1. Northern Hemisphere
The 500-hPa height
field during April 2009 featured positive anomalies over the
Gulf of Alaska
, the central
, and eastern
, and negative anomalies over the high latitudes of the
, the circulation continued to reflect a westward retraction of the East Asian
jet stream (Fig. T21)
and a westward shift of the downstream ridges and troughs. These conditions
partly reflect a lingering La Niņa signal.
temperature signals during April included above average temperatures in
, and below average temperatures in northwestern
and in the vicinity of the
The main precipitation signals included above-average totals in the central
and east of the
, and below-average totals in western
a. North Pacific/
At 200-hPa, La Niņa
again contributed to enhanced mid-Pacific troughs in both hemispheres flanking
the suppressed equatorial convection, and a westward retraction of the
subtropical ridges toward Australasia (Fig. T22).
Associated with this pattern, the East Asian jet stream was retracted westward
toward the western Pacific (Fig. T21).
These conditions contributed to a westward shift to the
Gulf of Alaska
of the mean ridge normally located over western
, and a westward shift of the mean
trough toward western
were associated with cooler (Fig. E1)
and drier (Fig. E3)
than average conditions from southern
. They were also associated with well above average precipitation in large
portions of the central and northern Plains regions of the
(Figs. E3, E6).
, the circulation during April featured a 4-celled pattern of 500-hPa height
anomalies (Fig. E9). This pattern reflected an
anomalously zonal jet stream configuration across the
, and a pronounced split-flow pattern over
. The anomalous ridge over
contributed to well above average temperatures in eastern Europe (Fig.
E1), and to an extensive area of significantly
below average precipitation across
Also during April an anomalous wave pattern contributed to cooler and wetter
than average conditions in the vicinity of the
, and to well above average temperatures in northeastern
2. Southern Hemisphere
The circulation during April continued to reflect
some aspects of La Niņa, with an enhanced ridge over the central South Pacific
and an anomalous trough over the high latitudes of the eastern South Pacific (Figs.
Also evident during the month was a zonal wave-4 pattern, with above average
heights over southern
, the eastern
, and the eastern
, and below average heights over the western
La Niņa signal was again evident in the pattern of below average precipitation
E3). This region has received below average
rainfall in every month since November 2008, and during April 2009 the totals
were in the lowest 10th percentile of occurrences (Fig.
, the rainy season extends from October
through April. Rainfall during April was slightly below-average for the region
as a whole, although portions of
recorded well below average totals (Figs.
E3, E4). Overall, the
2008-09 rainy season has been above average, which is consistent with La Niņa.