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A theory for the Indian Ocean dipole-zonal mode

机译：印度洋偶极子-纬向模式的理论

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Four fundamental differences of air-sea interactions between the tropical Pacific and Indian Oceans are identified based on observational analyses and physical reasoning. The first difference is represented by the strong contrast of a zonal cloud-SST phase relationship between the warm and cool oceans. The in-phase cloud SST relationship in the warm oceans leads to a strong negative feedback, while a significant phase difference in the cold tongue leads to a much weaker thermodynamic damping. The second difference arises from the reversal of the basic-state zonal wind and the tilting of the ocean thermocline, which leads to distinctive effects of ocean waves. The third difference lies in the existence of the Asian monsoon and its interaction with the adjacent oceans. The fourth difference is that the southeast Indian Ocean is a region where a positive atmosphere ocean thermodynamic feedback exists in boreal summer. A conceptual coupled atmosphere-ocean model was constructed aimed to understand the origin of the Indian Ocean dipole-zonal mode (IODM). In the model, various positive and negative air-sea feedback processes were considered. Among them were the cloud-radiation-SST feedback, the evaporation-SST-wind feedback, the thermocline-SST feedback, and the monsoon-ocean feedback. Numerical results indicate that the IODM is a dynamically coupled atmosphere-ocean mode whose instability depends on the annual cycle of the basic state. It tends to develop rapidly in boreal summer but decay in boreal winter. As a result, the IODM has a distinctive evolution characteristic compared to the El Nino. Sensitivity experiments suggest that the IODM is a weakly damped oscillator in the absence of external forcing, owing to a strong negative cloud-SST feedback and a deep mean thermocline in the equatorial Indian Ocean. A thermodynamic air-sea (TAS) feedback arises from the interaction between an anomalous atmospheric anticyclone and a cold SST anomaly (SSTA) off Sumatra. Because of its dependence on the basic-state wind, the nature of this TAS feedback is season dependent. A positive feedback occurs only in northern summer when the southeasterly flow is pronounced. It becomes a negative feedback in northern winter when the northwesterly wind is pronounced. The phase locking of the IODM can be, to a large extent, explained by this seasonal-dependent TAS feedback. The biennial tendency of the IODM is attributed to the monsoon-ocean feedback and the remote El Nino forcing that has a quasi-biennial component. In the presence of realistic Nino-3 SSTA forcing, the model is capable of simulating IODM events during the last 50 yr that are associated with the El Nino, indicating that ENSO is one of triggering mechanisms. The failure of simulation of the 1961 and 1994 events suggests that other types of climate forcings in addition to the ENSO must play a role in triggering an IODM event. [References: 36]
机译：根据观测分析和物理推理，确定了热带太平洋和印度洋之间海-气相互作用的四个基本差异。第一个差异是温暖和凉爽的海洋之间的纬向云-SST相位关系的强烈对比所代表的。在温暖的海洋中，同相云SST关系导致强烈的负反馈，而在冷的舌头中存在明显的相位差会导致热力学阻尼弱得多。第二个差异来自基本状态纬向风的逆转和海洋热跃层的倾斜，这导致了海浪的独特影响。第三个差异在于亚洲季风的存在及其与邻近海洋的相互作用。第四点不同是印度洋东南部是一个在北方夏季存在积极大气海洋热力学反馈的区域。构造了一个概念性的大气-海洋耦合模型，旨在了解印度洋偶极子-纬向模式（IODM）的起源。在模型中，考虑了各种正面和负面的海海反馈过程。其中包括云辐射-SST反馈，蒸发-SST-风反馈，温跃层-SST反馈和季风-海洋反馈。数值结果表明，IODM是一种动态耦合的大气-海洋模式，其不稳定性取决于基本状态的年周期。它在北方夏季趋于迅速发展，而在北方冬季则衰退。结果，与厄尔尼诺现象相比，IODM具有鲜明的演变特征。敏感性实验表明，由于存在强烈的负云SST反馈和赤道印度洋深部的温跃层，IODM是没有外部强迫的弱阻尼振荡器。热气-海（TAS）反馈是由异常的大气反气旋和苏门答腊附近的冷SST异常（SSTA）之间的相互作用引起的。由于它依赖于基本状态风，因此该TAS反馈的性质取决于季节。仅当北部夏季明显出现东南气流时，才产生正反馈。在北方冬季，当西北风明显时，它成为负面反馈。 IODM的锁相在很大程度上可以通过与季节相关的TAS反馈来解释。 IODM的两年期趋势归因于季风-海洋反馈和偏远的厄尔尼诺强迫，该强迫具有准两年期的成分。在存在现实的Nino-3 SSTA强迫的情况下，该模型能够模拟与厄尔尼诺现象有关的最近50年中的IODM事件，表明ENSO是触发机制之一。 1961年和1994年事件的模拟失败表明，除了ENSO之外，其他类型的气候强迫也必须触发IODM事件。 [参考：36]

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