El-Nino is associated with floods, droughts, and other disturbances in a range of locations around the world. These effects makes predicting it of high interest. It is the most prominent known source of inter-annual variability in weather and climate around the world (about 3 to 8 years), though not all areas are affected. It has signatures in the Pacific, Atlantic and Indian Oceans.

El-Nino is officially defined as sustained sea surface temperature anomalies of magnitude greater than 0.5°C across the central tropical Pacific Ocean. When the condition is met for a period of less than five months, it is classified as El-Nino conditions; if the anomaly persists for five months or longer, it is classified as an El-Nino episode. Historically, it has occurred at irregular intervals of 2-7 years and has usually lasted one or two years.

Weather scientists have known for long that El-Nino can influence the behaviour of the monsoon. India’s 132-year-old rainfall history has shown that severe droughts have always been accompanied by El-Nino events. But El-Nino events have not always produced severe droughts. This has baffled meteorologists who have been struggling to fathom the El-Nino-monsoon link.

In the natural course, Eastern Pacific waters remain cool because of Humboldt oceanic current. When El-Nino strongly warms the central Pacific the atmosphere above it heats up and rises. This induces a large, dry air mass to sink over India. This influences monsoons. But there are other factors, such as Indian ocean temperatures, which also play a role.