Laboratory for Climate Studies, National Climate Center
China Meteorological Administration, Beijing 100081, China
Dept. of Atmospheric Sciences and Inst. of Geophysics and Planetary Physics
University of California, Los Angeles
|Editor's note: This report appeared in the UCLA TROPICAL METEOROLOGY NEWSLETTER (No. 26) (Editor: Prof. Michio Yanai). Prof. Yanai and the authors of this report cordially allowed us to publish it on the AMON.|
Summary: An extremely severe flooding occurred over the Yangtze River during the summer of 1998. The heavy Meiyu rainfall in the Yangtze River valley was related to a weak East Asian summer monsoon and an intensive but southward shifted subtropical high over the western Pacific. At larger scales, this corresponds to the most intensive El Nino event over the Pacific and the anomalously extensive snow cover over the Tibetan Plateau.
1. The storm rainfall over the Yangtze River valley this summer and in history
An extremely severe flooding occurred over the Yangtze River valley this summer. The water level along most parts of the middle and lower reaches of the Yangtze River was record-breaking high and lasted for more than sixty days. The maximum rate of flow reached 72,300 m3s-1 at Hankou, which is located in the middle reaches of the Yangtze River, only next to that in 1954 during this century. By August 22, over 1300 people died with 14 million homeless, and the economic loss is estimated over $24 billion.
Although human factors such as deforestation are thought partly responsible, one of the main reasons for the flooding was the heavy and concentrated rainfall. Heavy rainfall stroke the lower and middle Yangtze River during the period of June 12 to 27 and in the last ten days of July the areas along the whole Yangtze River were hit again by excessive storm rainfall. Over the period from June 12 to August 23, the total rainfall in the Yangtze River valley was 577 mm, 142 mm above the normal. This rainfall amount was the second largest, only next to that in 1954 in the last 45 years.
According to historical records, 214 flooding hazards
occurred over the Yangtze River valley during the period from 185 B. C. to
1911. The recent most severe flooding events are summarized in the following
table (Table 1).
|Year||Maximum Rate of Flow (m3 s-1)||Affected Areas (km2)||Number of Death|
|1788||86,000 at Yichang||-||-|
|1860||110,000 at Zhicheng||-||-|
|1870||57,300 at Beibei||30,000||-|
|1931||64,600 at Yichang||115,000||455,000|
|1935||59,300 at Hankou||15,000||142,000|
|1954||76,100 at Hankou||32,000||30,000|
|1991||66,700 at Hankou||154,000||1,200|
|1998||72,300 at Hankou||154,000||>1,300|
2. Synoptic situations and large-scale environment:
The Weak South Asian monsoon and ITCZ;
The location of the western Pacific subtropical high south of its normal
position and a stable blocking high located over the northeast
Pacific and Sea of Ochotsk
During this summer, the South Asian monsoon and ITCZ, as well as the equator-crossing flow were all anomalously weak so that there had been only 5 numbered typhoons in the western Pacific and South China Sea this year until mid-September. Among them only two landed. This is exceptional since 1951, and is related to the southward shift of the subtropical high over the northwestern Pacific.
In the second and last ten days of June, since the location of subtropical
high was south to its normal position, the corresponding wet and warm air
flow met with the intensive and stable cold air flow from the back of the
blocking high in the middle and high latitudes over the Yangtze River
valley. The convergence of cold and warm air flow formed the excessive heavy
rainfall. At the end of June the ridge line of the subtropical high moved
toward north of 25 N and the rainy season in north China began ten days
earlier than normal. However, the clear and hot weather in southern China
only lasted only about ten days. After July 16 the subtropical high retreated
southward and the cold air mass strengthened again. The ridge line of
subtropical high moved to south of 20N and the heavy rainfall hit the
Yangtze River valley again.
3. Possible climatic factors related to the flooding over the Yangtze River valley during this summer
3.1 The most intensive El Nino event in this century
Some previous studies showed that there exists a
relationship between severe storm rainfall over the Yangtze River
valley and the SST anomaly in the middle and eastern equatorial Pacific. For
an example, 1931 and 1954 when severe flooding hit the Yangtze River valley
were all El Nino year. From the correlation between the SST over Nino3 and
the summer precipitation over China, it is found that the only significant
negative correlation located in the Middle Reaches of the Yellow River and
eastern parts of North China. Further studies revealed that in summer of the
year when El Nino events occurred before summer, the main rain belts tended
to locate between the Yellow River and the Yangtze River, or in the South
China and south of the Yangtze River. In the summer of next year, heavy
rainfall may strike the middle and lower reaches of the Yangtze River.
Given that this summer was a transition phase for this ENSO event, any possible
link between the heavy rainfall in China and ENSO this summer should be
Diagnostic and model studies are needed to delineate the relationships and
to understand the mechanisms.
3.2 The anomalously more snow over the Tibetan Plateau
In winter the Tibetan Plateau is a heat sink while in
summer the Tibetan Plateau acts as a heat source. This seasonal variation of
the heating effect of Tibetan Plateau plays an important role in the
progress of East Asian monsoon.
The snow cover was anomalously extensive over the Tibetan Plateau last
winter and this spring. This caused a slow progress of East Asian monsoon and a
weak summer monsoon by reducing the heating over the Plateau. Correspondingly,
the subtropical high over the western Pacific is intensive, but is located
to the south of its normal position. The main rain belt in China shifted
to the south so that more rainfall occurred over the
Yangtze River valley. However, the snow-climate feedback is of much
uncertainty, partly due to the lack of reliable snow data.