, and below average heights
extending from Canada to Greenland, over western Russia, and across the western
North Pacific (Fig. E9). This pattern projected strongly
onto several main teleconnection patterns, including the positive phase (+2.6)
of the North Atlantic Oscillation (NAO), and the negative phases (-1.9 and -1.8,
respectively) of the West Pacific and Pacific/ North American (PNA) patterns.
200-hPa, the circulation featured enhanced troughs in both hemispheres over the
tropical and subtropical central/eastern Pacific (Fig.
This pattern is consistent with La Niņa.
main surface temperature signals during April included well above average
temperatures across the south-central U.S. and eastern Siberia, and below
average temperatures in the northwestern U.S. and western Canada (Fig.
E1). The main precipitation signals included above
average totals in the northwestern, Midwestern and northeastern U.S., and
below-average totals along the U.S. Gulf Coast, much of Europe, and northeastern
China (Fig. E3).
a. North Pacific
and North America
the lower latitudes, the 200-hPa circulation during April featured a 4-celled
anomaly pattern of streamfunction anomalies characterized by amplified ridges
over Australasia and amplified mid-Pacific troughs in both hemispheres (Fig.
T22). These conditions are linked to
the La Niņa- related pattern of tropical convection, with enhanced subtropical
ridges flanking the region of deep convection over Indonesia and the eastern
Indian Ocean, and the amplified troughs flanking the region of suppressed
convection over the central Pacific (Fig.
the extratropics, the mean 500-hPa circulation during April featured a broad
trough across the entire Pacific Basin, along with deep trough across Canada.
These conditions were accompanied by above average heights across the eastern
Pacific and the southern U.S. This overall pattern was associated with enhanced
jet stream winds and increased storminess from the eastern Pacific to the
northeastern U.S. (Fig. T21).
It was also associated with an enhanced low-level frontal boundary that extended
across the Midwestern U.S. This combination of conditions led to well above
average precipitation across the central U.S., with the Great Lakes, Midwest,
and Ohio Valley regions each recording totals in the upper 90th
percentile of occurrences (Fig. E5).
Many locations recorded precipitation surpluses exceeding 150 mm, which was
generally more 200% of normal. As a result, by early May there was significant
flooding of the Mississippi River.
extensive upper-level trough across Canada also contributed to significantly
cooler (Fig. E1) and wetter than normal (Fig. E3) conditions in the northwestern
quadrant of the U.S., where many locations recorded temperatures in the lowest
10th percentile of occurrences and precipitation totals in the upper
90th percentile of occurrences.
the U.S. Gulf Coast continued to experience exceptionally dry conditions in
association with above average 500-hPa heights across the region. April marks
the 7th consecutive month with area-averaged totals in the lowest 10th
percentile of occurrences for this region.
North Atlantic and Europe
500-hPa circulation during April featured a large-amplitude trough centered just
west of Greenland, and above average heights across the western North Atlantic
and Europe (Fig. E9).
This pattern projected strongly onto the positive phase (+2.6) of the NAO.
circulation was associated with enhanced southwesterly winds across Scandinavia,
and with a strong poleward transport of heat into the high latitudes of Eurasia.
The ridge over Europe resulted in exceptionally warm and dry conditions, with
many areas recording temperatures in the upper 90th percentile of
occurrences and precipitation totals in the lowest 30th percentile of
occurrences. For northern Europe, this marks the second straight month with
area-averaged totals in the lowest 10th percentile of occurrences (Fig.
the past three months, the 500-hPa circulation over northern Russia and Siberia
has featured a trough in the west and a ridge in the east, accompanied by a deep
southwesterly flow of mild air into the high latitudes of Eurasia (Fig.
E9). These conditions led to an early snow-melt,
and to a continuation of well above average temperatures during April across
most of Siberia.
during April, precipitation was well below average in northeastern China. This
dryness continued from March, in response to upper-level convergence (Fig. T23) focused in the area
between an upstream amplified upper-level ridge and a downstream amplified
trough (Fig. E9). Much of the region was also
situated in enhanced sinking motion within an enhanced left entrance region of
the East Asian jet stream (Fig. T21).
anomalies reflect departures from the 1981-2010 base period.
500-hPa circulation during April featured above average heights throughout the
polar region, along with below average heights over southern South America and
southern Africa (Fig. E15).
In the subtropics, the upper-level (200-hPa) streamfunction pattern reflected an
amplified trough across the central South Pacific, and an amplified ridge over
Australia (Fig. T22).
These circulation anomalies are consistent with La Niņa.
The South African
rainy season lasts from October to April. During April, rainfall for the region
as a whole was above average, with area-averaged totals in the upper 80th
percentile of occurrences (Fig. E4).
Rainfall during the 2010-11 rainy season was above average in five of the seven
months, and below average in two months (October and February).
The South African rainy season is often above average during La Niņa.