热带太平洋背景态变化对中太平洋型El Niño现象的调制作用

Central Pacific (CP) El Niño is manifested as anomalous sea surface temperature occurring near the dateline in the tropical central Pacific. As far, there has been a common viewpoint that CP El Niño is dynamically different from the conventional eastern Pacific (EP) El Niño. However, there is no specific study on the topic that how and why these dynamic differences arise. In addition, the tropical Pacific is undergoing a La Niña-like mean state shift that represents as eastern (western) Pacific cooling down (warming up) in the period when CP El Niño frequently occurs. However, most state-of-the-art climate model failed to reproduce this mean state shift and the two types of El Niño, making it difficult to disentangle causal relationship between mean state and El Niño type in these models. Fortunately, early ENSO theory, as well as the present research that is carried out in an intermediate coupled model by us, has already pointed out that the mean state would favor the pattern or mode of ENSO, which can be supported by the observation to some extent. In order to avoid the mismatch between observational and modeling research using state-of-the-art climate models, and more importantly to inherit the aforementioned reasonable theory and the undergoing research, we will employ intermediate coupled model to investigate the modulation of mean state to the relative importance of each dynamical process in the air-sea coupled system. And also, we will validate the hypothesis that the La Niña-like mean state shift in the tropical Pacific is the main factor for the frequent occurrence of the CP El Niño. This study will help understand the dynamical mechanism of the CP El Niño.

中太平洋型(中太型)El Niño是发生在赤道中太平洋日界线附近的海温变暖的现象，目前普遍的观点认为中太型El Niño的动力机制与传统的东太平洋型(东太型) El Niño的有很大的区别。再者，近十几年热带太平洋经历了东太变冷西太变暖的La Niña型的背景态变化，但大多数气候模式对这种变化以及两类El Niño的模拟都不算成功，难以从中理顺背景态与两类El Niño的关系。有利的是，早期ENSO理论和自有的模式研究结果已指出热带太平洋背景态对海气动力过程的强度及ENSO的型态是有偏好的，且这在一定程度上与观测相符。为避免观测与大型模式结果的不一致,同时借鉴合理的理论并延续已开展的研究，本研究将采用中等复杂程度气候模式来考察热带太平洋背景态对海洋大气动力过程强度的调制作用，证实背景态变化是导致中太型El Niño现象频繁发生的原因，为解释中太型El Niño现象的机制提供一个新的思路。

中太平洋型(中太型)El Niño是发生在赤道中太平洋日界线附近的海温变暖的现象，这有别于发生的赤道东太平洋上的传统型El Niño，且在2000年之后中太型El Niño频繁发生。借助多套观测资料和一个改进的Zebiak-Cane海气耦合模式，本项目对两类El Niño的动力机制及其与热带太平洋背景态之间的联系进行了研究。研究发现，在相当大的背景态变化范围内，热带太平洋海气相互作用能容纳两个不稳定的El Niño-Southern Oscillation (ENSO)模态，分别是准两年振荡的中太型ENSO模态和准4年振荡的东太型ENSO模态，且分别受纬向平流反馈和温跃层反馈的控制，这两个模态的时空特征和动力机制与观测结果是吻合的。进一步发现，ENSO主模态对热带太平洋背景态变化很敏感，当温跃层较浅且信风较强(La Niña型背景态变化)时，中太型ENSO模态是主导模态，因为这样的背景态下纬向海温梯度更大，纬向平流反馈对海温增长的贡献更大；而在相反的背景态之下，东太型ENSO模态是主导模态。因此，2000年之后热带太平洋呈现的La Niña型背景态年代际变化就更有利于中太型ENSO模态成为主导模态，因而有利于中太型El Niño事件频繁发生。这些研究结果为解释中太型El Niño现象的机制提供一个新的思路。