A Decadal Decrease in May Precipitation Over Yunnan, China After 2009 and Its Associated Atmospheric Circulation

Abstract

In this study, the authors identify a decadal decrease in May precipitation over Yunnan, China, after 2009. Based on observed data from meteorological stations, the area-average precipitation in May over Yunnan has decreased from 135 mm during the period 1999–2008 to 83 mm during the period 2009–2019. The decline in precipitation is particularly pronounced in the southwestern and central regions of Yunnan, where it exceeds 100 mm. The moisture budget analysis indicates that the dynamical component of the vertical moisture advection, which is generated by the anomalous descending motion, is the largest contributor to the precipitation decrease after 2009 over Yunnan, followed by the thermodynamic component of the horizontal moisture advection. Further analysis suggests that the regional descending motion and low-level divergence may be attributable to the combined influence of Eurasian wave trains associated with the anomalous North Atlantic SST pattern and warm SST anomalies in the tropical western Indian Ocean (TWIO). On the one hand, the subtropical branch of the Eurasian wave train induces upper-level convergence anomalies over the upper Lancang–Mekong basin, which facilitates the local descending motion and low-level divergent circulation. On the other hand, the Indian Ocean has experienced a remarkable warming around 2009, particularly over the tropical western Indian Ocean. As a result, an anomalous vertical circulation is observed, with ascending motion over the TWIO and descending motion over Yunnan and neighbouring regions. These findings may provide further insight into the dynamic processes behind the observed precipitation decrease and the occurrence of frequent droughts over Yunnan in recent years.