Characteristics and Causes Analysis of Abnormal Meiyu in China in 2016

Based on the National Climate Center Meiyu monitoring data and NCEP reanalysis dataset, the characteristics of abnormal Meiyu and associated large scale circulation during the Meiyu period of 2016 in China are analyzed. The results are as follows:(1) There were obvious regional characteristics of Meiyu in 2016 in China. The Meiyu onset date over regions to the south of the Yangtze River was 14 days earlier than normal and was the earliest as that in 1995 since 1951. The Meiyu outset date in 2016 was 11 days later than normal with a longer duration and more Meiyu precipitation, whereas daily average precipitation was weaker. The Meiyu onset (outset) date over the Yangtze River valley was later than normal, while the Meiyu duration was close to normal with more than double of normal Meiyu precipitation. The amount of total Meiyu precipitation and average daily precipitation are ranked as the third and the second highest values since 1951, respectively. The Meiyu onset, outset, and duration over the Yangtze-Huaihe valley were close to normal, but the amount of total Meiyu precipitation was more than normal. (2) The significantly early Meiyu onset over regions to the south of the Yangtze River was induced by abnormally early transition of East Asian circulation from winter pattern to summer pattern. The southward shift of East Asian subtropical upper-level westerly jet and the western Pacific subtropical high (WPSH) were combined with the southward displacement of the East Asian summer monsoon surge in middle July, leading to late than normal Meiyu outset. (3) The decay of the extremely strong El Niño event in the previous winter and the persistent warming of the Indian Ocean Basin-Wide mode (IOBW) in spring and summer were the main reasons that caused the WPSH to be extremely strong during the Meiyu period in 2016. The abnormally strong moisture flux from the southwest of the WPSH and weak cold air from the north converged over the Yangtze-Huaihe valley, resulting in higher than normal Meiyu precipitation over this region.