Answer: El Nino Southern Oscillation (ENSO) is a recurring oceano-atmospheric condition in the Southern equatorial pacific due to change in the sea surface temperature. This alternating warming and cooling pattern, also known as the ENSO cycle, has a direct impact on rainfall distribution in the tropics and may have a significant impact on weather across the Indian subcontinent and other parts of the world. 

So this combination of El Niño, La Niña, and the neutral state between the two opposite effects is called the El Niño Southern Oscillation (ENSO). 

Different phases of El Nino-Southern Oscillation (ENSO): 

• El Nino:  El Nino is the warming of sea waters in the Central-east Equatorial Pacific that occurs every few years (Warm phase off the coast of Peru). 

• During El Nino, surface temperatures in the equatorial Pacific rise which weakens the trade winds — east-west winds that blow near the Equator. 
• Due to El Nino, warm western pacific mound created in normal years start receding back because of which  warm water gets accumulated near eastern pacific ocean due to el nino current.
• It thus brings warm water from the western Pacific towards America. 

• La Nina: La Nina is just the opposite of El Nino. It is the positive extreme of the walker cell. It sees cooler than average sea surface temperatures in the eastern equatorial Pacific region. Trade winds are stronger than usual, pushing warmer water towards Asia. 

•  Neutral (walker cell) : Neither El Nino nor La Nina. 
• The Walker circulation (walker cell) is caused by the pressure gradient force that results from a high pressure system over the eastern Pacific ocean, and a low pressure system over Indonesia and nearby regions.

How ENSO impacts monsoon and agriculture in India?

• Monsoon:  

• ENSO has a profound impact on summer monsoonal rainfall across India, as most major droughts occur during El Nino events. 
• During La Nina events, above average rainfall is witnessed in the Indian sub-continent. 

• Agricultural productivity:  

• El Nino has a direct impact on India's agricultural sector since it tends to reduce the production of summer crops. 

• For example, affected crops include rice, sugarcane, cotton, and oilseeds. 

• Rainfall during La Nina events might result in farmers losing their standing Kharif crops. 

• For example, while Kharif crop harvesting happens in September-end or early October, any rain immediately before that would be damaging to the standing crops. 

ENSO is one of the most important climate phenomena on Earth due to its ability to change the global atmospheric circulation, which in turn influences temperature and precipitation across the globe. Because of ENSO, we can often predict the arrival of many seasons in advance of its strongest impacts on weather and climate.