上世纪90年代中期，南海热带气旋（TC）活动发生了一次明显的年代尺度增强，然而东亚大气环流和南海环境场的年代尺度差异并未与这次TC突变表现出明确的对应关系。与此同时，1994年后南海季节内振荡（ISO）活动强度增大，其与TC在年际和年代时间尺度上表现出较好的一致性变化。本项目以ISO对上世纪90年代中期南海TC年代尺度增强影响的过程和机理为研究对象，采用观测资料统计检验、能量收支诊断分析和敏感性数值模拟试验等方法，研究南海TC活动与环境场变量年代差异非一致性变化的原因，分析突变前后的两个年代，局地ISO结构、强度和传播等特征对TC生成与移动影响过程的差异，揭示与ISO变化密切关联的对流层低层辐合增强及活跃对流调制南海TC年代尺度增强的动力学和热力学机理。本项目的研究有望深化对ISO调制TC活动物理过程及南海TC长时间尺度变化机制的认识，为南海和西北太平洋TC活动的季节预测提供科学依据。

TC activity over the South China Sea (SCS) experiences a significant decadal enhancement in the mid-1990s. However, the changes of large-scale circulation and environmental conditions over the East Asia hardly correspond to the decadal change of TC genesis simultaneously. Meanwhile, a significant decadal change of the SCS intraseasonal oscillation (ISO) activity occurred in 1994, and its intensity starts increasing since the mid-1990s on the decadal time scale. In particular, the interannual and decadal variations of the SCS TC genesis exhibit a close relationship with those of the ISO activity. This project focuses on the mechanism and influences of the ISO change on the decadal enhancement of TC activity over the SCS in the mid-1990s. By statistical analysis, diagnosis method and numerical simulation experiment, the cause of inconsistent changes between the SCS TC activity and local environmental variables will be studied. The differences of ISO impacts on the TC genesis and intensity between the two decades will be investigated. The dynamic and thermodynamic impacts of ISO activity in association with the intensified convergence and enhanced convection on the long-term TC change will be analyzed. All above-mentioned issues studied in this project will be helpful to enlarge the knowledge of the mechanism of the ISO influences on the TC activity, as well as the long-term TC activity over the SCS. This project might be also beneficial for the improvement of TC activity seasonal prediction over the SCS and the western North Pacific.