摘要:对比强厄尔尼诺次年2016年和1998年长江中下游梅雨季风环流异同点，并探讨其物理机制，结果表明：（1）2016年梅雨集中期和1998年两段梅雨期季风环流有诸多相似特征：西北太平洋副热带高压（副高）偏强偏西、南亚高压偏强偏东、孟加拉湾到南海西南季风偏弱；此外，华北东部到江淮均有冷槽维持；副高持续稳定地将西南季风引导至长江中下游形成强西南暖湿气流，并与来自冷槽的北方南下干冷空气辐合，在高层辐散形势配合下形成强降雨。（2）三段梅雨期，青藏高原附近均为高压脊控制，受暖平流及高原热源、梅雨凝结潜热等因素影响，青藏高原到江南、华南一带大气中高层呈大范围强温度正距平；印度尼西亚群岛附近洋面为海温正距平，对流和热源偏强；这是季风环流相似特征形成的两个重要因素。（3）2016年梅雨集中期，青藏高原暖脊最强，东部冷槽最浅，海温正距平范围最大最北，因而南亚高压和副高位置最北，梅雨雨带也最北；梅雨集中期的结束与冷空气减弱以及台湾以东洋面等海域海温正距平显著增强引起超强台风“尼伯特”登陆有关；7月第4候之后，菲律宾以东洋面、南海及东海海域海温正距平增强，对流活跃，导致7月21日之后副高显著偏北；因而没能出现第2段梅雨集中期。（4）1998年7月中旬至8月初，青藏高原上空高压脊较浅，北部呈位势高度负距平，冷空气势力较强，温度偏低，东部冷槽深，西北太平洋海温正距平区域维持不变，故南亚高压和副高异常偏南，从而出现第2段梅雨。
关键词:青藏高原热力作用/
海温异常/
季风环流/
梅雨
Abstract: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 Ni?o 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.
Key words:Tibetan Plateau thermal effect/
Sea surface temperature anomaly (SSTA)/
Monsoon circulation/
Meiyu
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