Indian monsoon's relation with the decadal part of PDO in observations and NCAR CCSM4 International Journal of Climatology

This study has investigated the influence of the decadal component of the Pacific Decadal Oscillation (PDO) on the Indian monsoon in observations and coupled climate model. A major part of the conventionally defined PDO is shown to be dominated by interannual variability. By extracting the pure decadal part of the North Pacific variability, this study has shown that the Indian monsoon rainfall exhibits different relations with the conventionally defined PDO and the pure decadal component of the PDO. This result may have implications for decadal prediction of the monsoon. The analysis suggests that the warm (cold) phase of pure decadal variability of PDO is associated with deficit (excess) rainfall over the west central part of India. In contrast, the conventional warm (cold) PDO index is associated with deficit (excess) rainfall over most of India. The warm phase of the pure decadal PDO opposes the moisture flow beyond 20°N over the Indian monsoon region via the meridional winds extending from the North Pacific and leads to reduced rainfall over west central India. The Community Climate System Model version 4 of the National Center for Atmospheric Research shows reasonable simulation of the decadal PDO mode in both the North Pacific sea surface temperature and the Indian monsoon rainfall and the relation between them. Further, the observed and simulated PDO–monsoon relation is substantiated through a regionally de‐coupled experiment. The coupled model experiment also provides supporting evidence for the mechanism involving the intermediary role played by the tropical Pacific Ocean in the PDO–monsoon relation. Grant no. NA09OAR4310058

本研究基于观测资料与耦合气候模式，探究了太平洋年代际振荡（Pacific Decadal Oscillation, PDO）的年代际分量对印度季风的影响。研究发现，传统定义下的PDO信号主体由年际变率主导。通过提取北太平洋变率中的纯年代际分量，本研究揭示印度季风降水与传统定义的PDO、以及PDO的纯年代际分量分别呈现出迥异的关联特征。该结果对季风的年代际预测具有重要参考价值。分析表明，PDO纯年代际变率的暖（冷）相位与印度中西部地区的降水偏少（偏多）显著相关。与之形成鲜明对比的是，传统暖（冷）PDO指数则与印度绝大多数区域的降水偏少（偏多）存在关联。PDO纯年代际暖相位通过源自北太平洋的经向风，抑制了印度季风区20°N以北的水汽输送，进而导致印度中西部降水减少。美国国家大气研究中心（National Center for Atmospheric Research, NCAR）的第4版社区气候系统模型（Community Climate System Model 4, CCSM4）能够合理模拟北太平洋海表温度中的PDO年代际模态、印度季风降水及其二者间的关联关系。进一步通过区域解耦试验，验证了观测与模拟得到的PDO-季风关联关系。耦合模式试验还为热带太平洋在PDO与季风关联中起到中介作用的机制提供了支撑证据。资助编号：NA09OAR4310058