500-hPa height pattern during June 2008 featured positive anomalies across
and the high latitudes of the North Pacific,
, and southern
, and negative anomalies over
(Fig. E9). Over the
, the subtropical circulation at 200-hPa continued to reflect La Niņa, with
enhanced mid-Pacific troughs and cyclonic circulation anomalies in both
hemispheres flanking the suppressed convection over the central equatorial
Pacific (Fig. T22).
main surface temperature departures during June reflected warmer than average
conditions across the southern and eastern US, eastern Europe and northeastern
, and below-average temperatures over the north-central US, western
, and northern
(Fig. E1). The main precipitation
anomalies included above average totals in the midwestern
, and southeastern
, and below-average totals in northern
states, the southeastern US, and northeastern
a. North Pacific/
La Niņa signal was again prominent across the subtropical
during June. La Niņa is associated with a westward retraction of deep tropical
, and with 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-hPasubtropical ridge toward
(Fig. T22). Over the central
equatorial Pacific, the reduction in convective heating contributes to an
increased strength of the mid-Pacific trough.
strength, structure, and position of the East Asian jet stream are strongly
linked to conditions in the tropics and subtropics. 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. During La Niņa, the core
of the East Asian jet stream is often retracted westward toward
, (Fig. T21). This relationship is
normally most evident in winter and spring, but was also pronounced in June in
response to the ongoing La Nina-related pattern of tropical convection.
with these conditions, the 500-hPa heights were above average over the high
latitudes of the eastern North Pacific, and below-average over the north-central
US. This overall pattern was
associated with a continuation of below-average precipitation in northern
and the Gulf coast region of the
. Both of these areas have recorded significant precipitation deficits during
the past several months. In northern
many locations had recorded almost no rainfall since late March, which set the
stage for numerous (1781) uncontrolled wildfires that burned over 702,000 acres
during June and early July.
500-hPa circulation over the
also exhibited considerable variability during June. In early June, the
circulation featured a trough in the west and a ridge in the east. This pattern
was associated with a persistent frontal boundary and significant low-level
moisture transport across the
. These conditions contributed to a series of major rainfall events and severe
. Overall, the
has recorded well above-average precipitation since February (Fig.
500-hPa circulation during June featured a north-south dipole pattern of height
, with below-average heights to the north and above-average heights to the south
(Fig. E9). This pattern mainly contributed to
well above-average temperatures in eastern Europe, with many areas recording
departures in the upper 90th percentile of occurrences (Fig. E1).
June, the eastern flank of the normal Asian monsoon ridge at 200-hPa was weaker
than average in association with a broad trough over eastern
This pattern favored an anomalous southward penetration of frontal boundaries
and mid-latitude storms which, in turn, favored enhanced convection and
above-average rainfall over southeastern and eastern
2. Southern Hemisphere
The 500-hPa height field during June exhibited a strong zonal symmetry, with
above average heights in the middle latitudes and below average heights over
(Fig. E15). In the subtropics, ongoing
positive 200-hPa streamfunction anomalies over the central
indicated an anomalous cyclonic circulation and enhanced mid-Pacific trough
consistent with La Niņa (Fig. T22).
an anomalous 500-hPa trough contributed to anomalously warm and dry conditions
during June (Figs. E1, E3).