1.
Climate in China
During
April, monthly mean surface air temperatures were observed below normal in
eastern Heilongjiang, the western Tibetan Plateau and northwestern Sichuan.
Temperatures were above normal in the rest regions of China and anomalies more
than 2℃
were in most of eastern and central China.
For
the month, precipitations were more than 30% above average in southern and
western Xinjiang, most of Tibet, western Sichuan, most of Northeast China with
maximum value more than 200% above average in southwestern Xinjiang, western
Tibet and eastern Heilongjiang. Precipitations were 30%~80% below average in
central and eastern Northeast China, eastern and central Inner Mongolia, most of
North China and regions south to the Yangtze River with no precipitation in
parts of above regions.
2.
Climate in the World
During
April, monthly mean surface air temperatures were more than 2℃
above normal in northwestern Europe, east-central Russia, eastern China, most of
Australia, northern Western Africa, central Arabia, northern North America with
anomalous maximum above 4℃
in parts of Russia and Australia. Temperatures were about 1℃
below normal in India
Peninsula, northwestern Russia and Far East, southern United States and western
Alaska, southern South America and southern Africa.
During
April, precipitations were observed more than 50% above average in southwestern
Europe, northern North Europe, the regions from western Siberia to northern
Korea Peninsula via Baikal, the western Tibetan Plateau, southern India, Arabia,
southwestern West Africa and South Africa, Northern North America, Central
America and south-central South America with more than 200% above average in
parts of above regions.
Meanwhile, precipitations were 30-100% below average in east-central Russia,
eastern China, central Asia, eastern West Asia, northern South Asia,
northwestern Africa, most of Australia, central United States and western South
America.
3.
General
Circulation Features
Northern Hemisphere: During April, the mean
500hPa geopotential height field showed that meridional circulation weakened
obviously compared with last month. In the height anomaly
field, positive anomalies above 4 dam were over northern North America with
central value 12 dam. Negative anomalies were over the central North Pacific,
regions from northwestern Russia to Barents Sea, parts of polar region and
northwestern Pacific. For the month, subtropical high in northern hemisphere and
the Northwest Pacific subtropical high were larger and stronger than normal.
Southern Hemisphere:
During April, the mean 500hPa
geopotential height field exhibited a 3-wave pattern at the mid-high latitudes.
In the height anomaly field, the central South Pacific, the eastern South India
Ocean and the ocean south to South America were covered by negative anomalies
below -4 dam. Meanwhile, positive anomalies above 4 dam covered southeastern
Australia and offshore, middle of South India Ocean, areas from polar region to
eastern South Pacific and the central South Atlantic.
4. Tropical
Features
Sea
Surface Temperatures (SSTs): During April, positive SST anomalies about 0.5℃ dominated the most of the equatorial Pacific
and the equatorial Indian Ocean; Meanwhile, below -0.5℃ SST anomalies controlled the sea nearby South
America. The indices of NINO Z, NINO3 and NINO1+2 were 0.4℃, 0.4℃ and –0.5℃ and
increased 0.2,0.5 and 0.4 from last month, respectively. NINO3.4 was 0.4℃. NINO4 is 0.6℃, which decreased 0.2 from last month.
Subsurface
Temperatures: During April, the abnormally warm water in the east-central Pacific and the
abnormally cold water in the western Pacific weakened and shifted eastward and
upward. The abnormally cold water near South America coasts shrank obviously.
Warm
Pools: During April, the areas of warm pool in the western equatorial Pacific and
in the equatorial Indian Ocean were near or slightly larger than normal, while
the intensities were stronger than normal.
Wind Field：During April, at the lower troposphere (850hPa), weak easterly anomalies were over the equatorial Pacific near dateline and westerly anomalies about 2m/s were over the rest of equatorial Pacific. At the upper troposphere (200hPa), easterly anomalies about 4m/s were over most of the equatorial Pacific and westerly anomalies about 4m/s controlled most of the equatorial Indian Ocean.
Southern Oscillation：During April, Southern Oscillation Index (SOI) was -0.6, and decreased 0.9 from March.
Convection
over the Tropics: For
the month, enhanced convections were over the western tropical Pacific. In the
lower troposphere (850hPa) divergent wind field, a convergent center was over
equator at 150°E,
and a divergent center was over near 100ºW,
10ºS. Correspondingly, at the upper
troposphere (200hPa), a divergent center was near 150°E,
2.5°S
and a convergent center was observed over 110ºW,
5ºS.
5.
ENSO
Monitoring and Forecasting
①
During
April, positive SST anomalies about 0.5℃ dominated most of the equatorial Pacific.
Compared with last month, SSTs
increased in the eastern equatorial Pacific while positive SSTA weakened in the
central equatorial Pacific. SSTA indices of NINO
Z, NINO3 and NINO3.4 were near normal.
② Abnormally
warm water in east-central Pacific and abnormally cold water in western Pacific
weakened and shifted eastward.
③ The Southern Oscillation Index (SOI) was -0.6 in April, and fluctuated during
past months.
④
At the lower troposphere
(850hPa), westerly anomalies controlled the eastern and western equatorial
Pacific while weak easterly anomalies were over the central equatorial Pacific. The
convections enhanced in the western tropical Pacific.
⑤
Results of some statistical models for ENSO show that SSTA index for NINO3 will
be near normal in the following
months.
Above results showed generally tropical atmospheric and oceanic conditions were near normal in April. The evolution will be monitored continuously.