How monsoon actually works in general ?
During summer, the landmass gets heated up and the air near the land gets warm and goes up. At the same time, the air above the ocean is relatively cool and travel towards land to fill the gap created by the warm air. This is like the landmass is taking a huge breathe. So, this cool moisture filled air from ocean gives rain to the landmass.
But the above situation will get disturbed, when the surface of the ocean also becomes hot. So, the warm ocean surface and warm landmass competes for cold air which results in deficit in monsoon. This happens during the El-Nino years when the entire pacific becomes warmer.
What are the things that impacts Indian monsoon?
The El-Nino and IOD are two of the many phenomenons that influence the Indian monsoon. Some years El-Nino will have strong influence on monsoon and in some other years IOD will have strong influence. So, monsoon prediction is difficult.
What is Indian Nino?
It is a phenomenon where sea surface temperature becomes warmer and colder alternatively between eastern and western part of Indian Ocean. It is also called Indian Ocean Dipole. It is irregular in nature.
How does it happen?
This phenomenon occurs in three phases. One is "positive" Indian Ocean Dipole, in this scenario the western part of the Indian ocean becomes warmer than usual. So, it brings heavy rains to eastern Africa and Indian subcontinent. The another ones is "negative" Indian Ocean Dipole, here the eastern part of the Indian ocean becomes warmer and leaves the eastern Africa and India in drought. This phenomenon also has a "neutral" phase.