Articles/Reports: Luo and Zeng

Yong Luo
Laboratory for Climate Studies, National Climate Center
China Meteorological Administration, Beijing 100081, China

Ning Zeng
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.

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).

**Table 1:** The maximum rate of flow, affected areas and number of death in the most severe flooding events over the Yangtze River valley in the recent 200 years.
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

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 indirect. Diagnostic and model studies are needed to delineate the relationships and to understand the mechanisms.

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 intensity and 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.

The Flooding in the Yangtze River valley during the Summer of 1998: Some Weather and Climate Aspects