1. Northern Hemisphere
The 500-hPa height pattern
during April featured positive anomalies over
, and negative anomalies across the high latitudes of the North Pacific, the
, and western
(Figs. E9, E11).
The 200-hPa streamfunction anomaly pattern showed a strong inter-hemispheric
symmetry across the subtropical
, with anomalous ridges west of the date line, and anomalous troughs east of
the date line (Fig. T22). This pattern was
also prevalent in March, and is consistent with enhanced equatorial
convection west of the date line and suppressed convection east of the date
line (Fig. T25).
The main surface temperature
departures during April reflected well above-average temperatures across
, and below-average temperatures over much of the
(Fig. E1). The main precipitation anomalies
included drier than average conditions in the southeastern
, and throughout
In the subtropics, the
upper-level circulation during April again reflected the pattern of
anomalous convection across the equatorial Pacific, with enhanced
subtropical ridges at 200-hPa flanking the region of enhanced convection and
enhanced mid-Pacific troughs flanking the region of suppressed convection (Figs.
The 500-hPa circulation
pattern featured an amplified trough over eastern
and the western
, which contributed to below-average temperatures over much of the
(Figs. E9, E11).
At 200-hPa, this trough was associated with an enhanced jet stream centered
over the mid-Atlantic region and an anomalous anticyclonic circulation
(Fig. T21). The persistence of these
circulation anomalies contributed to suppressed rainfall in both
, with monthly totals in
falling into the lowest 30th percentile of occurrences. Ongoing
has led to major water shortages and wildfires, especially in the south.
The 500-hPa circulation
pattern during April featured a persistent blocking ridge centered over
western Europe (Fig. E9). This feature led
to a continuation of exceptional warmth across much of the continent, with
temperatures in many locations exceeding the 90th percentile of
occurrences (Fig. E1). This persistent ridge
also led to a continuation of well below-average precipitation across much
, with most regions recording totals in the lowest 10th
percentile of occurrences (Figs. E3, E4).
, a persistent upper-level ridge also led to well above-average temperatures
during April, with departures in many regions exceeding the 90th
percentile of occurrences.
2. Southern Hemisphere
In the subtropics and lower mid-latitudes the upper-level
circulation featured an anomalous ridge over
and the western South Pacific, and an enhanced mid-Pacific trough east of
the date line (Fig. T22). This anomaly
pattern is consistent with the anomalous equatorial convection across the
Pacific basin (Fig. T25).
, the persistent upper-level ridge was associated with exceptionally dry
conditions, with totals in many areas falling into the lowest 10th
percentile of occurrences (Fig. E3). In
, the persistent ridge was associated with above-average surface
temperatures, and above-average precipitation in the area upstream of the
mean ridge axis.
, the mean upper-level ridge normally centered over the
was enhanced during April, and extended farther south into the lower
extratropics. This anomaly pattern appears to be associated with the
amplified mid-Pacific trough father upstream, and does not reflect a
response to local convection which was near-average during the month (Fig.
T25). This amplified ridge contributed to well
above-average surface temperatures in southeastern
(Fig. E1), and to below-average
precipitation over south-central
in the vicinity of the mean ridge axis (Fig. E3).
, the rainy season extends from October to April. Precipitation was well
above average during April, with the largest surpluses observed in eastern
(Figs. E3, E4). For
the entire 2006-07 rainy season, area-averaged totals were below-normal in
October, February and March, near-normal in November and December, and above
normal in January and April.