1. Northern Hemisphere
The 500-hPa circulation
pattern during May featured positive height anomalies across the central
North Pacific, northeastern
, and south-central
, and negative anomalies over the eastern
, the eastern
(Fig. E9). In the subtropics, cyclonic
200-hPa streamfunction anomalies over the central North Pacific reflected
the ongoing pattern of suppressed convection near the date line (Figs. T22,
T25). These conditions were again associated
with a pronounced westward retraction of the East Asian jet core and a
westward shift of the associated jet exit region to just east of
The main surface temperature
departures during May reflected warmer than normal conditions across the
(Fig. E1). The main precipitation departures
included above-average totals in central
, and eastern
, and below-average totals in the central and Mid-Atlantic regions of the
, and southern
(Figs. E3, E5, E6).
Some aspects of the
circulation across the North Pacific again reflected the ongoing pattern of
suppressed convection near the date line. In particular, the westward
retraction and confinement of deep tropical convection to the western
Pacific resulted in an amplified trough over the central and eastern
subtropical Pacific (Fig. T22).
Accompanying this pattern, the East Asian jet stream was retracted well west
of the date line, and the associated jet exit region was shifted westward to
just east of
an anomalous upper-level ridge-trough pattern, characterized by
above-average heights in the west and below-average heights in the east, was
evident during the month. This pattern contributed to exceptionally warm
surface temperatures in the southwestern
and to large areas of below-average temperatures in the Mid-Atlantic region
(Fig, E1). A large area of below-average
precipitation was evident in the area of anomalous descending motion between
the ridge and trough axes (Fig. E3).
Area-average totals in both the Inter-Mountain and
regions were in the lowest 10th percentile of occurrences (Fig.
E5), with many locations recording less than
25% of normal precipitation during the month (Fig. E6).
Other areas of the
, including the Southwest, the
, and the Mid-Atlantic region, recorded ongoing precipitation deficits
during May. Each of these regions has experienced well below-average
precipitation since January 2006, and the
has recorded below-average precipitation in every month since September
2005. In contrast,
experienced wetter-than-average conditions during the month. With some areas
recorded the wettest May on record.
The 500-hPa circulation
pattern during May featured a persistent north-south dipole pattern of
anomalies, with negative anomalies extending from the eastern
, and positive anomalies extending across southern
(Fig. E9). This pattern was associated with
enhanced westerly flow and above-average precipitation across northern
, and with well below-average precipitation and above-average temperatures
. Precipitation totals in this region have been in the lowest 10th
percentile of occurrences for the last two months.
recorded well above-average precipitation during May, with totals in many
areas exceeding the 70th percentile of occurrences. For the
region as a whole, area-averaged totals were above the 90th
percentile of occurrences. This enhanced precipitation occurred in the area
of anomalous ascending motion and increased storminess downstream of the
mean upper-level trough position and within the right entrance region of the
East Asian jet stream.
2. Southern Hemisphere
The mean 500-hPa circulation
pattern during May featured generally above-average heights in the middle
latitudes between 40°S-60°S, and below-average heights over southern
Africa, in the area east of both Australia and South America, and across
Antarctica (Fig. E15). Regionally,
was situated between the mean upper-level ridge and trough axes in an area
of in large-scale descending motion, and recorded well below-average
precipitation during the month (Fig. E3).
Accompanying this anomalous circulation, a strong on-shore flow at low
levels contributed to below-average temperatures over much of the region (Fig.
was also situated between the mean upper-level ridge and trough axes, and
experienced anomalous onshore flow from the
Great Australian Bight
. This combination of conditions led to anomalously cool and dry conditions,
with rainfall totals and temperatures in many areas dropping below the
lowest 10th percentile of occurrences.
the rainy season, which normally lasts from October to April, continued to
produce beneficial rains in May. For the 2005-06 rainy season precipitation
was above average in every month since October. An enhanced South African
rainy season is consistent with a La Niņa episode.