28-05-2025
This Word Means: Madden-Julian Oscillation
While the southwest monsoon arrived in Kerala on May 24, eight days earlier than the normal onset date, it reached Mumbai two weeks in advance on May 26. This was also the earliest the monsoon arrived in Mumbai on record.
Several large-scale atmospheric-oceanic and local factors developed and favoured the early monsoon onset but one of the key drivers was the Madden-Julian Oscillation (MJO), according to the India Meteorological Department (IMD).
What is MJO?
The MJO is a moving system of winds, cloud, and pressure that brings rain as it circles around the equator. The phenomenon takes its name from the two scientists who identified it in 1971 — Roland Madden and Paul Julian, who then worked at the National Centre for Atmospheric Research in Boulder, Colorado.
The system typically travels eastward at a speed of 4-8 metres per second. It goes around the globe in 30-60 days on average, but can sometimes take 90 days.
As it moves, strong MJO activity often splits the planet into two parts — one in which the MJO is in active phase and brings rainfall, and the other in which it suppresses rainfall. In the active phase, MJO results in higher-than-average rainfall for that time of the year, while in the suppressed phase, the area receives less than average rainfall.
An active phase is generally followed by a weak or suppressed phase, in which there is little MJO activity.
The effect of the MJO is witnessed mainly in the tropical region, in the band between 30 degrees North and 30 degrees South of the equator, even though the mid-latitude regions in both hemispheres also feel its impact. India falls in this band. As the MJO cycle lasts only 30-60 days, there can be multiple MJO events in a season.
There is some evidence that the MJO and El Niño — the unusual warming of sea-surface Pacific waters off the South American coast — are correlated. Thus, strong MJO activity, like this year, is witnessed in a year of strong El Niño. However, the correlation is not exclusive: a strong El Niño year — the strength being a measure of the increase of temperature of sea surface water — is generally associated with a bad monsoon.
How did the MJO contribute to the early monsoon onset?
In the tropics, MJO in its active phase brings frequent cyclonic activity, and can initiate the onset of the monsoon. For instance, in June 2015, the MJO resulted in about 20 days of very good rainfall in most parts of the country.
This is what could have happened this year as well. That is because around May 22, the MJO, which had originated in the Indian Ocean, was in Phase 4 with an amplitude greater than 1, which is indicative of strong rainfall and storms, according to IMD's extended range forecast.
