Indian Ocean Dipole: Processes and impacts
Indian Ocean Dipole: Processes and impacts
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发表时间：
2009
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通讯作者：
P. Vinayachandran;P. A. Francis;Samrat Rao
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作者：
P. Vinayachandran;P. A. Francis;Samrat Rao
赤道印度洋东部较温暖，有较深的温跃层和混合层，并支持比西部更强的对流大气。在某些年份，东印度洋变得异常寒冷，反常的风沿赤道自东向西吹，苏门答腊岛海岸向东南吹，温跃层和混合层抬升，大气对流受到抑制。同时，西印度洋变暖，增强了大气对流。这种对流、风、海表温度和温跃层积极参与的海洋-大气耦合现象被称为印度洋偶极子（IOD）。斜压Kelvin波和Rossby波在距平风激发下的传播对海温异常的发展起着重要作用。由于印度洋的平均温跃层比太平洋深，长期以来，人们认为印度洋是被动的，仅仅对大气强迫作出反应。印度洋裂谷的发现和随后的研究表明，印度洋能够维持其自身固有的海洋-大气耦合过程。近100年来，约50%的IOD事件与厄尔尼诺-南方涛动（ENSO）共同发生，另一半事件独立发生。耦合模型已经能够再现IOD事件，并通过这样的模型处理实验——打开和关闭ENSO——支持基于观察的假设，即IOD事件在ENSO存在或不存在的情况下发生。不同的耦合模式以及对数据的分析普遍认为，在ENSO期间共同发生的IOD事件是由Walker单体下降分支向东印度洋的纬向转移所强迫的。在没有ENSO的情况下，IOD的启动过程尚不清楚，尽管一些研究表明哈德利环流的异常是最有可能的强迫功能。印度洋附近和偏远地区都能感受到印度洋大地震的影响。在IOD事件期间，东印度洋的生物生产力增加，这反过来导致大面积珊瑚死亡。此外，IOD影响了海洋大陆、印度次大陆、澳大利亚和东非的降雨。海洋大陆和澳大利亚的降雨量不足，而印度和东非的降雨量过剩。尽管通过一个耦合模式成功地预测了2006年的IOD，但要了解近几十年来IOD事件频率增加背后的原因，预测IOD事件及其对降雨变率的影响仍然是一个主要挑战。
Equatorial Indian Ocean is warmer in the east, has a deeper thermocline and mixed layer, and supports a more convective atmosphere than in the west. During certain years, the eastern Indian Ocean becomes unusually cold, anomalous winds blow from east to west along the equator and southeastward off the coast of Sumatra, thermocline and mixed layer lift up and the atmospheric convection gets suppressed. At the same time, western Indian Ocean becomes warmer and enhances atmospheric convection. This coupled ocean-atmospheric phenomenon in which convection, winds, sea surface temperature (SST) and thermocline take part actively is known as the Indian Ocean Dipole (IOD). Propagation of baroclinic Kelvin and Rossby waves excited by anomalous winds, play an important role in the development of SST anomalies associated with the IOD. Since mean thermocline in the Indian Ocean is deep compared to the Pacific, it was believed for a long time that the Indian Ocean is passive and merely responds to the atmospheric forcing. Discovery of the IOD and studies that followed demonstrate that the Indian Ocean can sustain its own intrinsic coupled ocean-atmosphere processes. About 50% percent of the IOD events in the past 100 years have co-occurred with El Nino Southern Oscillation (ENSO) and the other half independently. Coupled models have been able to reproduce IOD events and process experiments by such models – switching ENSO on and off – support the hypothesis based on observations that IOD events develop either in the presence or absence of ENSO. There is a general consensus among different coupled models as well as analysis of data that IOD events co-occurring during the ENSO are forced by a zonal shift in the descending branch of Walker cell over to the eastern Indian Ocean. Processes that initiate the IOD in the absence of ENSO are not clear, although several studies suggest that anomalies of Hadley circulation are the most probable forcing function. Impact of the IOD is felt in the vicinity of Indian Ocean as well as in remote regions. During IOD events, biological productivity of the eastern Indian Ocean increases and this in turn leads to death of corals over a large area.Moreover, the IOD affects rainfall over the maritime continent, Indian subcontinent, Australia and eastern Africa. The maritime continent and Australia suffer from deficit rainfall whereas India and east Africa receive excess. Despite the successful hindcast of the 2006 IOD by a coupled model, forecasting IOD events and their implications to rainfall variability remains a major challenge as understanding reasons behind an increase in frequency of IOD events in recent decades.