Understanding the Association of Tropical SST Anomalies on the ISMR During Extreme IOD Events

The synergistic effects of the Indian Ocean Dipole (IOD) and El Nino Southern Oscillation (ENSO) have been investigated in this study over various subregions of India during 1981–2015. The IOD, is aperiodic and erratic, still this phenomenon is intrinsic in nature dependent on coupled ocean and atmospheric characteristics of the Indian ocean. About 60% of the IOD events showed its coexistence with ENSO during the 35-year recent climatology, still 40% of IOD events are independently occurring. For this analysis, the strongest three positive IOD (1994, 1997, 2006) and three negative IOD (1996, 1998, 2010) events have been identified that co-occurred with El-Nino and La-Nina events respectively. This study reveals that pIOD events are stronger and but the nIOD events are frequent and effecting Indian Summer Monsoon Rainfall (ISMR). The Sea Surface Temperature (SST) anomalies helps in explaining how strong pIOD favors moisture transport towards the western ghats, central India, northwest India, and anticyclonic conditions in the Bay of Bengal area whereas nIOD produces less rainfall in central India and the western ghats, but greater rainfall in the northeastern region. The reduced ENSO-IOD coupling in the 2000s may have strengthened the connection between ENSO and the Indian Summer Monsoon (ISMR), which had experienced a weakening in preceding decades, as reported by Kumar et al. (Science 284:2156–2159, 1999) and Ashok et al. (Geophys Res Lett 28:4499–4502, 2001). This study also investigates whether a regional climate model (RegCM4.7) accurately incorporate the realistic ENSO-IOD coupling mechanisms, and simulate its impact on regionalized precipitation during summer monsoon. The high-resolution RegCM4.7 simulations are close to the observation of rainfall during 2006, 1996 and 2010 extreme IOD events simulation that indicate RCM’s higher sensitivity towards the abrupt change in boundary condition such as SST. Overall, the monsoon core regions i.e., central India, western ghats and northeast India, both IOD and southern oscillations are the synergistic predictor for the ISMR.