2019–20 Australian bushfire as seen by Himawari 8 2020-01-04

The Australian Northwest Cloudband (NWCB), or just Northwest cloudband, is a

band Band or BAND may refer to: Places *Bánd, a village in Hungary *Band, Iran, a village in Urmia County, West Azerbaijan Province, Iran * Band, Mureș, a commune in Romania *Band-e Majid Khan, a village in Bukan County, West Azerbaijan Province, I ...

of a broad, continuous, moisture-laden cloud that stretches from the tropical east

Indian Ocean The Indian Ocean is the third-largest of the world's five oceanic divisions, covering or ~19.8% of the water on Earth's surface. It is bounded by Asia to the north, Africa to the west and Australia to the east. To the south it is bounded by th ...

to southern Australia, which would form in the cool season and may generally bring sporadic precipitation across the

Australia Australia, officially the Commonwealth of Australia, is a Sovereign state, sovereign country comprising the mainland of the Australia (continent), Australian continent, the island of Tasmania, and numerous List of islands of Australia, sma ...

n continent. The NWCB is the third most commonly occurring cloudband in the world and the fourth most frequently occurring cloudband in the Southern Hemisphere.

Formation

Developing a NW-SE orientation and influenced by the

Indian Ocean dipole The Indian Ocean Dipole (IOD), also known as the Indian Niño, is an irregular oscillation of sea surface temperatures in which the western Indian Ocean becomes alternately warmer (positive phase) and then colder (negative phase) than the eastern ...

(IOD) and the Madden-Julian oscillation (MJO), the cloudband forms when moist and warm tropical air on the Indian ocean (near

Indonesia Indonesia, officially the Republic of Indonesia, is a country in Southeast Asia and Oceania between the Indian and Pacific oceans. It consists of over 17,000 islands, including Sumatra, Java, Sulawesi, and parts of Borneo and New Guine ...

) travels towards the southeast and passes over Australia towards the western side of a high pressure system in the southeast, where they ascend over the cooler air in the

mid-latitudes The middle latitudes (also called the mid-latitudes, sometimes midlatitudes, or moderate latitudes) are a spatial region on Earth located between the Tropic of Cancer (latitudes 23°26'22") to the Arctic Circle (66°33'39"), and Tropic of Caprico ...

within the content and thus create the cloudband. A negative IOD typically causes an increase in northwest cloudbands.Massive northwest cloudband from Indonesia to Tasmania
by Anthony Sharwood from Weatherzone. 29 April 2022. The cloud band occurs when the

high pressure system A high-pressure area, high, or anticyclone, is an area near the surface of a planet where the atmospheric pressure is greater than the pressure in the surrounding regions. Highs are middle-scale meteorological features that result from interpl ...

in the southwest thrusts the cold air from the

Southern Ocean The Southern Ocean, also known as the Antarctic Ocean, comprises the southernmost waters of the World Ocean, generally taken to be south of 60° S latitude and encircling Antarctica. With a size of , it is regarded as the second-small ...

to northwest Australia, while the northeastern high pulls warm, moist air from the

Coral Sea The Coral Sea () is a marginal sea of the South Pacific off the northeast coast of Australia, and classified as an interim Australian bioregion. The Coral Sea extends down the Australian northeast coast. Most of it is protected by the Fre ...

towards the center of Australia. When the warm northeasterly air clashes with the cold southwesterly airflow, their friction creates a large area of unstable rising air, thus forming clouds. The damp air and cloud that formed over the Indian Ocean then crossbeam the continent. Sea surface

temperature gradient A temperature gradient is a physical quantity that describes in which direction and at what rate the temperature changes the most rapidly around a particular location. The temperature gradient is a dimensional quantity expressed in units of degree ...

s in the eastern Indian Ocean, as well as low pressure anomalies (i.e.

monsoon A monsoon () is traditionally a seasonal reversing wind accompanied by corresponding changes in precipitation but is now used to describe seasonal changes in atmospheric circulation and precipitation associated with annual latitudinal oscil ...

lows and

tropical depression A tropical cyclone is a rapidly rotating storm system characterized by a low-pressure center, a closed low-level atmospheric circulation, strong winds, and a spiral arrangement of thunderstorms that produce heavy rain and squalls. Depend ...

s) can form the cloudband.

Background

On the warm tropical side, the clouds are generally heavy and

convective Convection is single or multiphase fluid flow that occurs spontaneously due to the combined effects of material property heterogeneity and body forces on a fluid, most commonly density and gravity (see buoyancy). When the cause of the convec ...

, which gain a lot of moisture into the

upper atmosphere Upper atmosphere is a collective term that refers to various layers of the atmosphere of the Earth above the troposphere and corresponding regions of the atmospheres of other planets, and includes: * The mesosphere, which on Earth lies between the ...

, whereas the lower

troposphere The troposphere is the first and lowest layer of the atmosphere of the Earth, and contains 75% of the total mass of the planetary atmosphere, 99% of the total mass of water vapour and aerosols, and is where most weather phenomena occur. From ...

is consistently warm and humid. The convection is centered along the NW cloud band since the

trade wind The trade winds or easterlies are the permanent east-to-west prevailing winds that flow in the Earth's equatorial region. The trade winds blow mainly from the northeast in the Northern Hemisphere and from the southeast in the Southern Hemisph ...

inversion is topically weaker. The NW cloudband usually has a high cloud base. A tropical-extratropical feature, the cloudband provides up to 80% of the annual rainfall for northwestern Australia and up to 40% of the annual rainfall for southwestern Australia, including most of the winter rainfall over northwest

Western Australia Western Australia (commonly abbreviated as WA) is a state of Australia occupying the western percent of the land area of Australia excluding external territories. It is bounded by the Indian Ocean to the north and west, the Southern Ocean to th ...

and

Central Australia Central Australia, also sometimes referred to as the Red Centre, is an inexactly defined region associated with the geographic centre of Australia. In its narrowest sense it describes a region that is limited to the town of Alice Springs and i ...

, which tend to have a drying trend in that season. Southern Australia, though, would receive most of its cloudband-associated rainfall when the band connects with cold front systems. The band increases rainfall over the northwest (

Kimberley region The Kimberley is the northernmost of the nine regions of Western Australia. It is bordered on the west by the Indian Ocean, on the north by the Timor Sea, on the south by the Great Sandy Desert, Great Sandy and Tanami Desert, Tanami deserts ...

and

Pilbara The Pilbara () is a large, dry, thinly populated region in the north of Western Australia. It is known for its Aboriginal peoples; its ancient landscapes; the red earth; and its vast mineral deposits, in particular iron ore. It is also a glo ...

), central (

Great Victoria Desert The Great Victoria Desert is a sparsely populated desert ecoregion and interim Australian bioregion in Western Australia and South Australia. History In 1875, British-born Australian explorer Ernest Giles became the first European to cros ...

region) and southeastern Australia (

South Australia South Australia (commonly abbreviated as SA) is a state in the southern central part of Australia. It covers some of the most arid parts of the country. With a total land area of , it is the fourth-largest of Australia's states and territories ...

Victoria Victoria most commonly refers to: * Victoria (Australia), a state of the Commonwealth of Australia * Victoria, British Columbia, provincial capital of British Columbia, Canada * Victoria (mythology), Roman goddess of Victory * Victoria, Seychelle ...

New South Wales ) , nickname = , image_map = New South Wales in Australia.svg , map_caption = Location of New South Wales in AustraliaCoordinates: , subdivision_type = Country , subdivision_name = Australia , established_title = Before federation , es ...

and

Tasmania ) , nickname = , image_map = Tasmania in Australia.svg , map_caption = Location of Tasmania in AustraliaCoordinates: , subdivision_type = Country , subdi ...

). The cloudband is an extensive layer of cloud that tends to be to long (with a width of at least ) and would link a

cold front A cold front is the leading edge of a cooler mass of air at ground level that replaces a warmer mass of air and lies within a pronounced surface trough of low pressure. It often forms behind an extratropical cyclone (to the west in the Norther ...

cut-off low A cut-off low (or cutoff low), sometimes referred to as the weatherman's woe, is defined as "a closed upper-level low which has become completely displaced (cut off) from basic westerly current, and moves independently of that current" by the Na ...

s at their southeastern end to the

ITCZ The Intertropical Convergence Zone (ITCZ ), 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 thermal ...

at their northwestern end. The

South Pacific Convergence Zone The South Pacific Convergence Zone (SPCZ), a reverse-oriented monsoon trough, is a band of low-level convergence, cloudiness and precipitation extending from the Western Pacific Warm Pool at the maritime continent south-eastwards towards French Pol ...

would often be connected to a NW cloud band. The band is generally around than long, where it can stretch from

Jakarta Jakarta (; , bew, Jakarte), officially the Special Capital Region of Jakarta ( id, Daerah Khusus Ibukota Jakarta) is the capital and largest city of Indonesia. Lying on the northwest coast of Java, the world's most populous island, Jakarta ...

in Indonesia down to

Hobart Hobart ( ; Nuennonne/Palawa kani: ''nipaluna'') is the capital and most populous city of the Australian island state of Tasmania. Home to almost half of all Tasmanians, it is the least-populated Australian state capital city, and second-small ...

, Tasmania. In extreme cases, it can even stretch from a tropical disturbance around the equator just south of

Sri Lanka Sri Lanka (, ; si, ශ්‍රී ලංකා, Śrī Laṅkā, translit-std=ISO (); ta, இலங்கை, Ilaṅkai, translit-std=ISO ()), formerly known as Ceylon and officially the Democratic Socialist Republic of Sri Lanka, is an ...

to Tasmania.

Occurrence

They generally occur between March and October, but more during late autumn and early winter when the oceans around the country are warm and the subtropical

jet stream Jet streams are fast flowing, narrow, meandering thermal wind, air currents in the Atmosphere of Earth, atmospheres of some planets, including Earth. On Earth, the main jet streams are located near the altitude of the tropopause and are west ...

is strong. Lasting from a few days to a week, they are more common when temperatures in the eastern Indian Ocean (near Australia) are higher than those in the western Indian Ocean (near

Eastern Africa East Africa, Eastern Africa, or East of Africa, is the eastern subregion of the African continent. In the United Nations Statistics Division scheme of geographic regions, 10-11-(16*) territories make up Eastern Africa: Due to the historica ...

and the

Arabian Peninsula The Arabian Peninsula, (; ar, شِبْهُ الْجَزِيرَةِ الْعَرَبِيَّة, , "Arabian Peninsula" or , , "Island of the Arabs") or Arabia, is a peninsula of Western Asia, situated northeast of Africa on the Arabian Plate ...

). The cloudbands would have a declining trend in late winter and early spring because the country's surrounding oceans would be cooler.Smith, R.K., M.J. Reeder, N.J. Tapper, and D.R. Christie, 1995. Central Australian cold fronts. Mon. Wea. Rev., 123, 16-38.

References

{{reflist

External links

Influence of Northwest Cloudbands on Southwest Australian Rainfall
Meteorological phenomena Weather events Atmospheric dynamics Atmospheric circulation Climate of Australia Indian Ocean Climate of Indonesia