Understanding the collusion between El Nino and Monsoon

Indian Meteorological Department (IMD) is anticipating an El Nino developing in the Pacific and so there is considerable worry over its impact on this year’s monsoon. These concerns seem to escalate with the delayed arrival of rains over Kerala and a retarded monsoon progression towards north.
Around a century back Sir Gilbert Walker, then Director-General of Observatories in India, found indications that changes in the Pacific affects the monsoon. With more studies, scientific understanding of the ‘El Nino Southern Oscillation’ (ENSO) has increased significantly in recent decades.
It must be noted that not every El Nino hinders the monsoon. But when the Pacific Ocean becomes exceptionally warm the probability of monsoon turning deficient rises sharply. When the Pacific is neither too warm nor cool, there is only a 16% chance of a monsoon ending in a drought. As per rainfall data for 126 years, the possibility of a drought soars to over 40% when there is an El Nino.
Some cases of El Nino’s impact: 

• 1997 was a year of one of the most powerful El Nino events in the last century. The waters of the equatorial Pacific Ocean had warmed sharply even before the onset of monsoon. As such a phenomenon typically stymies monsoon rains over India, a severe drought was widely predicted. However, the monsoon that year ended with above average rains.
• In 2002, a moderate El Nino surprisingly broke down the monsoon which resulted into a massive drought.