热带印度洋偶极子 (Indian Ocean Dipole) 是印度洋海域内海洋和大气环流年际变化的主要特征模态之一, 在热带海气耦合系统中起到非常重要的作用。同热带太平洋的ENSO现象类似, 热带印度洋偶极子也呈现出显著的不对称性。本文利用中国科学院大气物理研究所发展的全球海洋环流模式, 在观测风应力距平的强迫下, 评估了模式对热带印度洋季节变化、 热带印度洋偶极子 (IOD) 模态及其不对称性的模拟能力, 并且通过数值试验分析了IOD模态不对称性特征及其对气候平均态的影响。对照观测资料, 模式较好地再现了热带印度洋SST在季风驱动下的季节变化特征。在年际时间尺度上, 模式不仅能够再现IOD指数的变化趋势, 而且可以成功模拟出IOD模态的空间分布特征, 即表层和次表层海温在西印度洋表现为正异常, 在东印度洋表现为负异常。可见, 对于热带印度洋而言, IOD模态主要是对风应力异常的响应。热带印度洋海温与Niño3.4指数的相关性分析表明, 模式能够模拟出超前热带太平洋ENSO现象2～4个月时海温的偶极子型分布, 但是不能模拟出滞后ENSO现象2个月左右的全海盆增暖模态, 可能是因为模式试验中没有考虑热通量年际异常的强迫。同时, 模式模拟的IOD模态具有同观测结果相类似的不对称性, 进一步的敏感性试验表明风应力的不对称性对偶极子指数的不对称性贡献较小, 次表层及以下海温的不对称性可能主要受到海洋内部非线性动力过程的影响。通过数值试验, 本文还发现热带印度洋海温的不对称性对气候平均态会有影响, 而这种不对称性长期积累后, 会导致上层热带印度洋温度层结趋于稳定状态。
The tropical Indian Ocean Dipole (IOD), which plays an important role in the tropical ocean-air coupled system, is one of the main characters on interannual variability of the Indian Ocean and atmospheric circulation, the IOD exhibits significant asymmetry which is similar to the ENSO phenomenon in the tropical Pacific Ocean. Forced by the observed wind stress anomaly, the seasonal cycle of the tropical Indian Ocean, the IOD mode, and asymmetry of the IOD are evaluated by an OGCM of LASG, IAP/CAS, in the study. Asymmetry of the IOD and its impact on the climatological mean state have also been simulated by the numerical experiments. The simulation results are similar to the observational ones, reproducing seasonal change of SST character forced by the monsoon. On interannual timescales, the model can successfully reproduce not only the trends of the time series of the IOD index, but also the spatial distribution of temperature anomalies, which is characterized by dipole with the reverse signs in the western and eastern tropical Indian Ocean (the western Tropical Indian Ocean shows positive anomaly, while the eastern tropical Indian Ocean shows negative anomaly) for both the surface and subsurface sea temperature. Therefore, for the tropical Indian Ocean, the results manifest that the IOD mode is mainly response to the asymmetry of wind stress anomaly. The analysis of correlation between the sea temperature in the tropical Indian Ocean and the Niño 3.4 index show that 2-4 months leading ENSO in the tropical Pacific Ocean the simulated IOD mode well agrees with the observed, while almost 2 months lagging ENSO in the tropical Pacific Ocean the simulated Indian Ocean basin mode disagrees with the observed, which is probably due to disconsidering the influence of net heat flux interannual anomaly in the numerical experiments. Meanwhile, the asymmetry of IOD simulated by the OGCM is similar to the observed. The experimental results forced by the wind stress anomaly illustrate that the asymmetry of wind stress anomaly slightly contributes to the asymmetry of the IOD index, and the asymmetry of temperature anomaly in the subsurface and deep ocean mainly result from the internal nonlinear dynamics in the tropical Indian Ocean. From the numerical experiments it is obviously found that the asymmetry of temperature impacts the climatological mean state, which makes temperature stratification of the upper ocean system more stable in the tropical Indian Ocean.