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

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. 

A recent study conducted by Dr. LI Xinyuand Professor LU Riyu, from the Institute of Atmospheric Physics, Chinese Academy of Sciences, indicates the WNP–YRB rainfall relationship does indeed experience a subseasonal change. Specifically, the relationship is strong during early summer (the mei-yu period) but much weaker after the mei-yu period. YRB rainfall is mainly affected by tropical circulation anomalies during the mei-yu, i.e., the subtropical high. However, it is 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 explains. 

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

The study is published in Adv. Atmos. Sci.

Reference

Li, X. Y., and R. Y. Lu, 2018: Subseasonal change in the seesaw pattern of precipitation between the Yangtze River basin and the tropical western North Pacific during summer. Adv. Atmos. Sci., 35(10), 1231–1242, https://doi.org/10.1007/s00376-018-7304-6 .