The 500-hPa height
pattern during April 2008 featured positive anomalies over the high latitudes of
the North Pacific, eastern
, and northeastern
, and negative anomalies over western
, the eastern
, and central
(Fig. E9). The circulation over the North
was again consistent with marked a weakening and westward retraction of the
East Asian jet stream typical of La Nina (Fig. T21).
The subtropical circulation at 200-hPa was also consistent with La Niņa, with
enhanced mid-Pacific troughs in both hemispheres flanking the suppressed
convection over the central equatorial Pacific (Fig. T22).
The main surface
temperature departures during April reflected significantly warmer than average
conditions over eastern
and eastern Europe, and below-average temperatures over western
(Fig. E1). The main precipitation anomalies
included above average totals in the midwestern
and over southern
, and below-average totals in the northwestern and
areas of the
a. North Pacific/
The La Niņa signal
was again prominent across the
during April. La Niņa is associated with a westward retraction of deep
tropical convection toward
, and a complete disappearance of tropical convection from the central
equatorial Pacific (Fig. T25).
These conditions result in a westward retraction of deep tropospheric heating,
and hence a westward retraction of the 200-hPa subtropical ridge toward
(Fig. T22). Over the central
equatorial Pacific, the reduction in convective heating contributes to an
increased strength of the mid-Pacific troughs in both hemispheres.
structure, and position of the East Asian jet stream are strongly linked to
conditions in the tropics and subtropics. For example, the jet core coincides
with the strongest north-south height gradient at 200-hPa, which is heavily
influenced by the height anomalies in the subtropics. The jet exit region
coincides with the area of strong diffluence between the subtropical ridge and
trough axes. During La Niņa, the core of the East Asian jet stream is often
retracted westward toward Asia, and the heart of the jet exit region is shifted
westward to west of the date line (Fig. T21).
, the circulation during April featured a westward shift of the mean 500-hPa
ridge and trough axes, such that the mean
trough was located over the western US (Fig. E9).
This shift, which is consistent with the anomalous East Asian jet stream,
contributed to below-average temperatures over western
and above-average temperatures in eastern
(Fig. E1). It also contributed to
above-average precipitation across the midwestern and
regions of the
(Figs. E3, E5). In
contrast, below-average precipitation was observed in the
regions of the
. This marks the sixth consecutive month with below-average precipitation in the
region, which is consistent with La Nina (Fig. E5).
circulation during April featured an anomalous trough over the eastern
and an anomalous ridge over
. This pattern reflected a strong negative phase (-1.07) of the North Atlantic
Oscillation (Table E1). These conditions were
associated with increased storminess and above-average precipitation over
portions of southern and central
2. Southern Hemisphere
The 500-hPa circulation
pattern during April generally featured an anomalous zonal wave-3 pattern in the
middle latitudes, with above-average heights over the high latitudes of the
eastern South Pacific, the eastern
, and the polar region, and below-average heights east of the three continents (Fig.
E15). In the subtropics, the ongoing pattern of
positive streamfunction anomalies over the central Pacific reflected an enhanced
mid-Pacific trough consistent with La Niņa (Fig. T22).
In eastern Australia, a persistent upper-level trough led to a continuation of significantly cooler
and drier than average conditions (Figs. E1, E3).
The South African rainy season extends from
October to April, and is often stronger than average during La Niņa. The region
received near-normal precipitation during March and April, which followed
below-average totals in February (Fig. E4).
Overall the 2007-08 rainy season has been above average, with significant
surpluses recorded from October through January.