Enlightenment from heavy autumn rain of West China in 2017: synergic role of atmospheric circulation at mid-high latitudes and oceanic background

Autumn rain of West China (ARWC), a typical climate phenomenon characterized by continuous rainfall, is prone to flooding and secondary disasters. The ARWC in 2017, the greatest one since the last 30 years, resulted in an economic loss of 1.9 billion dollars and serious social impacts. It is thus urgent to understand the cause for this anomaly. In this article, the atmospheric circulations affecting the 2017's ARWC are identified. They are (1) a dipole pattern with the blocking over Europe and the trough over Lake Balkhash, which favors the southward outbreak of cold airs into West China, (2) increased water vapor transportation toward West China from the Pacific and Indian Oceans, and (3) a strengthening and northward displacement of the East Asian jet stream. The cold sea surface temperature anomaly (SSTA) in the equatorial central Pacific provides a superimposed effect. Further analysis reveals a synergic role in the interannual variability of the ARWC from the anomalies of the atmospheric circulation over Lake Balkhash and the SST in the equatorial central Pacific. The combination of anomalous trough over Lake Balkhash with cold SSTA in the equatorial central Pacific is most favorable for the increase of ARWC, while that of anomalous ridge with warm SSTA generally results in a decrease of ARWC. The combination of anomalous ridge with cold SSTA or that of anomalous trough with warm SSTA has no specific indication for ARWC anomalies.