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Major Factors Affecting Summer Rainfall over Yangtze River Basin Experience Subseasonal Change

Oct 19, 2018

The Yangtze River Basin (YRB) has been subject to severe floods throughout history, often resulting in heavy losses of life and economy. With the region’s rainfall exhibiting large interannual variability in summer, a better understanding of YRB’s rainfall during that season is important.

A recent study conducted by Dr. LI Xinyu and Prof. LU Riyu from the Institute of Atmospheric Physics (IAP) of the Chinese Academy of Sciences indicated that the WNP-YRB rainfall relationship experienced a subseasonal change. Specifically, the relationship was strong during early summer (the mei-yu period) but much weaker after the mei-yu period.

Located in the subtropical monsoon region, YRB rainfall is affected by both tropical and extratropical circulations. In particular, there is a well-known seesaw pattern of rainfall anomalies between the tropical western North Pacific (WNP) and the YRB, which is the most important source for predicting YRB rainfall.

On the other hand, the circulation over East Asia during summer experiences remarkable subseasonal evolution. Specifically, the East Asian westerly jet in the upper troposphere jumps northward around mid-July. It would therefore be interesting to test whether the WNP–YRB rainfall relationship experiences a change under distinct circulation backgrounds.

YRB rainfall was mainly affected by tropical circulation anomalies during the mei-yu, i.e., the subtropical high. However, it was mainly affected by extratropical circulation anomalies after this period.

"The East Asian westerly jet in the upper troposphere jumps northward after the mei-yu period, and the water vapor transported by the subtropical high extends northward to the Huang and Huai rivers, resulting in a weaker impact of the high on the YRB," Dr. LI said.

Given the implication of this work that the prediction of summer floods and droughts in eastern China is highly complex, Prof. LU says that more attention should be paid in future to study intraseasonal oscillation and midlatitude circulation.

Their study entitled "Subseasonal Change in the Seesaw Pattern of Precipitation between the Yangtze River Basin and the Tropical Western North Pacific during Summer" was published in Adv. in Atmos. Sci.

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