| 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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