1. Northern Hemisphere
The 500-hPa circulation pattern during June featured above-average heights
over the eastern North Pacific, northeastern
Canada
,
Europe
and throughout the polar region, and below-average heights over the western
United States
and the central
North Atlantic
(Figs. E9, E11).
Surface temperatures were above-average temperatures across
Alaska
, the northeastern quadrant of the
U.S.
,
Europe
, and both the high latitudes and subtropical latitudes of the
North Atlantic
, and below-average in western
North America
(Fig. E1). The main precipitation anomalies
during June reflected above-average totals in the north-central
U.S.
, the
Gulf of Mexico
, across the tropical
Atlantic
/
Caribbean Sea
, and throughout southeastern
Asia
, and below-average totals in the south-central
U.S.
and northern
Europe
(Figs. E3, E5, E6).
a. Pacific/
North America
The main 500-hPa circulation anomalies affecting the Pacific/ North American
sector during June were above-average heights over
Alaska
and eastern
Canada
, and below-average heights over the western
United States
(Fig. E9). Over eastern
North America
, this anomaly pattern reflected a complete disappearance of the
climatological mean Hudson Bay Low. The monthly surface temperature
departures were consistent with this anomaly pattern, with above-average
temperatures across
Alaska
and the north-central
U.S.
, and below-average temperatures over the western
United States
.
The mean upper-level ridge axis during June was centered over the
east-central
United States
. Upstream of the ridge axis , broad southwesterly flow and anomalous
ascending motion contributed to well above-average precipitation in the
northern Plains states, with totals in many areas exceeding the 90th
percentile of occurrences. Conversely, anomalously dry conditions were
observed along and downstream of the mean ridge axis, with portions of the
Great Lakes
region recording totals in the lowest 30th percentile of
occurrences. This area has recorded below-average precipitation for the last
four consecutive months (Fig. E5).
Above-average precipitation was also observed along the
Gulf
Coast
region of the
United States
and throughout the
Gulf of Mexico
. Two tropical storms contributed to these large rainfall totals. Tropical
storm Arlene formed in the northern
Caribbean Sea
on 9 June. It subsequently tracked through the
Gulf of Mexico
and made landfall along the central
Gulf
Coast
. The second tropical storm of the season (Bret) formed over the western
Gulf of Mexico
and made landfall in eastern
Mexico
.
b.
North Atlantic
,
Eurasia
The 500-hPa circulation pattern during June featured above-average heights
over central and southern
Europe
(Fig. E11), which led to anomalously warm
and dry conditions over much of the continent. Area-average precipitation
totals in both northern and southern
Europe
were in the lowest 20th percentile of occurrences, while monthly
mean temperatures were above the 70th percentile of occurrences.
Exceptionally warm SSTs were again observed throughout the high latitudes
and subtropical latitudes of the
North Atlantic
, with departures in both regions exceeding the 90th percentile (Figs.
T18, E1). This
anomaly pattern reflects the ongoing warm phase of the Atlantic
multi-decadal mode that began approximately in 1995 (Goldenberg et al. Science,
2001).
2. Southern Hemisphere
In the middle latitudes, the
mean 500-hPa circulation pattern during June featured an anomalous zonal
wave 3-4 pattern, with above-average heights over the three ocean basins and
below-average heights in the vicinity of the three continents (Fig. E15).
The circulation pattern at high latitudes featured above-average heights
throughout the polar region and the central South Pacific, which extended
well into the middle stratosphere (Fig. S1).
At lower latitudes, the 200-ha subtropical ridges were stronger than
average throughout the Southern Hemisphere, with the largest anticyclonic
anomalies occurring over tropical Australia and the eastern South Pacific (Fig.
T22). These anomalies were related to a
regional strengthening of the local Hadley circulations in association with
amplified monsoon systems over southeastern
Asia
and
Central America
, respectively (Fig. T23).
This anomaly pattern was
associated with enhanced upper-level westerlies and a nearly uniform South
Pacific jet stream from
Australia
to southern
South America
(Fig. T21). Much of southern
South America
was situated in the diffluent exit region of this jet. This region
experienced increased storminess and above-average precipitation, with
monthly totals generally exceeding the 70th percentile of
occurrences. In east-central
South America
, located equatorward of the jet exit region, monthly mean temperatures
ranged from 2°-3°C above average and exceeded the 90th
percentile of occurrences.
In
Australia
,
a strong upper-level ridge covered the northern part of the continent while
an amplified trough prevailed across the southwest (Fig. T22).
These conditions were associated with enhanced jet stream winds across the
continent, and an anomalous jet exit region in the east (Fig. T21).
Eastern
Australia
was situated downstream of the mean upper-level trough axis and within the
cyclonic shear side of the anomalous jet exit region, and therefore
experienced increased storminess and enhanced precipitation during the
month. Also downstream of the mean trough axis, anomalous low-level poleward
flow from the interior of the continent resulted in above-average
temperatures to much of southern
Australia
(Fig. T20). This anomalous flow reflected a
nearly complete disappearance of the normal inland flow of cooler marine air
from the
Great
Australian Bight
.
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