1. Northern Hemisphere
The 500-hPa circulation
pattern during June featured a persistent positive height anomalies in the
middle latitudes, and negative anomalies in the polar region (Figs. E9,
E11). The main centers of positive height
anomalies were observed over western
North America
, the high latitudes of the
North Atlantic
, northern
Europe
, and
Siberia
, and coincided with areas of significantly above-average surface
temperatures (Fig. E1). The main
precipitation departures during June included well above-average totals in
the Mid-Atlantic and northeastern
U.S.
, and below-average totals in the central
U.S.
and both central and northern
Europe
(Figs. E3, E5, E6).
a.
North America
Over
North America
, an anomalous upper-level ridge-trough pattern, characterized by
above-average heights in the west and an amplified trough over the eastern
U.S.
, was evident during much of June. This pattern contributed to exceptionally
warm surface temperatures in the western
U.S.
and much of
Canada
(Fig, E1). It also contributed to a large
area of below-average precipitation in the area between the ridge and trough
axes, with area-averaged totals across the central
U.S.
generally in the lowest 30th percentile of occurrences. In
contrast, precipitation was significantly above-average downstream of the
mean trough axis, with area-averaged totals in the Mid-Atlantic and
northeastern regions of the
U.S.
exceeding the 90th percentile of occurrences (Fig. E5).
In the Northeast, periods of exceptionally heavy rainfall resulted in
severe flooding during the month. Elsewhere,
the
Gulf
Coast
region of the
U.S.
recorded ongoing precipitation deficits during June, and has recorded
below-average precipitation in every month since September 2005.
b.
North Atlantic
/
Eurasia
The 500-hPa circulation pattern during June featured persistent
above-average heights across the high latitudes of the
North Atlantic
, northern
Europe
, and
Siberia
(Fig. E9). This pattern was associated with
above-average temperatures across
Eurasia
, with mean departures exceeding 2°C over large parts of
Europe
and 3°C over central
Russia
. It was also associated with below average precipitation over much of
Europe
, with many areas recording totals in the lowest 10th percentile
of occurrences. In southern
Europe
precipitation has been below-average since April 2006.
2. Southern Hemisphere
The mean 500-hPa
circulation pattern during June featured persistent above-average heights
south of
Australia
and
South
America
, and across the subtropical South Pacific, and
below-average heights north of
New Zealand
, over the eastern South Pacific, and over the southern
Indian Ocean
(Fig. E15).
Regionally, central
South America
recorded well above-average temperatures in
association with an anomalous low-level flow of tropical origin from the
Amazon
Basin
(Figs. E3, T20).
In
Australia
the persistent anomalous anticyclonic circulation
south of the continent was associated with extensive onshore flow at low
levels from the south and east, and with anomalous descending motion across
the south. This combination of conditions led to anomalously cool
temperatures across the continent, and to well below-average precipitation
across the southeast and west where totals were in the lowest 10th
percentile of occurrences. Similar conditions also contributed to
anomalously cool and dry conditions over much the continent during May.
|