What is the Australian monsoon?
The monsoon is a seasonal reversal of winds over parts of the tropics. The word comes from the Arabic term 'mausam', meaning season.
This change in winds is one of Australia's climate drivers and part of our northern seasons. The northern Australian wet season includes the monsoon months but can extend several months on either side.
The monsoon can bring:
- cloudy conditions
- long periods of heavy rain
- occasional thunderstorms, and
- fresh to strong squally winds.
This can lead to floods in affected areas.
Video: Ask the Bureau: What is the monsoon?
When an active monsoon arrives the sky is filled with dark clouds, we see widespread rain showers and thunderstorms, and these can last for a few days or even a week or more. The Australian monsoon develops in response to summertime heating over the northern Australian region, when the continent warms at a faster rate than the surrounding oceans. This sets up a giant sea breeze circulation, drawing in moisture from these oceans over the lower pressure on the land. As the humidity progressively builds, a monsoon trough (which is the focus for the rainfall and cloudiness) becomes established over the Australian region. True monsoonal flow, with deep low-level westerly winds, exists north of the trough; so when the trough moves south over a location, this area becomes affected by monsoonal conditions. An active monsoon period can be followed by an inactive or 'break' monsoon period, when we return to pre-monsoon or build-up type weather conditions – dominated by isolated showers and thunderstorms. Northern Australia typically sees about three monsoon bursts each wet season, sometimes occurring as late as mid-April.
The end of the final monsoon bursts of the year is called the 'monsoon retreat'. Before the onset of the much-anticipated monsoon, northerners endure the so-called build-up: a time when we can go a bit troppo due to the extreme heat and humidity. The cooling, refreshing rains of the monsoon provide welcome change from the build-up and herald the annual transformation from a dry, brown landscape to a lush, watery oasis. It's these monsoonal rains that deliver the vast majority of the annual rainfall across most of tropical northern Australia.
Farmers, communities and the ecosystems which have evolved across Australia's north depend on these monsoonal rains to replenish water storages and recharge natural aquifers. These rains can of course create substantial flooding and restrict movement across large areas. Climate factors such as whether it is El Niño or La Niña can have a significant impact on monsoonal variability. La Niña typically means earlier-than-normal monsoon onset, while El Niño is often associated with rainfall during the monsoon season being less than average. Another important climate driver is the Madden–Julian Oscillation or MJO. It can have a significant impact on the timing of the active and inactive monsoon phases.
We often get asked why the monsoon is important for the rest of Australia. During extended monsoon bursts, massive amounts of moisture build up throughout the atmosphere of northern Australia. Under the right conditions, weather systems across southern Australia tap into this tropical moisture – meaning high humidity, showers and thunderstorms often leading to widespread rainfall and even flooding across southern Australia. But for northern Australia, the life cycle of the people and the environment is intimately related to the annual monsoon.
How the Australian monsoon works
In northern Australia, the prevailing wind is from the east or south-east for most of the year. During active monsoon periods – any time from November to April – the winds shift to become north-westerly at the surface.
As summer approaches, the continent heats up. Low pressure draws the monsoon trough – a zone of low pressure and rising air – over northern Australia. This trough draws in moist air from the Indian Ocean and southern Asian waters.
This influx of moist air is what we call the monsoon. The monsoon trough frequently spawns individual low pressure systems, producing heavy rain and flooding.
Monsoon phases
The monsoon has active and inactive phases.
A typical wet season consists of a prolonged inactive phase during the build-up (the period before initial onset). After this, there are usually 2 to 4 active–inactive cycles.
Inactive phases are usually longer than active ones. Each full cycle lasts about 4–8 weeks.
Transitions from active to inactive monsoon phases may be associated with the Madden–Julian Oscillation.
Active phase
The active phase is usually associated with broad areas of cloud and rain. There are sustained moderate to fresh north-westerly winds on the north side of the trough.
If the trough is close to or over land, there may be widespread heavy rain.
Inactive phase – a break in the monsoon
An inactive phase happens when the monsoon trough temporarily weakens or retreats north of Australia.
Also called a break in the monsoon, this phase brings light winds and isolated shower and thunderstorm activity, sometimes with gusty squall lines.
When is the Australian monsoon?
The northern wet season extends from about October to April in the far north of the Northern Territory. It generally starts later and ends earlier elsewhere. Active monsoon phases may happen at any time during this period.
The initial monsoon onset – the reversal of the winds – normally happens in late December around Darwin.
Later than normal onset is often associated with a positive phase of the Indian Ocean Dipole or El Niño conditions in the Pacific. La Niña is often associated with an early onset. Learn more about El Niño and La Niña.