El Nino-Southern Oscillation（ENSO）是热带太平洋海气耦合最强的年际信号，其变化会引起全球气候异常，对东亚季风具有重要影响。El Nino是ENSO的暖位相，其规则的年际变化由大尺度海洋动力过程和海气相互作用决定。然而，每次El Nino事件的持续时间、强度及其类型等都不相同。其中，赤道太平洋季节内Kelvin波被认为是这种不规则性产生的主要原因之一。以往学者分析了温跃层及以上的季节内Kelvin波在El Nino演变中的作用。本项目拟利用Argo等观测资料和数值实验，研究中层（温跃层以下－1500米）季节内Kelvin波的水平/垂直传播特征，揭示其在两类El Nino（中部型和东部型）中的差异及其对两类El Nino演变的影响和机制，对深刻理解ENSO机理具有重要的科学意义，为改进ENSO和东亚季风预测水平提供可靠的理论基础，具有重要的实践意义和应用价值。

ENSO is the strongest air-sea coupled interannual signal in the tropical Pacific. Its variation can induce severe global climate change, and has significant impact on the eastern Asian monsoon. El Ni?o is the warm phase of ENSO. The regular year-to-year variability associated with the El Ni?o is governed by large-scale ocean dynamics and coupled ocean-atmosphere interactions. No two El Ni?o events are exactly alike though, and the cycle between each El Ni?o exhibits considerable irregularity in amplitude, duration, temporal evolution, and spatial structure. One factor contributing to this irregularity from the point view of ocean is the intraseasonal Kelvin wave, which is the dominant mode of the oceanic variation in the equatorial Pacific on the time scale of intraseasonal. The role of the intraseasonal Kelvin wave along and above the thermocline is discussed in the previous studies. This proposed program aims to understand the horizontal/vertical propagation characters of the intraseasonal Kelvin wave in the mid-depth ocean (beneath thermocline to 1500 m) along the equatorial Pacific, and elucidate its difference during two types of El Ni?o (eastern type and central type), and hence reveal the mechanism of the mid-depth intraseasonal Kelvin wave influencing the two types of El Ni?o, through observations (Argo etc.) and numerical modeling. The study of this program is important to understand the ENSO variability, hence to improve the prediction of the ENSO and eastern Asian monsoon.