Extratropical Highlights – April 2012
1. Northern Hemisphere
The 500-hPa circulation during April
featured a persistent anomalous wave pattern from North America to central
Russia (Figs. E9,
E11). Regional aspects of this pattern included above-average
heights over central North America, the central North Atlantic, and western/ central
Russia, and below-average heights over Europe.
In the subtropics, the 200-hPa circulation
featured cyclonic streamfunction anomalies in both hemispheres near 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 April included well above-average temperatures across the central United
States and western/ central Russia (Fig.
E1). The main precipitation signals (Figs. E3, E6) included above-average totals in the U.S. Pacific
Northwest, western Canada, and much of Europe, and below-average totals in the eastern
U.S. and southwestern 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
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. During April, the East
Asian jet core remained focused near Japan, and the jet exit region was shifted
to well west of the date line (Fig. T21).
Over North America, the mean
500-hPa circulation during April featured an amplified wave pattern, with troughs
along the west and east coasts, a strong ridge over the center of the continent
These conditions were associated with exceptionally warm surface temperatures
across the central U.S., with many areas recording departures in excess of +3oC
and above the 90th percentile of occurrences (Fig. E1). They were also associated with an
enhanced storm track (Fig. E13) and
above-average precipitation across the Pacific Northwest (Figs. E3, E5), and with
above-average precipitation in western Canada.
Below-average precipitation was
recorded across the eastern one-third of the U.S. in the area downstream of the
mean ridge axis. Monthly totals in the Great Lakes region were in the lowest 10th
percentile of occurrences, and the mid-Atlantic and Northeast regions received
below-average totals for the sixth straight month (Fig. E5). By the end of April, extreme or
exceptional drought was present across Georgia, southeastern Alabama, and much
of northern Florida. Severe drought conditions developed across the Delmarva
Peninsula and portions of New England. Exceptional drought conditions persisted
in western Texas and southeastern New Mexico.
b. Europe and Russia
The 500-hPa circulation during April
featured a deep trough across Europe and a strong ridge over western/ central Russia
(Figs. E9, E11). This pattern contributed to exceptionally wet
conditions across Europe (Fig. E3), with
area-averaged totals in both Northern and Southern Europe near the 90th
percentile of occurrences (Fig. E4). The
persistent upper-level ridge was associated with exceptionally warm surface
temperature across western/ central Russia, with many areas recording
departures exceeding +5oC and in the upper 90th
percentile of occurrences.
2. Southern Hemisphere
In the extratropics, the mean 500-hPa
circulation during April featured above-average heights in the middle latitudes
and below-average heights across the high latitudes of the South Pacific Ocean (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 April, rainfall was below average for the region as a whole, with
many areas recording totals in the lowest 30th and 10th
percentile of occurrences (Figs. E3, E4). For the 2011-12
rainy season, precipitation was near average during
October-January and March, and below average during February and April (Fig. E4). Seasonal
rainfall in this region is often above average during La Niña.