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Extratropical Highlights - July 2001

1. Northern Hemisphere
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 (2-4C 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 30S latitude band, and portions of southern Argentina (Fig. E3).

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