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Extratropical Highlights - June 2002

1. Northern Hemisphere
Amplified subtropical ridges were observed across the Northern Hemisphere during June (Fig. T22). In the extratropics the circulation featured above-average 500-hPa heights across the United States, southern Europe, the Middle East, and eastern Siberia, and below-average heights over the high latitudes of both the North Pacific and North Atlantic, and central Russia (Figs. E10, E12). Prominent temperature (Fig. E1) and precipitation (Fig. E3) anomalies included warmer and drier than normal conditions across the southwestern and central United States and Europe, and a second month of excessive rainfall in southeastern China.

a. North Pacific and North America

Over the North Pacific the circulation during June featured above-average heights in the subtropics and below-average heights at high latitudes. This anomaly pattern contrasts markedly with the strong negative phase (-1.5) of the PNA teleconnection pattern that had persisted for the past four months (Table E1, Figs. E7, E8).

In the United States above-average heights overspread most of the country during June, resulting in a continuation of exceptionally warm temperatures in the west (Fig. E1). Also, the mean upper-level ridge and troughs exhibited a northeast-southwest orientation during the month, with the ridge axis extending from Minnesota to western Texas and the trough extending from the northeastern U.S. to Alabama (Fig. T22, E10).

Below-average rainfall was observed in the Inter-Mountain, Southwest, Great Plains, Midwest, and Ohio Valley regions (Figs. E3, E5). Precipitation has been below normal in the Inter-Mountain and Southwest regions since June 2001, in the Great Plains region since October 2001, and in the Ohio Valley since August 2001 (Fig. E5).

Above-normal rainfall was observed in the Gulf Coast region during June primarily in response heavy rains across Florida (Fig. E6). Near-normal rains were recorded in the Southeast, Mid-Atlantic and Northeast Regions. These areas were situated within the mean upper-level trough axis, while Florida was located slightly downstream of the trough axis in an area of anomalous upper-level divergence (Fig. T23, bottom). In the Southeast large precipitation deficits have been recorded in seven of the last nine months (Fig. E5). June marks only the second month of near-normal precipitation in that region since October 2001.

b. North Atlantic and Europe

Over the North Atlantic and western Europe the 500-hPa anomaly dipole of below-average heights at high latitudes and above-average heights in the middle latitudes (Fig. E10) persisted for a third consecutive month. During June these anomalies reflected a strong positive phase (+2.0) of the East Atlantic Jet teleconnection pattern (EA-JET, Table E1), and were associated with a doubling of the climatological mean jet stream winds across northern Europe (Fig. E11). This circulation contributed to a continuation of above-average temperatures throughout the continent, with large areas of central Europe and western Scandinavia recording temperatures above the 90th percentile (Fig. E1 bottom). It also contributed to significantly below-average rainfall across southern Europe, with area-averaged totals reaching only the 10th percentile during the month (Fig. E5).

c. China

Significantly above-average convective precipitation has been observed across central and southeastern China for the past two months, which has resulted in widespread flooding in the Yangtze River Valley. This heavy rainfall has been associated with an amplification of the Asian monsoon ridge along its western flank, and a persistent downstream trough over China (Fig. T22). These conditions were also accompanied by enhanced jet stream winds upstream of China and reduced jet stream winds over northern China, which resulted in a well-defined jet exit region centered over the country (Fig. T21).

During May the mean upper-level trough was located over western China, which led to large-scale ascending motion and above-average rainfall over large portions of the country. During June the mean upper-level trough was located over eastern China, which allowed transient upper-level disturbances and their associated frontal boundaries to strengthen within the anomalous jet exit region as they propagated southeastward toward the downstream trough axis. These systems produced a continuation of enhanced precipitation across southern and central China (Fig. T25).

2. Southern Hemisphere

The upper level circulation during June featured amplified subtropical ridges across the Southern hemisphere (Fig. T22), with the largest anomalies found over Australia and the South Atlantic. At high latitudes a zonal wave-2 pattern of 500-hPa height anomalies was evident during the month, with persistent above-average heights observed over the eastern South Pacific and Indian Ocean, and below-average heights observed in the areas south of Australia and South America (Fig. E18).

Over Australia warmer and drier than normal conditions were again recorded in the southeast and southwest. In the northeast near-normal rainfall was observed during June (Fig. E4) following six consecutive months (December 2001-May 2002) of significantly below normal rains in that region. Over South America above-average temperatures over southeastern Brazil and below-average temperatures across southern South America (Fig. E1) have persisted since April.

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