The Usual Suspect: What Is El Niño?
El Niño is a naturally occurring climate pattern characterized by the unusual warming of sea surface temperatures in the central and eastern equatorial Pacific Ocean. Though it happens thousands of kilometres away, it disrupts global weather systems.
Typically, atmospheric circulation, known as the Walker Circulation, involves rising moist air over the warm western Pacific (near Indonesia) and sinking dry air over the cooler eastern Pacific. This pattern helps drive the winds that bring moisture to India. During an El Niño, this engine shifts. The warming of the central and eastern Pacific pulls the rain-making activity away from the Indian region, often weakening the monsoon winds and increasing the chances of below-average rainfall. Historically, some of India’s most severe droughts have coincided with strong El Niño events.
The Monsoon’s Guardian: The Indian Ocean Dipole
While El Niño is a powerful player, it doesn't act alone. The Indian Ocean has its own climate see-saw called the Indian Ocean Dipole (IOD). The IOD measures the temperature difference between the western Indian Ocean (near the Arabian Sea) and the eastern Indian Ocean (near Indonesia). It has two crucial phases. In a 'negative' phase, the eastern Indian Ocean is warmer, which can worsen El Niño's drying effect. However, a 'positive' phase is the monsoon's saving grace. During a positive IOD, the waters of the Arabian Sea become warmer than usual, increasing moisture and strengthening the monsoon winds blowing toward India. A strong positive IOD can actively counteract the negative effects of an El Niño, sometimes turning a potential drought year into one with normal or even surplus rain. This happened in 1997 and 2019, when a positive IOD compensated for El Niño conditions and helped deliver a healthy monsoon.
Not All El Niños Are Created Equal
Another layer of complexity is that there isn't just one type of El Niño. Scientists now distinguish between a 'classic' El Niño, where warming is concentrated in the eastern Pacific near South America, and an 'El Niño Modoki', where the warming is focused in the central Pacific. The name 'Modoki' is Japanese for 'similar, but different'. This geographical distinction is critical. Research suggests that central-Pacific El Niños (Modoki events) have a stronger link to Indian droughts. The warming in this specific region can cause air to sink over India, suppressing the formation of rain clouds. The location of the Pacific warming—not just its intensity—plays a huge role in determining how severely the monsoon is affected.
The Wild Cards: MJO and Other Factors
Other, shorter-term phenomena also stir the pot. The Madden-Julian Oscillation (MJO) is a large, eastward-moving pulse of clouds and rainfall that travels around the equator every 30 to 60 days. When the MJO's 'active' phase is over the Indian Ocean, it can enhance monsoon rainfall and trigger wet spells. Conversely, its 'suppressed' phase can lead to breaks in the monsoon. The MJO can either amplify or weaken El Niño's impact depending on its phase and timing. Furthermore, some research points to atmospheric disturbances from as far away as the North Atlantic. These can send air currents, known as Rossby waves, toward the subcontinent, sometimes suppressing rainfall in the crucial late-monsoon period of August, independent of an El Niño.
