1. Northern Hemisphere
The 500-hPa height pattern
during May featured positive anomalies over the high latitudes of the North
Pacific, eastern
North America
, the eastern
North Atlantic
, and south-central
Russia
, and negative anomalies across the high latitudes of the
North Atlantic
to western
Siberia
(Figs. E9, E11).
The main surface temperature
departures during May reflected above-average temperatures across much of
the
United States
,
Europe
, and most of southern and eastern
Asia
(Fig. E1). The main precipitation anomalies
included above average totals in the central
U.S.
, northwestern
Europe
, and north-central
Russia
, and below average totals across the eastern
U.S.
and portions of the western
U.S.
(Fig. E3).
a.
North America
The 500-hPa circulation
pattern featured a strong ridge over eastern
North America
and a broad trough over the western
United States
(Figs. E9, E11).
This anomalous wave pattern strongly controlled the monthly temperature and
precipitation patterns. Above-average precipitation in the Plains States
coincided with the area of large-scale ascending motion downstream of the
mean trough axis, while anomalously warm and dry conditions across the
eastern half of the U.S. coincided with the mean upper-level ridge. The most
significant precipitation deficits occurred in the Southeast,
Ohio
Valley
,
Great Lakes
, and Mid-Atlantic, regions, where area-averaged totals were in the lowest
10th percentile of occurrences (Fig. E5).
By the end of the month, ongoing precipitation deficits had resulted in
moderate to severe drought across the southeastern
U.S.
, with the extreme drought conditions observed in
Mississippi
,
Alabama
,
Georgia
,
Tennessee
, and
Florida
.
b.
Eurasia
The 500-hPa circulation
pattern during May featured a north-south dipole of height anomalies, with
negative anomalies across the high latitudes of the
North Atlantic
and positive anomalies farther south (Fig. E9).
This pattern was associated with an enhanced westerly flow of marine air
into northern
Europe
and southern
Scandinavia
, which contributed to above-average surface temperatures and increased
precipitation throughout the region.
A similar north-south dipole
pattern of height anomalies was also observed over central
Asia
. The persistent positive height anomalies across the southern latitudes
contributed to above-average surface temperatures extending from the
Middle East
to
China
. Farther north, enhanced zonal flow led to fewer cold-air outbreaks and
warmer surface temperatures in northeastern
Asia
and eastern
Siberia
.
2. Southern Hemisphere
The
500-hPa height anomaly pattern during May featured a pronounced zonal
symmetry, with positive height anomalies in the middle latitudes (especially
extending from eastern
Australia
to
South America
) and over
Antarctica
, and negative height anomalies in the southern extratropics. The
circulation across southern
Australia
featured above-average heights at 500-hPa centered just west and east of the
continent, and below-average heights centered south of the continent. At
850-hPa, this wave pattern was associated with anomalous southwesterly flow
across eastern Australia, along with a complete disappearance of the normal
onshore flow of cooler air from the Great Australian Bight. This combination
of conditions contributed to suppressed rainfall along both the west and
east coasts, and to exceptionally warm surface temperatures (exceeding the
90th percentile of occurrences) across the eastern half of
Australia
.
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