一次El Nino背景下副高异常事件的基本特征

On the Basic Characteristics of an Abnormal Event of Subtropical High Under the Background of an El Nino Episode

分析了2019年一次El Nino事件发展的基本特征,并探讨了该次事件背景下副高偏强偏南的原因。研究发现:赤道中东太平洋纬向流反馈偏弱和西太平洋海域区域性的东风异常,都对该次事件表现出向中部型转变的趋势有一定程度的贡献。西太平洋海域上层大气的强烈下沉运动以及对流层低层存在的异常反气旋环流在一定程度上促进了副高的发展,在该次El Nino事件发展期间,导致西太平洋副高向南增强,位置偏南;Hadley环流下沉支的异常进一步加强了西太平洋副高的强度。2018年期间,从东太平洋海域西传的冷性Rossby波到达西太平洋海域且维持,产生异常Rossby波响应,同时2019年5月,MJO来到印度洋海域第8位相,西太平洋海域为反气旋性环流异常,二者共同影响,有利于热带西太平洋及我国南海地区出现异常的反气旋环流,进一步促进西太平洋副高加强西伸、强度偏强、位置偏南。

The basic characteristics are analyzed of an evolving El Nino episode in 2019 and the causes are examined of a stronger and more southward subtropical high under its background.The result is shown as follows.The fact that the feedback of zonal flows was relatively weak in the equatorial central and eastern Pacific and there were regional easterly anomalies in the area of the western Pacific contributed to some extent to the shifting of the episode towards the central pattern.There was intense descending motion in the upperlevel atmosphere over the western Pacific and anomalous anti-cyclonic circulation in the lower troposphere,driving the subtropical high to develop to some extent.During the evolution of the El Nino episode,they caused the subtropical high in the western Pacific to grow more to the south and move into a more southward location.In addition,the anomalies of a descending branch of the Hadley cell further strengthened the subtropical high.In 2018,cold Rossby waves transporting westward from the eastern Pacific arrived in the western Pacific and stayed there,producing anomalous Rossby-wave response.Then in May 2019,the Madden-Julian Oscillation shifted to Phase 8 in the Indian Ocean region and the western Pacific region was covered with anticyclonic circulation anomalies.Their joint effect made it favorable for anomalous anticyclonic circulation to happen in tropical western Pacific and the South China Sea,further making the subtropical high in the western Pacific strengthen,extend to the west and move to the south.