The word 'monsoon' comes from an Arabic word meaning 'season' and when we talk about monsoons we're generally referring to a seasonal wind that generates widespread, persistent rainfall across a broad region. There are monsoonal regions across most of the world's tropical areas, including northern Australia. In northern Australia we have two distinct seasons: A dry season, which roughly coincides with the southern winter; and a wet season, which peaks during the summer months.

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.

Ask the Bureau: What is the monsoon?

Climatologist Greg Browning explains how the Australian monsoon delivers most of the annual rainfall across our tropical north.

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