The influence of the Madden-Julian Oscillation activity anomalies on Yunnan＇s extreme drought of 2009-2010

Yunnan Province of China suffered a record-breaking drought that persisted from autumn 2009 into spring 2010. The present study examined the physical causes of this extreme drought event in terms of persistent anomalies of intraseasonal oscillation in the tropical atmosphere （the Madden-Julian Oscillation hereafter the MJO）. The results show that the occurrence of severe drought in Yunnan was caused directly by deficient rainfall lasting from the summer of 2009 to the spring of 2010. Further ex- ploration reveals a persistent positive variation of MJO index 1 from June to October. Accordingly, the convective activity over the Bay of Bengal （BOB） weakened continuously, and then an anomalous descending airflow was induced over the tropi- cal Indian Ocean, resulted in the anomalous weakening of vertical Asian monsoon circulation in South Asia. Consequently, the transport of water vapor from the tropical Indian Ocean to Yunnan decreased abnormally, leading to persistent below-normai rainfall over Yunnan from summer to autumn in 2009. As a result, a severe drought began to appear in autumn. In the winter of 2009-2010, MJO index 1 remained persistently positive, indicating the continuous weakening of convective activity over the BOB. The atmospheric circulation associated with the persistent positive anomalies in the MJO also demonstrated anomalous patterns. Specifically, there was an anomalous high-pressure ridge stretching from South Asia through the Tibetan Plateau and into the western part of southwestem China. This indicates that the atmospheric circulation over Yunnan was dominated by vertical descending airflow in the high-pressure ridge. Simultaneously, the India-Burma trough was weakened, which resulted in unfavorable conditions for the transport of water vapor from the BOB to Yunnan, causing the observed persistent deficient precipitation in winter and the subsequently intensified drought. Therefore, the persistent anomalies in MJO activity in the tropical atmosphere played an important role in the occurrence of the extreme drought event in Yunnan in 2009-2010.