The Madden-Julian Oscillation (MJO) is a slow, eastward-moving pulse of cloud, rain and wind that circles the tropics roughly every 30 to 60 days, and it is one of the main reasons India’s monsoon alternates between wet “active” spells and dry “break” spells. Unlike a cyclone or a single thunderstorm, the Madden-Julian Oscillation is a large-scale pattern spanning thousands of kilometres, but its rhythm shapes when heavy rain reaches India, when cyclones spin up over the seas, and how forecasters plan weeks ahead.
For a country whose farming, water supply and disaster planning depend on the timing of rain, the Madden-Julian Oscillation sits between day-to-day weather and the season as a whole. Last updated: 30 October 2026. This explainer sets out what the MJO is, how it travels, and how it touches the Indian monsoon, the Kerala onset and the storms of the Bay of Bengal and Arabian Sea.
| Attribute | Detail |
|---|---|
| Full name | Madden-Julian Oscillation (abbreviated MJO) |
| Type | Eastward-moving tropical intraseasonal (sub-seasonal) disturbance of cloud, rainfall and winds |
| Discovered | 1971, by Roland Madden and Paul Julian of the US National Center for Atmospheric Research |
| Typical cycle | About 30 to 60 days for one full trip around the globe |
| Direction of travel | Eastward, from the Indian Ocean towards the Pacific, at a few metres per second |
| Most common measure | Real-time Multivariate MJO (RMM) index with eight phases, plus outgoing longwave radiation |
| Main Indian impact | Modulates active and break spells of the southwest monsoon and the timing of cyclone formation |
| Indian forecasting use | India Meteorological Department (IMD) extended range forecasts covering the following two to four weeks |
Key Takeaways
- The Madden-Julian Oscillation is an eastward-travelling tropical pulse of enhanced and suppressed rainfall that repeats roughly every 30 to 60 days.
- Roland Madden and Paul Julian identified the Madden-Julian Oscillation in 1971 while studying tropical wind records from the Pacific.
- During the Indian summer monsoon, the Madden-Julian Oscillation helps drive the alternation of active spells and break spells in rainfall over the subcontinent.
- An active MJO phase over the equatorial Indian Ocean makes cyclone formation over the Bay of Bengal and Arabian Sea more likely in the following days and weeks.
- Forecasters track the Madden-Julian Oscillation with an eight-phase index, and it is a key source of skill in extended range forecasts beyond about ten days.
- The Madden-Julian Oscillation does not decide the total seasonal rainfall, which is more closely tied to El Nino, La Nina and the Indian Ocean Dipole.
What is the Madden-Julian Oscillation?
The Madden-Julian Oscillation is a coupled system of tropical convection (towering rain clouds) and large-scale winds that drifts slowly eastward along the equator and repeats on a cycle of about a month or two. It is the dominant source of variability in the tropics on time scales between roughly two weeks and three months, which meteorologists call the intraseasonal or sub-seasonal range.
At any moment, one stretch of the tropics has an active zone of deep clouds and heavy rain, while the regions on either side have clear, suppressed conditions. This pairing moves east like a slowly rolling wave. A typical active zone is thousands of kilometres wide, so when the MJO sits over the equatorial Indian Ocean, the entire northern Indian Ocean, including the Bay of Bengal and the Arabian Sea, feels its influence.
The oscillation is not a single storm and has no eye or fixed shape. It is better imagined as an envelope of many clusters of thunderstorms that grow, merge and fade as the whole package advances. Air rises and cools in the wet zone, flows eastward and westward at upper levels, sinks in the dry zone and returns along the surface, forming a giant overturning circulation that travels with the cloud mass.
Who discovered the Madden-Julian Oscillation?
The Madden-Julian Oscillation was discovered in 1971 by the American atmospheric scientists Roland Madden and Paul Julian, who worked at the US National Center for Atmospheric Research. While analysing years of upper-air wind and pressure records from a small Pacific island station, they noticed a repeating oscillation in the surface pressure and zonal (east-west) winds with a period of roughly 40 to 50 days.
Their finding was surprising because most known tropical variations at the time were tied to the seasons or to daily cycles. A repeating, eastward-moving signal on a sub-seasonal time scale pointed to a previously unrecognised mode of the atmosphere. In later decades, satellite observations of cloudiness and rainfall confirmed that the pattern is global in reach, and the phenomenon was named after its two discoverers.
Indian researchers have contributed to understanding how the pattern interacts with the monsoon, including work at the Indian Institute of Tropical Meteorology (IITM) in Pune. The MJO is now a standard topic in monsoon research and operational forecasting around the world.
How does the MJO move around the tropics?
The Madden-Julian Oscillation moves eastward at about 4 to 8 metres per second, which means one full circuit of the globe takes around 30 to 60 days. It tends to form over the Indian Ocean or the western Pacific, travels across the Maritime Continent (the islands of Indonesia and surrounding seas), and often weakens as it crosses the cooler central and eastern Pacific, though a weaker signal can continue around the world.
Meteorologists divide each cycle into eight phases using the RMM index, which combines winds and outgoing longwave radiation. Outgoing longwave radiation is the heat energy escaping to space; very cold, high cloud tops reduce it, so low values mark deep convection. The table below summarises where the wet phase sits in each part of the cycle and what that usually means for India.
| RMM phase | Where enhanced rain is centred | General tendency for India |
|---|---|---|
| Phases 1 and 8 | Africa and the Western Hemisphere, or the western Pacific | Convection is mostly elsewhere; India is often in a transition period |
| Phases 2 and 3 | Equatorial Indian Ocean | Rain builds over the sea; cyclone formation becomes more likely; monsoon revival follows |
| Phases 4 and 5 | Maritime Continent | The wet zone moves away from India; the monsoon may weaken |
| Phases 6 and 7 | Western Pacific | Suppressed conditions over the Indian Ocean; break-like spells are more common |
These tendencies are statistical, not guarantees. The same phase can produce different outcomes depending on the season, the background state of the ocean and the strength of the pulse, which is measured by the RMM amplitude. A pulse with an amplitude below one is generally treated as too weak to matter.
How does the MJO affect the Indian monsoon?
The Madden-Julian Oscillation affects the Indian monsoon by pulsing rainfall up and down on a 30 to 60 day rhythm, so that the season’s rain arrives in surges rather than as a steady drizzle. A wet pulse corresponds to an active monsoon spell with a vigorous monsoon trough and widespread heavy rain, while a dry pulse tends to coincide with a break in the monsoon, when the trough shifts towards the Himalayan foothills and rainfall drops across most of the country.
Northward propagation in summer
In the northern summer, the pattern takes a distinctive form over the Indian region. Convection that forms over the equatorial Indian Ocean often moves northward as well as eastward, creeping across the Bay of Bengal and then over the Indian landmass at roughly a degree of latitude per day. Scientists often call this the boreal summer intraseasonal oscillation (BSISO), which is a close relative of the classical MJO.
As the rain band reaches India, the monsoon enters an active phase. Weeks later, the band has moved on towards the Himalayas and the equatorial ocean is dry, which sets up a break in the monsoon. A fresh pulse then begins to build in the south, and the cycle repeats. Over a monsoon season from June to September, India typically experiences several such transitions.
Why the rhythm matters on the ground
Seasonal totals can look normal even when the monsoon behaves badly. If a month’s rain falls in a few days and then stops for weeks, crops may suffer drought stress in the dry stretch and flooding in the wet one. The MJO is therefore a major reason a “normal” monsoon can still bring waterlogging in one fortnight and parched fields in the next.
How does the MJO influence monsoon onset over Kerala?
The Madden-Julian Oscillation influences monsoon onset over Kerala because the arrival of an active MJO pulse over the equatorial Indian Ocean in late May or early June often provides the burst of convection that pushes the monsoon flow onto the Kerala coast. The onset is declared by the India Meteorological Department when specific rainfall, wind and cloud criteria are met, and it is common for onset to follow soon after a wet MJO phase reaches the southern Arabian Sea and Bay of Bengal.
This link helps explain why the date of onset can vary by a week or more from the normal date of around 1 June, even when the larger seasonal pattern looks similar. A strong pulse can bring onset early, while a weak or poorly timed pulse can stall the monsoon near Sri Lanka and the Andaman Sea. Forecasters watch the MJO phase in the weeks before the expected date for this reason.
The MJO also affects the monsoon’s advance after onset. Surges across Maharashtra, Gujarat and central India are frequently tied to the passage of the wet pulse, and stalls in the advance are often linked to a dry phase.
How does the MJO affect cyclones in the Bay of Bengal and Arabian Sea?
The Madden-Julian Oscillation affects cyclones by changing the ingredients that storms need, so cyclone formation over the north Indian Ocean is considerably more likely when the wet MJO phase is over the Indian Ocean. During that phase, the air is moist, the winds at upper and lower levels are favourably arranged and large rotating cloud masses called cyclonic gyres are more easily generated.
Cyclones in the north Indian Ocean mostly form in two seasons: April to June before the monsoon, and October to December after it. The MJO tends to set the precise timing within these windows. A sequence of storms in a single season, such as several cyclones within a few weeks, often traces back to successive MJO passes that repeatedly primed the same ocean basin.
The oscillation also relates to the monsoon onset vortex, a rotating system that can form over the Arabian Sea near the time of onset and, in some years, develops into a cyclone. Several well-known pre-monsoon Arabian Sea storms formed during the active MJO phase. Meanwhile, the dry phase generally suppresses new development, though already-formed storms can persist.
How is the Madden-Julian Oscillation measured and tracked?
The Madden-Julian Oscillation is measured mainly with satellite data on cloud-top temperature and outgoing longwave radiation, combined with wind observations at two heights in the atmosphere. The most widely used product is the Real-time Multivariate MJO index, proposed by Matthew Wheeler and Harry Hendon in 2004, which plots the MJO’s position and strength on a two-axis diagram.
On that diagram, a point far from the centre means a strong MJO, and the angle shows the phase. Over days, the point circles anticlockwise as the pulse moves from phase 1 through phase 8. Forecasting centres, including IMD, publish these diagrams alongside model forecasts of how the pulse should evolve.
- Satellite cloud data: infrared imagery shows where deep convection is present.
- Outgoing longwave radiation: low values indicate towering, rain-bearing clouds.
- Wind data: reversals of the east-west winds at the surface and in the upper troposphere mark the oscillation’s wave structure.
- Ocean data: sea-surface temperature and moored buoys in the Indian Ocean help show how the ocean responds to and feeds the pulse.
How do El Nino and the Indian Ocean Dipole interact with the MJO?
El Nino, La Nina and the Indian Ocean Dipole set the background state of the tropical ocean, and the Madden-Julian Oscillation operates within that background rather than replacing it. A strong El Nino year, for example, tends to be associated with a weaker monsoon overall, but the MJO can still deliver wet and dry pulses inside the season.
The relationship runs in both directions. Strong MJO events, especially westerly wind bursts in the western Pacific, can help trigger or strengthen El Nino events. Meanwhile, a positive Indian Ocean Dipole, in which the western Indian Ocean is warmer than the east, can alter how readily the MJO convection forms and moves across the basin. These interactions are an active area of research and are one reason sub-seasonal prediction remains difficult.
The practical point is that seasonal outlooks and sub-seasonal MJO forecasts answer different questions. The seasonal outlook estimates total rain, while the MJO helps estimate when within the season rain surges and pauses are likely.
Can the MJO help forecast India’s weather weeks ahead?
The Madden-Julian Oscillation helps forecast India’s weather weeks ahead because it is one of the few sources of predictability beyond the roughly ten-day limit of ordinary weather forecasts. Since the pulse moves in a fairly regular, trackable way, models that capture it well can offer useful guidance for the following two to four weeks.
The India Meteorological Department issues extended range forecasts on a weekly cycle that describe the expected rainfall and temperature anomalies for the coming weeks. These products draw on coupled ocean-atmosphere models, and correct simulation of the MJO is a major factor in their skill. Forecasts of monsoon active and break spells, as well as the likelihood of cyclone formation, are among the most valuable outputs.
Limits remain. Many models still weaken the MJO too quickly or struggle to carry it across the Maritime Continent, and the northward progression over India is hard to reproduce. Forecasters therefore treat these outlooks as probabilities rather than certainties.
Why does the Madden-Julian Oscillation matter for farmers and disaster planning?
The Madden-Julian Oscillation matters for farmers and disaster planners because it signals, weeks in advance, whether rain is likely to surge or pause during critical decision points. Sowing, transplanting, irrigation and fertiliser application all depend on the timing of rain, not only its total.
- Agriculture: extended range forecasts help farmers choose sowing dates and plan irrigation for the dry stretch of a break spell.
- Water management: reservoir operators can anticipate inflows after an active pulse and prepare for dry gaps.
- Flood preparedness: a strong wet phase warns urban and river authorities to expect heavy bursts.
- Cyclone readiness: an MJO phase favourable for storms prompts coastal agencies and fishing communities to monitor sea conditions more closely.
- Energy and health: utilities plan for heat during dry breaks, and health departments watch for rain-linked disease spikes after wet bursts.
Bottom Line
The Madden-Julian Oscillation is the tropics’ monthly heartbeat, a travelling wave of rain that sets the tempo of India’s monsoon and nudges cyclones into being. It does not determine how much rain the season brings, but it strongly influences when that rain arrives, which is exactly what farmers and disaster managers need to know.
Frequently Asked Questions
What is the Madden-Julian Oscillation in simple words?
The Madden-Julian Oscillation is a giant pulse of rainy weather that travels eastward around the tropics and returns every 30 to 60 days. Wherever the pulse is, clouds and heavy rain increase, and where it has just passed, conditions turn calmer and drier. It affects India mainly through the timing of monsoon rain surges and cyclones.
How long does one MJO cycle last?
One full Madden-Julian Oscillation cycle typically lasts about 30 to 60 days, which is the time the pulse takes to circle the globe. Individual cycles vary, and some are faster or slower than the average. The oscillation is also stronger in some seasons than others, and it can be weak or hard to detect in certain periods.
Does the MJO cause the active and break spells of the Indian monsoon?
The Madden-Julian Oscillation is one of the main influences on active and break spells of the Indian monsoon, but not the only one. Its northward-moving rain bands over the Bay of Bengal and India produce the wet and dry alternation within the season. Other factors, including the monsoon trough position, local weather systems and the background state of the oceans, also play a part.
Can the MJO trigger cyclones in the Bay of Bengal and Arabian Sea?
The Madden-Julian Oscillation does not create cyclones on its own, but an active phase over the Indian Ocean makes their formation more likely. It supplies moisture, favourable winds and large rotating cloud clusters that can organise into storms. Cyclones in the pre-monsoon and post-monsoon seasons often form during such active phases.
Is the MJO the same as El Nino?
No, the Madden-Julian Oscillation is not the same as El Nino. El Nino is a seasonal-to-yearly warming of the central and eastern Pacific that lasts many months, while the MJO is a faster travelling pulse of rain that lasts weeks per pass. The two interact, however, because MJO wind bursts can help trigger El Nino and El Nino affects how the MJO behaves.
Can the MJO be forecast in advance?
Yes, the Madden-Julian Oscillation can be forecast several weeks in advance with reasonable skill, which makes it valuable for extended range forecasting. The India Meteorological Department and other centres use model forecasts of the MJO’s phase and strength to guide outlooks for rainfall and cyclone risk. Skill is lower when the pulse is weak or is forecast to cross the Maritime Continent.
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