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Extratropical Highlights - May 2000

1. Northern Hemisphere

The circulation during May featured above-average heights across the east-central North Pacific and United States, over the eastern North Atlantic and Europe, and from northeastern Russia to western Alaska (Fig. E9). The circulation also featured below-average heights over Canada, the polar region and central Russia. In the subtropics and lower mid-latitudes, there remains considerable symmetry of the upper-level circulation anomalies in both hemispheres (Fig. T22, bottom). During May, this pattern was dominated by anticyclonic circulation anomalies extending across both hemispheres in the 20-50 latitude band.

Since mid-1998 the most notable aspects of this pattern have been anticyclonic anomalies extending eastward in the Northern Hemisphere from the Atlantic Ocean to Asia and in the Southern Hemisphere from the Atlantic Ocean eastward to Australia. This pattern is consistent with ongoing La Nia conditions, and is the leading mode of atmospheric variability on both the interannual and interdecadel time scales.

a. North America

Above-average heights covered most of the United States, and below-average heights covered western Canada during May (Fig. E9). This anomaly pattern was accompanied by enhanced westerlies into the western United States (Fig. E10) and with anomalously zonal flow across most of the country. These enhanced westerlies were associated with an anomalous jet stream from northern Oregon northeastward to the Great Lakes region (Fig. E10, right), and with an overall poleward shift of the mean jet stream to the northern tier of the United States (Fig. E10, left). Most of the United States was situated south of the mean jet stream and recorded well above-average temperatures during the month, while western Canada was situated north of the mean jet stream and recorded below-average temperatures.

In the United States, temperatures in the Southwest and in Texas averaged 2-3C above normal during the month (Fig. E1, top), which is above the 90th percentile (Fig. E1, bottom). Elsewhere, temperatures over much of the remainder of the U.S. were above the 70th percentile.

The anomalous circulation also contributed to enhanced precipitation across much of central Canada and the Great Lakes region during May (Fig. E3, bottom), and to below-average rainfall over large portions of the southeastern United States. For the Great Lakes region, the above-average rainfall during May broke a 9-month period of significantly below-average precipitation that began in August 1999 (Fig. E5).

Considerable dryness has prevailed in the Gulf Coast region since November 1999, with area-average totals often below the 10th percentile during this 7-month period (Fig. E5). This dryness was particularly acute in Florida during May, which recorded its lowest May rainfall total on record dating back to 1895. The Southeast region has also recorded generally below-average rainfall since November 1999, with area-average totals below the 10th percentile occurring during November 1999, February 2000 and May 2000 (Fig. E5).

b. Europe/ Asia

Most of Europe again experienced milder-than-average temperatures during May (Fig. E1). Temperatures were above the 90th percentile (2-3C above normal) across northeastern Europe and central Europe. This warmth was associated with above-average heights (Fig. E9) and enhanced westerly flow over the eastern North Atlantic and Europe (Fig. E10, right). Farther east, an amplified upper-level trough contributed to anomalously cold temperatures over central Russia during the month, with values averaging more than 3C below-average in the region north of the Caspian Sea.

2. Southern Hemisphere

The Southern Hemisphere circulation during May (Fig. E15) featured above-average heights over the middle latitudes of both the central South Pacific and South Atlantic, and below-normal heights over the high latitudes of both the western and eastern South Pacific. Much of the central latitudes have experienced above-average heights for more than a year, in association with an extremely persistent pattern of anticyclonic circulation anomalies in both hemispheres extending from the Atlantic ocean eastward to Australasia (See Northern Hemisphere Highlights above).

In Australia, below-average temperatures were observed across the western half of the continent during May, with temperatures averaging in the lowest 10th percentile throughout the region. This cooling was associated with enhanced easterly winds at 850 hPa (Fig. T20), and much higher-than-normal pressure (Fig. T19). Temperatures have been below normal in this area since November 1999, in conjunction with a persistent pattern of above-average rainfall. Farther south, below-average rainfall was observed over both southwestern and southeastern Australia during May. Much of this dryness occurred in association with the large region of above-normal 500-hPa heights that extended across southwestern Australia.


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