The Dipole Mode Index (DMI), an Index used to represent the Indian Ocean Dipole (IOD), shows a back to back occurrence of positive and negative IOD events during the period 1994 to 1998. The year 1994 was a strong positive IOD year and was followed by a strong negative IOD in 1996. The positive IOD in 1997 and negative IOD in 1998 were co-occurred with the strong El-Nino and strong La-Nina, respectively. In the present study, we have looked into the evolution of consecutive IOD events during the period 1994 to 1998 and analyzed the differences in their formation, features and association with Indian Summer Monsoon Rainfall (ISMR) for various homogenous regions of the Indian subcontinent. We have found that the El-Nino and La-Nina enhances the IOD-induced positive SST anomalies in the western and eastern parts of the Indian Ocean during the co-occurred years, respectively. The evolution of positive IOD in 1997 is driven by the strong easterly wind anomalies associated with strong El-Nino. The positive anomalies of SST in the western equatorial Indian Ocean is maintained by the El-Nino which results in the long life span of positive IOD in the year 1997 than that of pure positive IOD year 1994. The westward propagating Rossby wave generated due to PIOD is maintained by strong El-Nino in the year 1997. The vertical dynamics, represented by D20, showed minor differences during the pure and co-occurred IOD events. The positive IOD years with El-Nino had good correlation with the Peninsular India and Central North East India regions rainfall. Since we have used only limited samples or cases under the different categories, our present results are preliminary in nature.

中文翻译：

---

1994-1998年连续IOD事件对比阶段的演化机制

偶极子模式指数 (DMI)，一个用来代表印度洋偶极子 (IOD) 的指数，显示 1994 年至 1998 年期间正负 IOD 事件背靠背发生。 1994 年是强正 IOD 年，紧随其后的是 1996 年的强负 IOD。1997 年的正 IOD 和 1998 年的负 IOD 分别与强厄尔尼诺和强拉尼娜同时发生。在本研究中，我们研究了 1994 年至 1998 年期间连续 IOD 事件的演变，并分析了印度次大陆各个同质区域在其形成、特征和与印度夏季风降雨 (ISMR) 的关联方面的差异。我们发现厄尔尼诺和拉尼娜分别增强了印度洋西部和东部在共同发生的年份中IOD诱导的正海温异常。1997 年正 IOD 的演变是由与强厄尔尼诺现象相关的强东风异常驱动的。西赤道印度洋海温正异常由厄尔尼诺现象维持，导致1997年正IOD寿命比1994年纯正IOD寿命长。 PIOD产生的向西传播的Rossby波由强厄尔尼诺现象在 1997 年维持。以 D20 为代表的垂直动态在纯 IOD 事件和同时发生的 IOD 事件期间显示出微小的差异。厄尔尼诺的正IOD年份与印度半岛和印度东北部中部地区的降雨量具有良好的相关性。由于我们仅使用了不同类别下的有限样本或案例，因此我们目前的结果本质上是初步的。