What is the Madden–Julian Oscillation?
The Madden–Julian Oscillation, or MJO, is a pulse of cloud and rainfall near the equator. It circles the globe eastward every 30–60 days. The MJO is an important climate driver of tropical weather.
The MJO can bring changes in wind, cloudiness and rainfall. Its effects are most obvious over the Indian Ocean and western equatorial Pacific. It influences the timing, development and strength of the major global monsoon patterns. This includes the Indian and Australian monsoons.
The MJO can change the likelihood of a wetter or drier period on weekly to monthly timescales. This is different to the El Niño–Southern Oscillation and Indian Ocean Dipole. These drive changes in Australia's climate on seasonal to annual timescales.
Video: Understanding the Madden–Julian Oscillation (MJO)
While the El Niño–Southern Oscillation and the Indian Ocean Dipole drive changes in Australia's climate on seasonal to annual timescales, the MJO can change the odds of having a wetter or drier period on weekly to monthly timescales.
So what is the MJO?
It is an eastward moving zone of wind and cloud that circles the tropical areas of the globe in around 30– 60 days.
When that extra cloud arrives, it can increase the chance of receiving above-average rainfall.
However, ahead and behind the extra tropical cloudiness are regions where it might be drier than usual.
The relationship between the MJO and Australian weather patterns changes with the season.
Its greatest effect on our rainfall occurs during the northern Australian wet season, which runs from October to April.
During this period, the MJO can influence the timing, variability and strength of the Australian monsoon.
Late in every year a belt of low pressure, known as the monsoon trough, forms in the tropics and becomes the focus for enhanced cloud and rainfall for the wet season.
The monsoon is at its most active when this trough is close to, or over northern Australia, and typically brings widespread rainfall to tropical areas.
When the MJO is in the Australian region, it can strengthen the monsoon trough and bring even more rainfall.
And at the start of the season it can encourage the monsoon trough to form.
The monsoon trough can also go through periods when it is weaker or has moved north of Australia. This is called a break in the monsoon.
As the MJO moves away from Australia it can bring higher pressure systems, and less cloud, causing this break.
Another way the MJO effects our weather is its influence on tropical cyclones.
During the tropical cyclone season, cyclones are more likely to develop when the MJO is nearby.
They are less likely to develop when it is further away from Australia.
The MJO also has a relationship with the El Niño–Southern Oscillation, which swings between La Niña, neutral and El Niño.
An MJO can bring periods of weaker trade winds over the Pacific. These can help trigger or strengthen El Niño – potentially bringing less rainfall for eastern Australia.
Likewise, it can weaken or disrupt La Niña – a climate driver often associated with above-average Australian rainfall.
While the MJO has its biggest impact in the tropics, this is not the only region it can affect.
In the southern winter and spring – the dry season in northern Australia – the tropical cloudiness of the MJO shifts north of the equator. But it can still influence rainfall across parts of central and even southern Australia due to changes it can trigger in weather patterns.
The Madden–Julian Oscillation plays an important role in driving Australia's climate.
As with all our climate drivers, no two MJO events, and no two sets of impacts are exactly the same.
Understanding this key Australian climate driver can help you better understand rainfall and temperature outlooks – and make smarter climate decisions.
MJO impact on Australia
The MJO has most effect on Australia's tropical areas, with only some influence on parts of southern Australia.
Influencing our monsoon
The MJO's greatest effect on our rainfall is during the northern Australian wet season, which runs from October to April. During this time, it can influence the timing, variability and strength of the Australian monsoon.
When the MJO is in the Australian region, it can strengthen the monsoon trough and bring even more rainfall. At the start of the season, it can encourage the monsoon trough to form.
The monsoon trough can also go through periods when it is weaker or moves north of Australia. This is called a break in the monsoon. As the MJO moves away from Australia it can bring higher pressure systems and less cloud. This can cause the break.
Impact on tropical cyclones
During the tropical cyclone season, cyclones are more likely to develop when the MJO is nearby. They are less likely to develop when it's further away from Australia.
Effect on the El Niño Southern Oscillation
When the MJO pulse of rainfall is over the Pacific Ocean, it can mean weaker trade winds for the Pacific region. This can help trigger or strengthen El Niño, potentially bringing less rainfall for the months ahead. It can also weaken or disrupt La Niña, which is often associated with above average Australian rainfall. Learn more about El Niño and La Niña.
Monitoring the MJO
Most tropical rainfall comes from tall thunderstorms which have very cold tops. These cold tops can be detected by satellites, so we use satellite observations to monitor the MJO in the tropics.
For the MJO forecast, view the Climate driver update on our current website.