The Australian monsoon (AUM), also known as the Australian summer monsoon (ASM), and the Australian-Indonesian monsoon (AIM), is a

monsoon A monsoon () is traditionally a seasonal reversing wind accompanied by corresponding changes in precipitation but is now used to describe seasonal changes in Atmosphere of Earth, atmospheric circulation and precipitation associated with annu ...

system that increases thunderstorms and rainfall over many areas of

Indonesia Indonesia, officially the Republic of Indonesia, is a country in Southeast Asia and Oceania, between the Indian Ocean, Indian and Pacific Ocean, Pacific oceans. Comprising over List of islands of Indonesia, 17,000 islands, including Sumatra, ...

and

northern Australia The unofficial geographic term Northern Australia includes those parts of Queensland and Western Australia north of latitude 26th parallel south, 26° and all of the Northern Territory. Those local government areas of Western Australia and Q ...

, from the far northern tropics of the region to the semi-arid zone of Australia, typically between November and mid-March, which is the

wet season The wet season (sometimes called the rainy season or monsoon season) is the time of year when most of a region's average annual rainfall occurs. Generally, the season lasts at least one month. The term ''green season'' is also sometimes used a ...

of many parts of northern Australia and Indonesia. The origin of the Australian monsoon (AUM) is comparable the North African monsoon, since both develop from the seasonal motion of the

Intertropical Convergence Zone The Intertropical Convergence Zone (ITCZ , or ICZ), known by sailors as the doldrums or the calms because of its monotonous windless weather, is the area where the northeast and the southeast trade winds converge. It encircles Earth near the t ...

(ITCZ) and the connected meridional shift in the overturning Hadley circulation, which lead to a pronounced rainfall seasonality. From the end of the 19th century, the force of the Australian monsoon has been measured by the summer precipitation at Darwin.

Mechanism

In northern Australia, the predominant wind is from the east or southeast in most occasions, which usually bring dry conditions. Though during monsoon periods (between November and April), the winds change to northwesterly, where

atmospheric pressure Atmospheric pressure, also known as air pressure or barometric pressure (after the barometer), is the pressure within the atmosphere of Earth. The standard atmosphere (symbol: atm) is a unit of pressure defined as , which is equivalent to 1,013. ...

decreases over an area extending to

Java Java is one of the Greater Sunda Islands in Indonesia. It is bordered by the Indian Ocean to the south and the Java Sea (a part of Pacific Ocean) to the north. With a population of 156.9 million people (including Madura) in mid 2024, proje ...

Sumatra Sumatra () is one of the Sunda Islands of western Indonesia. It is the largest island that is fully within Indonesian territory, as well as the list of islands by area, sixth-largest island in the world at 482,286.55 km2 (182,812 mi. ...

Timor Sea The Timor Sea (, , or ) is a relatively shallow sea in the Indian Ocean bounded to the north by the island of Timor with Timor-Leste to the north, Indonesia to the northwest, Arafura Sea to the east, and to the south by Australia. The Sunda Tr ...

and eastward to

Papua New Guinea Papua New Guinea, officially the Independent State of Papua New Guinea, is an island country in Oceania that comprises the eastern half of the island of New Guinea and offshore islands in Melanesia, a region of the southwestern Pacific Ocean n ...

. When the Australian continent heats up much rapid than the surrounding ocean (the Timor,

Banda Sea The Banda Sea (, , ) is one of four seas that surround the Maluku Islands of Indonesia, connected to the Pacific Ocean, but surrounded by hundreds of islands, including Timor, as well as the Halmahera Sea, Halmahera and Ceram Seas. It is about ...

, and

Arafura Sea The Arafura Sea (or Arafuru Sea) lies west of the Pacific Ocean, overlying the continental shelf between Australia and Western New Guinea (also called Papua), which is the Indonesian part of the Island of New Guinea. Geography The Arafura Sea is ...

), low pressure systems may form, which efficaciously draw in the

monsoon trough The monsoon trough is a convergence zone between the wind patterns of the southern and northern hemispheres. It is a portion of the Intertropical Convergence Zone in the Western Pacific,Bin WangThe Asian Monsoon.Retrieved 2008-05-03. and is dep ...

(which is an area of low pressure and rising air) above the hot and dry areas of northern Australia, thereby increasing

humidity Humidity is the concentration of water vapor present in the air. Water vapor, the gaseous state of water, is generally invisible to the human eye. Humidity indicates the likelihood for precipitation (meteorology), precipitation, dew, or fog t ...

prior to the rains (this is known as the “build-up”). The

trough Trough may refer to: In science * Trough (geology), a long depression less steep than a trench * Trough (meteorology), an elongated region of low atmospheric pressure * Trough (physics), the lowest point on a wave * Trough level (medicine), the l ...

attracts moist air from the encompassing oceans and this inflow of moist air is referred to as the monsoon. During the "active" phase or "bursts", large regions of cloud and rain are formed, where there is a constant northwesterly wind on the north area of the trough, alongside heavy rainfall on the land, which last from about four to eight weeks. An inactive phase or "breaks" is when the monsoon trough diminishes and withdraws to the north of Australia, although light winds, sporadic showers and thunderstorm activity may occur. Later than normal monsoon arrivals, and drier ones, are generally associated with

El Niño EL, El or el may refer to: Arts and entertainment Fictional entities * El, a character from the manga series ''Shugo Chara!'' by Peach-Pit * Eleven (''Stranger Things'') (El), a fictional character in the TV series ''Stranger Things'' * El, fami ...

conditions in the Pacific, while

La Niña LA most frequently refers to Los Angeles, the second most populous city in the United States of America. La, LA, or L.A. may also refer to: Arts and entertainment Music *La (musical note), or A, the sixth note *"L.A.", a song by Elliott Smit ...

is mostly associated with an early monsoon season and wetter ones as well. The monsoon's thickness is normally less than 1,500 metres (4,900 feet) over the sea and 2,000–2,500 metres (6,600–8,200 feet) over the land. Although the northern Australian monsoonal bursts peak between mid-November and mid-December, they can extend well into March. The Australian monsoon rainfall variability (AUMRV) has a similar ratio in rainfall over much of the country, going as far south as the southern Murray–Darling Basin. About 28% of inter-annual AUMRV is linked with oceanic irregularity in the tropical Pacific and

Indian Ocean The Indian Ocean is the third-largest of the world's five oceanic divisions, covering or approximately 20% of the water area of Earth#Surface, Earth's surface. It is bounded by Asia to the north, Africa to the west and Australia (continent), ...

s. Moreover, compared to the Asian, African and

North American North America is a continent in the Northern and Western hemispheres. North America is bordered to the north by the Arctic Ocean, to the east by the Atlantic Ocean, to the southeast by South America and the Caribbean Sea, and to the sou ...

monsoonal regions, the AUM seems to possess the highest variation, as it is linked to

El Niño–Southern Oscillation El Niño–Southern Oscillation (ENSO) is a global climate phenomenon that emerges from variation in winds and sea surface temperatures over the tropical Pacific Ocean. Those variations have an irregular pattern but do have some semblance of cyc ...

, especially over northeastern Australia.

Effects

The monsoon seasons are generally associated with overcast conditions, extended periods of heavy rain, episodic thunderstorms and fresh to strong squally winds, which frequently lead to flooding in affected areas in the

Northern Territory The Northern Territory (abbreviated as NT; known formally as the Northern Territory of Australia and informally as the Territory) is an states and territories of Australia, Australian internal territory in the central and central-northern regi ...

, northern

Western Australia Western Australia (WA) is the westernmost state of Australia. It is bounded by the Indian Ocean to the north and west, the Southern Ocean to the south, the Northern Territory to the north-east, and South Australia to the south-east. Western Aust ...

and northern

Queensland Queensland ( , commonly abbreviated as Qld) is a States and territories of Australia, state in northeastern Australia, and is the second-largest and third-most populous state in Australia. It is bordered by the Northern Territory, South Austr ...

. Most of the

fresh water Fresh water or freshwater is any naturally occurring liquid or frozen water containing low concentrations of dissolved salt (chemistry), salts and other total dissolved solids. The term excludes seawater and brackish water, but it does include ...

for the sparsely populated northern Australia comes from the Australian monsoonal rains. The Australian monsoon can also have a high influence on rainfall on the southeastern seaboard during the warmer months, such as in

southeast Queensland South East Queensland (SEQ) is a bio-geographical, metropolitan and statistical region of the state of Queensland in Australia, with a population of approximately 4.0 million people out of the state's population of 5.5 million. The area covere ...

and as well as the northern half of

New South Wales New South Wales (commonly abbreviated as NSW) is a States and territories of Australia, state on the Eastern states of Australia, east coast of :Australia. It borders Queensland to the north, Victoria (state), Victoria to the south, and South ...

(

Northern Rivers Northern Rivers is the most northeasterly Regions of New South Wales, region of the Australian state of New South Wales, located between north of the state capital, Sydney, and encompasses the catchments and fertile valleys of the Clarence Ri ...

to metropolitan

Sydney Sydney is the capital city of the States and territories of Australia, state of New South Wales and the List of cities in Australia by population, most populous city in Australia. Located on Australia's east coast, the metropolis surrounds Syd ...

), where summer is the wettest season and winter is the driest (the precipitation contrast between the two seasons decrease farther to the south as the effect of the Australian monsoon lessens).

References

{{reflist Climate of Australia Winds Regional climate effects Environmental science Weather events in Australia Climate oscillations Climate of Indonesia

Mechanism

Effects

See also

References