Similarities and Differences of Monsoon Circulations between 2016 and 1998 Meiyu Periods in the Middle and Lower Reaches of the Yangtze River and the Comparison of Their Physical Mechanisms

The similarities and differences of monsoon circulations between 2016 and 1998 Meiyu periods in the middle and lower reaches of the Yangtze River following the previous strong El Niño years, together with the comparison of their physical mechanisms, are analyzed in this paper. The results show that: (1) The monsoon circulations during the Meiyu concentrated period in 2016 and the two Meiyu periods in 1998 have several similarities: the western Pacific subtropical high (WPSH) is stronger and is positioned westward more than usual; the South Asia high (SAH) is stronger and positioned more eastward; and the southwest monsoon from the Bay of Bengal to the South China Sea is weaker than normal. In addition, a cold trough is maintained from eastern North China to Jianghuai. The WPSH continuously guides the southwest monsoon to the middle and lower reaches of the Yangtze River, forming a strong warm and humid southwest airflow. The airflow converges with the dry and cold air flown from north to south out of the cold trough, causing heavy rainfall under the condition of high-level divergence. (2) For the three periods of Meiyu, they all have a warm ridge over Tibetan Plateau and the nearby area. The similar characteristics of monsoon circulations are due to (a) the positive temperature anomalies at the middle-upper levels over most of the regions from Tibetan Plateau to Jiangnan and South China, which are resulted from the warm advection, the heat sources from the Plateau, and the latent heat of condensation of Meiyu, and (b) the positive sea surface temperature anomaly (SSTA) with strong convection and heat sources over Indonesian archipelago. (3) During the 2016 Meiyu concentrated period, the combined effect of the strongest warm ridge over Tibetan Plateau, the shallowest eastern cold trough, and the greatest and the most northerly positive SSTA leads to the most northerly SAH and WPSH with the most northerly Meiyu rain belt. The end of the Meiyu concentrated period is related to the weakening of cold air and the significant increase in positive SSTA in the seas east of Taiwan and the Taiwan Strait, which caused the super typhoon “Nibert” to land. From the 4th pentad of July, the positive SSTA over the South China Sea, the eastern ocean of the Philippines, and the East China Sea increases rapidly with active convection which lead to a strong and obviously northerly WPSH after July 21, so there is no longer a second Meiyu concentrated period. (4) From the middle July to early August in 1998, the ridge over Tibetan Plateau is weak with negative geopotential height anomalies, which results in the strong cold air over northern Tibetan Plateau. The cold trough in the East is deep. Also, there is no significant changes of positive SSTA in Northwest Pacific. These factors make SAH and WPSH continue to be strong and abnormally southward, which results in the second period of Meiyu.