First Transition of the Asian Summer Monsoon and the Taiwan Meiyu

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First Transition of the Asian Summer Monsoon and
the Taiwan Meiyu

• Chih-wen Hung and Huang-Hsiung Hsu
• Dept. of Atmospheric Sciences
• National Taiwan University, Taiwan

• Introductions to Asian monsoon and Meiyu
• Data and method
• First transition of the Asian summer monsoon and
  Taiwan Meiyu
• First transition of the Asian summer monsoon and
  ISO
• Interannual variations of Taiwan Meiyu
• Conclusions

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Asian Monsoon
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Meiyu, Baiu, Changma (plum rain)

• Meiyu in Taiwan
• usually from mid-May to mid-June
• Meiyu in the Yangtze River
• usually from mid-June to mid-July
• Baiu in Japan, Changma in Korea
• usually from late-June to late-July

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(No Transcript)
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Data

• ERA 40 (European Centre for Medium-Range Weather
  Forecast Re-Analysis 40)
• 1958 to 2002, 2.5x 2.5 resolution
• 850hPa, 200hPa
• Rain gauge data on Taiwan (CWB)
• 21 Taiwan weather stations (18 of them are
  well-spread at the main island of Taiwan, and the
  rest are at the small islands surrounding Taiwan
• 1950 now

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Hsu et al. 1999

• Onset of the Asian summer monsoon
• 5-day running means of 850hPa streamfunction from
  April to June
• Apply EOF (30E-180, 30S-50N)
• Day 0 The date when the EOF1 falls below zero

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EOF1
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Blue shading 5-day running mean of 21 CWB
weather station rainfall Red line PC1
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Onset types
Dates when PC1 changes from to -
Type 1 Sharp onset PC1 falls from positive to
negative value sharply only once in a 60-day
period (30 days before and 30 days after the
onset in April-June). Type 2 PC1 drops twice
in April-June. The period between two drops is
less than 30 days. Type X rest cases
Onset date
Late onset Onset in June
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• For almost half of years in 1958-2002, onset of
  the Asian summer monsoon can be characterized to
  one major type, the sharp onset.
• Sharp onset has an abrupt large-scale circulation
  change, i.e. the reversal of wind direction from
  northeasterly to southwesterly.
• For the rest of years (none sharp onset cases),
  the wind directions of monsoonal flow swings back
  and forth several times in April-June, and no
  clear abrupt change of the large-scale
  circulation is seen.

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1958-2002 (45 years) Type 1 (24 cases, including
5 late onset cases) Type 2 (15 cases) Type X (6
cases)
CWB rain PC1
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Composite for sharp onset (19 cases)
E
E
DIV
W
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Onset
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• The timing of the sharp onset is associated with
  a clear large-scale eastward moving ISO.
• When the convection of ISO moves to the region
  near Maritime continent (120E), it is also about
  the time for the sharp onset.

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Onset
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Onset
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• The heavy Meiyu rainfalls in Taiwan occurs after
  the sharp onset of the Asian summer monsoon.
• When the moisture supply zone is established all
  the way from the Indian Ocean to the South China
  Sea area, the moist transport is abruptly
  enhanced.
• The moist air quickly arrives the South China Sea
  and provides necessary moisture for the Meiyu
  front (or rain belt) near Taiwan.

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• Most wet (active) Taiwan Meiyu years are sharp
  onset cases, but few dry (inactive) Taiwan Meiyu
  years are characterized as sharp onset.
• Sharp onset

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• Conclusions
• The sharp onset of the Asian summer monsoon
  implied
• (1) a clear abrupt change in the large-scale
  monsoon circulation
• (2) a continuous monsoonal flow after the onset
• (3) the moisture can be transported more
  efficiently following the monsoon flow to
  the SCS and Taiwan areas.

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Onset