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Climate modesDraft — pending scientific review

The Madden–Julian Oscillation

A travelling pulse of tropical rainfall that circles the globe in one to two months.

This explanation is written conservatively from textbook-level science and has not yet been reviewed by a meteorologist. Treat it as a draft.

The mechanism

Follow the chain: each step causes the next.

  1. 01

    Convection organises

    A large envelope of enhanced thunderstorm activity forms, usually over the Indian Ocean.

  2. 02

    It moves east

    The envelope drifts eastward across the Maritime Continent and into the western Pacific, with a suppressed, drier region on either side.

  3. 03

    Circulation responds

    Winds near the surface and aloft adjust around the moving convection, and the signal continues around the globe mainly in the winds.

  4. 04

    The cycle repeats

    A full cycle typically takes about 30 to 60 days, after which a new active phase can begin over the Indian Ocean.

Explore it

Step through the mechanism one stage at a time.

Step 1 of 4

Convection organises

A large envelope of enhanced thunderstorm activity forms, usually over the Indian Ocean.

In short

A moving pattern, not a storm

The Madden–Julian Oscillation (MJO) is the dominant source of sub-seasonal variability in the tropics. Rather than a single weather system, it is a planetary-scale pattern of alternating wetter and drier conditions that travels eastward along the equator.

Deeper science

Open a section for more detail.

Phase and amplitude

The MJO is commonly tracked with the Real-time Multivariate MJO (RMM) index, which combines outgoing longwave radiation and winds. It is shown on a phase diagram with eight phases describing where the active convection is; the distance from the centre is the amplitude, and values above 1 are usually considered an active MJO.

Why it matters

The MJO can modulate the timing of monsoon rainfall, the likelihood of tropical cyclone formation, and heavy rain episodes across Indonesia. Because it evolves over weeks, it is one of the main sources of predictability between weather forecasts and seasonal outlooks.

Related topics

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    El Niño and La Niña: how the tropical Pacific reshapes weather around the world.

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  • The Indian Ocean Dipole

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