Madden–Julian oscillation

The MJO is a coupled tropical disturbance in which enhanced rainfall and deep convection alternate with suppressed convection as the system moves eastward. The disturbance was identified in the early 1970s by Roland Madden and Paul Julian from pressure and wind records at equatorial stations, especially Canton Island in the central Pacific.¹ Unlike El Niño–Southern Oscillation, which varies mainly on interannual timescales and is closely tied to sea-surface temperatures, the MJO is an intraseasonal phenomenon whose active and quiet phases recur several times each year.

Its period is not perfectly regular: individual events may last from about 30 to 60 days, while propagation can slow, stall, or weaken over the Maritime Continent. The oscillation is strongest in the zonal winds and rainfall of the equatorial Indian and Pacific oceans, but its circulation anomalies extend into the subtropics.

The MJO consists of a moving convective envelope accompanied by a characteristic progression of wind and pressure anomalies. An enhanced-rainfall phase is followed to its west by low-level westerly winds and to its east by low-level easterlies; the pattern reverses aloft, reflecting a large-scale overturning circulation.²

Moisture accumulation helps organize the disturbance: shallow clouds and moisture often increase before deep convection becomes prominent, while evaporation, radiation, surface fluxes, and atmospheric moisture transport help sustain or terminate it. The system is therefore not simply a wave in sea-surface temperature. Its behavior emerges from interactions among convection, circulation, moisture, and the underlying ocean.

Forecast centers commonly describe its position with the Real-time Multivariate MJO index, which combines near-equatorial outgoing longwave radiation with zonal-wind fields. The index represents phase and amplitude rather than a single physical measurement.

The MJO changes the probability of important weather events on subseasonal timescales, filling part of the forecasting gap between short-range weather prediction and seasonal outlooks. Its convective phases modulate monsoon rainfall in Asia, Africa, and Australia and can alter the likelihood, location, and timing of tropical cyclone formation.³

As the disturbance moves through the tropics, its altered winds can trigger teleconnections that affect mid-latitude circulation, including the jet stream and storm tracks. These remote responses depend on the MJO phase, season, background climate, and the state of phenomena such as El Niño. An active MJO phase can also influence atmospheric rivers, cold-air outbreaks, and extreme precipitation in otherwise temperate regions.

Operational prediction is most useful for broad changes in risk rather than deterministic forecasts of a particular storm. Skill generally declines after several weeks, especially when the convective signal is weak or disrupted by competing tropical variability.

The MJO is often strongest over oceanic regions but can be disrupted by the Maritime Continent, where the many islands, shallow seas, and complex coastlines interfere with the usual ocean–atmosphere coupling. This “barrier” effect is a major challenge for models and can cause propagation to slow or temporarily stall.⁴

Its influence also extends beyond rainfall totals. MJO-related circulation changes affect the timing of tropical cyclone genesis, the vertical distribution of ozone and other trace gases, and the organization of other tropical waves. Some events propagate eastward only through part of the tropics, while others weaken over the Indian Ocean or Pacific before completing a circuit.

Climate models have historically struggled to reproduce the MJO’s speed, amplitude, and persistence. Improving its simulation matters not only for climate projections but also for subseasonal-to-seasonal prediction, disaster preparedness, water management, and agricultural planning.⁵

Convection

The vertical transport of heat and moisture in the atmosphere, often expressed in the tropics through deep clouds and heavy rain.

Outgoing longwave radiation

Infrared radiation emitted to space; unusually low values over the tropics commonly indicate high, cold cloud tops associated with deep convection.

Teleconnection

A geographically distant atmospheric response linked to a source disturbance through large-scale circulation patterns.

Subseasonal-to-seasonal prediction

Forecasting on timescales from several weeks to several months, between conventional weather forecasts and seasonal climate outlooks.