The Northern Hemisphere 500-hPa
circulation during July featured above-average heights over much of the middle latitudes,
including the high latitudes of the North Pacific, the central United States and Canada,
the central North Atlantic, and western Russia (Fig. E10).
All of these anomalies were quite persistent (Fig. E12, left),
and generally occupied the respective regions for 80% of the month.In North America
these anomalies reflected an amplification of the climatological mean ridge from Texas
northward to northern Hudson Bay, and contributed to above-average surface temperatures
across the region (Fig. E1). Over the North Atlantic the
above-normal heights were associated with a northward shift of the mean subtropical ridge,
which contributed to well above-average (exceeding the 90th percentile)
sea-surface temperatures along the axis of the subtropical ridge from Florida
northeastward to Portugal. Over western Russia a very strong upper level ridge also
contributed to well above-average (2°-4°C anomalies) surface temperatures that also exceeded the
90th percentile.
2. Southern Hemisphere
The mean July circulation in the Southern
Hemispheric reflected an anomalous wave-3 pattern at 500-hPa (Fig.
E16). This pattern featured above-average heights poleward of the three
continents, and below-average heights over the high latitudes of the central ocean basins.
The most persistent of these anomalies reflected a large area of above-average heights in
the area south of Australia (Fig. E18, left), which was
accompanied by a strong poleward shift of the main belt of westerlies in that region.
Farther east, above-average heights over the high latitudes of the eastern South Pacific
were associated with recurring blocking activity during the month.
Over the central latitudes of the South Pacific, below-average heights during the month
were associated with a pronounced eastward extension of the wintertime jet stream. This
jet extended from the central Indian Ocean eastward to central Chile, and contributed to
increased storminess and above-average rainfall (exceeding the 90th percentile)
from the western South Pacific to central Chile along the 30°S
latitude band, and portions of southern Argentina (Fig. E3).
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