– May 2013
1. Northern Hemisphere
The 500-hPa circulation during May featured
above-average heights from eastern Siberia to eastern Canada, across the high
latitudes of the North Atlantic, and over western Russia, and below-average
heights throughout the polar region, Europe, and central Russia (Figs. E9, E11).These conditions projected strongly onto the
negative phases of the East Atlantic/ West Russia teleconnection pattern (-2.1)
and the West Pacific pattern (-1.1) (Table
E1, Fig. E7).
The main land-surface temperature signals
during May included above-average temperatures in western North America,
western Russia, Scandinavia, eastern Siberia, and Mongolia, and below-average
temperatures in Alaska, the southeastern U.S., and central Europe (Fig. E1). The
main precipitation signals included well above-average totals in the north-central
U.S., central Europe, and most of China (Fig.
E3). In the U.S., long-term precipitation
deficits led to a continuation of extreme and exceptional drought in the central
and southern Plains states, with moderate or severe drought also continuing in
much of the West.
a. North America
The mean 500-hPa circulation during
May featured above-average heights across Canada and in the extreme western U.S., and a broad trough across the central and southern
U.S. (Figs. E9,
E11). This pattern was associated with weaker
than average westerly winds across the continent (Figs. T21, E10).
These conditions contributed to above-average
temperatures in western North America, and to below-average temperatures in the
southeastern U.S. (Fig. E1). They also contributed to well above-average
precipitation in the upper mid-western U.S.
Even though the
Great Plains region of the U.S. has recorded near-average precipitation since
January (Fig. E5), these
totals have not been enough to erase prior severe rainfall deficits. Large
portions of the U.S. central and southern Plains continued to be impacted by
extreme or exceptional drought. At the end of May, the “U.S. Drought Monitor”
indicated exceptional drought from central Texas northward to western Nebraska,
including eastern Wyoming and southeastern Colorado. Much of western U.S. recorded
moderate or severe drought, with extreme and exceptional drought expanding
across New Mexico.
North Atlantic and western Eurasia
The 500-hPa circulation featured a
north-south dipole pattern of height anomalies across the North Atlantic Ocean,
with above-average heights extending across the high latitudes of the North
Atlantic, and below-average heights extending from Greenland to northern Europe
(Fig. E9). This pattern was associated with enhanced jet
stream winds extending across the high latitudes of the North Atlantic into western Europe.
The circulation also featured a
strong wave pattern from Europe to central Russia, with a ridge extending
southeastward from Scandinavia to the Caspian Sea, and troughs located over
central Europe and central Russia (Fig. E9). This wave pattern reflected a strong negative
phase of the East Atlantic/ West Russia teleconnection pattern (-2.1) (Table E1, Fig. E7).
resulting flow pattern produced well above-average precipitation across much of
Europe, with many areas recording departures in the upper 90th
percentile of occurrences (Fig. E3). In northern Europe, area-averaged rainfall
totals exceeded the 90th percentile of occurrences, while in
southern Europe they reached the 80th percentile of occurrences (Fig. E4).
Also, the mean trough axis in
Europe delineated areas of below-average surface temperatures in western Europe from well above-average surface temperatures
in southeastern Europe, the eastern Mediterranean Sea, and western Russia (Fig. E1).
Temperature departures in these regions were generally above the 90th
percentile of occurrences, with the largest departures (3oC-5oC)
observed in western Russia.
c. China and Mongolia
eastern Asia experienced anomalously warm and wet conditions during May. The
most significant temperature departures were observed in northern China,
Mongolia, and eastern Siberia, with departures in portions of eastern Siberia
exceeding +4oC (Fig. E1). Much of China and northern Mongolia also
recorded well above-average precipitation during May, with many areas recording
totals in the upper 90th percentile of occurrences. For China as a
whole, area-averaged totals exceeded the 90th percentile of occurrences,
marking an early start to that regions summertime rainy season (Fig. E4).
2. Southern Hemisphere
The mean 500-hPa circulation during
May featured an anomalous zonal wave 3 pattern of height anomalies, with
above-average heights observed south of Australia, south of South Africa, and across the central South Pacific, and
below-average heights observed over the high latitudes of the South Pacific and
south of South America (Fig. E15).
At 200-hPa, the circulation over
Australia featured an anomalous wave pattern, with an amplified ridge over the
middle of the country and amplified troughs along both the west and east coast
This pattern contributed to well above-average temperatures across southern
Australia, with monthly departures exceeding the 90th percentile of
occurrences along the south-central coast (Fig.
E1). Also, above-average precipitation was
observed in the west downstream of the mean trough axis, while below-average
precipitation was observed in the east downstream of the mean ridge axis (Fig. E3).