Application Laboratory, Japan Agency for Marine-Earth Science and Technology, Yokohama, Japan.
Sci Rep. 2018 Jan 9;8(1):123. doi: 10.1038/s41598-017-18396-6.
The El Niño/Southern Oscillation has been traditionally linked to the extremes in the Indian summer monsoon rainfall (ISMR) affecting more than a billion people in the region. This trans-oceanic influence is seen to be moderated by the Indian Ocean Dipole (IOD) phenomenon in recent decades. In the presence of a positive IOD (pIOD), the otherwise subdued ISMR in an El Niño year remains close to normal even in the face of record breaking El Niños. While this general influence of pIOD on ISMR is understood, the influence of negative IOD (nIOD) on ISMR is not yet recognized. In this study, it is revealed that those opposite phases of IOD are associated with distinct regional asymmetries in rainfall anomalies. The pIOD is associated with a tripolar pattern in rainfall anomalies with above normal rainfall in central parts of India and below normal rainfall to north and south of it. Conversely, the nIOD is associated with a zonal dipole having above (below) normal rainfall on the western (eastern) half of the country. This spatial quasi-asymmetry arises from the differences in the atmospheric responses and the associated differences in moisture transports to the region during contrasting phases of the IOD.
厄尔尼诺/南方涛动现象与印度夏季季风降雨（ISMR）的极端情况传统上相关联，影响了该地区超过十亿人口。在最近几十年中，印度洋偶极子（IOD）现象被认为缓和了这种跨洋影响。在正印度洋偶极子（pIOD）的情况下，厄尔尼诺年中本来较弱的 ISMR 仍然接近正常，即使面对破纪录的厄尔尼诺现象也是如此。虽然人们已经了解 pIOD 对 ISMR 的一般影响，但 nIOD 对 ISMR 的影响尚未得到认可。在这项研究中，揭示了相反的IOD 阶段与降雨异常的明显区域不对称性有关。pIOD 与印度中部地区的异常降水呈三极模式相关，而其北部和南部的异常降水则低于正常水平。相反，nIOD 与纬向偶极子相关，其特征是该国西部（东部）半部分的异常降水高于（低于）正常水平。这种空间准不对称性是由于大气响应的差异以及在 IOD 相反阶段期间向该地区的水汽输送的差异所致。
