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

Botao Zhou,Zunya Wang

doi:10.1007/s00704-019-02809-9

Abstract

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.

Highlights

With the northward advance of the East Asian summer monsoon (EASM), peaks of precipitation appear successively in different regions of China
4 Laboratory for Climate Studies, National Climate Center, China Meteorological Administration, Beijing, China thereby forming the pre-summer rainy season in South China, the Meiyu season in the mid-lower reaches of the Yangtze River valley, and the rainy season in North China (e.g., Ding 1992; Wang and Lin 2002; Wu and Wang 2001; Chen et al 2004; Ding and Chan 2005; Huang et al 2012). Another typical phenomenon related to the advance/retreat of the EASM is the so-called Bautumn rain of West China^ (ARWC), which features the last rainy season in China (Gao and Guo 1958; Ding 2007; Wang and Ding 2008; Yuan and Liu 2013)
Three criteria are applied for the determination of the beginning dates of the Autumn rain of West China (ARWC): (1) the day starting from August 21, with daily precipitation up to 0.1 mm occurring in at least 50% of stations in West China, is considered as one autumn rainy day; (2) if the consecutive autumn rainy days persists no less than 5 days, this process is taken as one active autumn rainy episode

Summary

Introduction

With the northward advance of the East Asian summer monsoon (EASM), peaks of precipitation appear successively in different regions of China. Gu et al (2015) further highlighted that the sea surface temperature (SST) in the tropical eastern and western Pacific contributes to the autumn precipitation in South China. This study is motivated to investigate the cause of the excessive ARWC in 2017 and further to explore the synergic role of the key circulation at the mid-high latitudes and the SST in the tropical Pacific in the interannual variability of ARWC.

Data and methods

Characteristic of the ARWC in 2017 and related background

Findings

Summary