Why is the Indian Ocean Dipole in the news | Explained

In the monsoon of 1997, the Central Pacific was running one of the fiercest fevers in its recorded history in the form of an El Niño so violent that, by every rule of thumb, India ought to have stared at one of its worst droughts. However, in the greatest of meteorological surprises, June-September monsoon rainfall ended up being surplus, about 2% above normal. The Director-General of the India Meteorological Department (IMD), Mrutyunjay Mohapatra, observes that it has “happened only once ever.” The agent of that escape sat not in the Pacific but in India’s backyard sea and is called the Indian Ocean Dipole (IOD). With a ‘Super El Niño’ forecast for this year, it is an open meteorological question if a 1997 situation could recur. Currently, the monsoon is running a 40% deficit in India, with the IMD expecting June-September rain to be 90% — a tad above ‘deficient’ --of the long-period average.

The equatorial oceans and the air directly above them are two layers in endless conversation, forever trading moisture. Where the sea surface is warm, it breathes water vapour upward; the moist air rises, cools, condenses into clouds, and falls back as rain. Where the surface is cool, the air sinks dry, and the sky withholds. The Pacific runs the largest version of this engine. Trade winds drag sun-warmed water and heap it near Indonesia, where the air ascends in towering rain, drifts back east at altitude, and subsides over the cool eastern Pacific off South America.