The Australian Northwest Cloudband (NWCB), or just Northwest cloudband, is a band of a broad, continuous, moisture-laden cloud that stretches from the tropical east Indian Ocean to southern Australia, which forms in the cool season and generally brings sporadic precipitation across the Australian continent. [1] The NWCB is the third most commonly occurring cloudband in the world and the fourth most frequently occurring cloudband in the Southern Hemisphere. [2]
Developing a NW-SE orientation and influenced by the Indian Ocean dipole (IOD) and the Madden-Julian oscillation (MJO), the cloudband forms when moist and warm tropical air on the Indian ocean (near Indonesia) 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 within the content and thus create the cloudband. A negative IOD typically causes an increase in northwest cloudbands. [3]
The cloud band occurs when the high pressure system in the southwest thrusts the cold air from the Southern Ocean to northwest Australia, while the northeastern high pulls warm, moist air from the Coral Sea 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. [3] Sea surface temperature gradients in the eastern Indian Ocean, as well as low pressure anomalies (i.e. monsoon lows and tropical depressions) can form the cloudband. [4]
On the warm tropical side, the clouds are generally heavy and convective, which gain a lot of moisture into the upper atmosphere, whereas the lower troposphere is consistently warm and humid. The convection is centered along the NW cloud band since the trade wind inversion is topically weaker. The NW cloudband usually has a high cloud base. [5]
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 and Central Australia, 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. [6] The band increases rainfall over the northwest (Kimberley region and Pilbara), central (Great Victoria Desert region) and southeastern Australia (South Australia, Victoria, New South Wales and Tasmania). [7] [8]
The cloudband is an extensive layer of cloud that tends to be 3,000 km (1,900 mi) to 8,000 km (5,000 mi) long (with a width of at least 450 km (280 mi)) and would link a cold front or cut-off lows at their southeastern end to the ITCZ at their northwestern end. The South Pacific Convergence Zone would often be connected to a NW cloud band. [9] The band is generally around than 5,000 km (3,100 mi) long, where it can stretch from Jakarta in Indonesia down to Hobart, Tasmania. [3] In extreme cases, it can even stretch from a tropical disturbance around the equator just south of Sri Lanka to Tasmania. [10]
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 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 and the Arabian Peninsula). The cloudbands would have a declining trend in late winter and early spring because the country's surrounding oceans would be cooler. [11]
In meteorology, precipitation is any product of the condensation of atmospheric water vapor that falls from clouds due to gravitational pull. The main forms of precipitation include drizzle, rain, sleet, snow, ice pellets, graupel and hail. Precipitation occurs when a portion of the atmosphere becomes saturated with water vapor, so that the water condenses and "precipitates" or falls. Thus, fog and mist are not precipitation; their water vapor does not condense sufficiently to precipitate, so fog and mist do not fall. Two processes, possibly acting together, can lead to air becoming saturated with water vapor: cooling the air or adding water vapor to the air. Precipitation forms as smaller droplets coalesce via collision with other rain drops or ice crystals within a cloud. Short, intense periods of rain in scattered locations are called showers.
The 1998 Pacific hurricane season was a fairly average Pacific hurricane season. Despite this, it had nine hurricanes and six major hurricanes, which was well above average. The season officially started on May 15 in the eastern Pacific and on June 1 in the central Pacific, and ended on November 30; these dates conventionally delimit the period during which most tropical cyclones form in that region. The first tropical cyclone developed on June 11, about ten days later than the normal start of the season. The final storm of the year, Hurricane Madeline, dissipated on October 20. Storm activity in the Central Pacific Hurricane Center's warning zone was low, with just one tropical depression observed in the region. Two tropical cyclones from the eastern Pacific also entered the central Pacific; the former did so as a hurricane.
A rainband is a cloud and precipitation structure associated with an area of rainfall which is significantly elongated. Rainbands in tropical cyclones can be either stratiform or convective and are curved in shape. They consist of showers and thunderstorms, and along with the eyewall and the eye, they make up a tropical cyclone. The extent of rainbands around a tropical cyclone can help determine the cyclone's intensity.
A mesoscale convective system (MCS) is a complex of thunderstorms that becomes organized on a scale larger than the individual thunderstorms but smaller than extratropical cyclones, and normally persists for several hours or more. A mesoscale convective system's overall cloud and precipitation pattern may be round or linear in shape, and include weather systems such as tropical cyclones, squall lines, lake-effect snow events, polar lows, and mesoscale convective complexes (MCCs), and generally forms near weather fronts. The type that forms during the warm season over land has been noted across North and South America, Europe, and Asia, with a maximum in activity noted during the late afternoon and evening hours.
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 and then colder than the eastern part of the ocean.
The 1997–98 South-West Indian Ocean cyclone season was fairly quiet and had the latest start in 30 years. The first tropical disturbance originated on January 16, although the first named storm, Anacelle, was not upgraded until February 8, a record late start. The last storm to dissipate was an unusually late tropical depression in late July. Many of the storms suffered from the effects of wind shear, which contributed to there being only one tropical cyclone – equivalent to a minimal hurricane. The season also occurred during a powerful El Niño.
In meteorology, the different types of precipitation often include the character, formation, or phase of the precipitation which is falling to ground level. There are three distinct ways that precipitation can occur. Convective precipitation is generally more intense, and of shorter duration, than stratiform precipitation. Orographic precipitation occurs when moist air is forced upwards over rising terrain and condenses on the slope, such as a mountain.
The 2005–06 South-West Indian Ocean cyclone season was the fifth least-active on record. The Météo-France office on the island of Réunion tracked 13 tropical disturbances, of which six intensified into a moderate tropical storm. Three of these systems proceeded to attain tropical cyclone status – reaching 10 minute maximum sustained winds of at least 120 km/h (75 mph). The American-based Joint Typhoon Warning Center also tracked eight storms in the basin. Activity was below normal due to a powerful Walker circulation, which increased convection over the neighboring Australian basin, but suppressed activity in the western Indian Ocean. As a result, most of the storms developed near or entered from the Australian basin, crossing 90°E to enter the South-West Indian Ocean.
The 2002–03 Australian region cyclone season included Cyclone Inigo, which tied with Cyclone Gwenda in 1999 as the most intense tropical cyclone on record in the Australian basin. It began on 1 November 2002 and ended on 30 April 2003. The regional tropical cyclone operational plan also defines a tropical cyclone year separately from a tropical cyclone season, which runs from 1 July 2002 to 30 June 2003. These dates conventionally delimit the period of each year when most tropical cyclones form in the Australian region, which is defined as in the southern hemisphere between 90°E and 180°E. The season's ten tropical depressions existed within these dates, with the first, designated as Tropical Cyclone 07S, entering the basin on 27 December 2002. The last system, Cyclone Epi, dissipated on 6 June 2003. Tropical cyclones in this area were monitored by four Tropical Cyclone Warning Centres (TCWCs): the Australian Bureau of Meteorology (BOM) in Perth, Darwin, and Brisbane and one in Port Moresby, Papua New Guinea.
The 1998–99 South-West Indian Ocean cyclone season was a quiet season that had the fourth-lowest number of days with tropical storm or tropical cyclone activity. Most of the storms formed either in the Mozambique Channel or in the far eastern portion of the basin, with five storms crossing from the adjacent Australian basin east of 90° E. As a result, few storms impacted Madagascar, and none made landfall on the African continent. Throughout most of the season, there was below-normal sea surface temperatures in the Indian Ocean east of Madagascar. In February, typically the peak in activity, Réunion island recorded its highest average monthly pressure since 1953. Due to generally unfavorable conditions, there were only six tropical storms tracked by the Météo-France office (MFR) on Réunion. There were only two tropical cyclones – a storm with winds of at least 120 km/h (75 mph).
Rain is water droplets that have condensed from atmospheric water vapor and then fall under gravity. Rain is a major component of the water cycle and is responsible for depositing most of the fresh water on the Earth. It provides water for hydroelectric power plants, crop irrigation, and suitable conditions for many types of ecosystems.
A cold-core low, also known as an upper level low or cold-core cyclone, is a cyclone aloft which has an associated cold pool of air residing at high altitude within the Earth's troposphere, without a frontal structure. It is a low pressure system that strengthens with height in accordance with the thermal wind relationship. If a weak surface circulation forms in response to such a feature at subtropical latitudes of the eastern north Pacific or north Indian oceans, it is called a subtropical cyclone. Cloud cover and rainfall mainly occurs with these systems during the day.
The 2013–14 Australian region cyclone season was a slightly below-average tropical cyclone season, with 10 tropical cyclones occurring within the Australian region. It officially started on 1 November 2013, and ended on 30 April 2014. The regional tropical cyclone operational plan defines a "tropical cyclone year" separately from a "tropical cyclone season"; the "tropical cyclone year" began on 1 July 2013 and ended on 30 June 2014.
The 2016–17 Australian region cyclone season, despite a very high number of tropical lows, was a slightly below-average season in terms of activity, with nine tropical cyclones, three of which intensified further into severe tropical cyclones; though it was much more active than the previous season. The season was the first to have a severe tropical cyclone since the 2014–15 season. It was the period of the year when most tropical cyclones form in the Southern Indian Ocean and Pacific Oceans between 90°E and 160°E. The season officially ran from 1 November 2016 to 30 April 2017, however, a tropical cyclone could form at any time between 1 July 2016 and 30 June 2017 and would count towards the season total. The first named storm, Yvette, developed during 21 December, and the final named storm, Greg, left the region on 3 May as a remnant low. This season was also the second-costliest tropical cyclone season on record in the Australian region basin, behind only the 2010–11 season, with a total of AUD$3.7 billion in damages incurred by the various storms, mostly from Cyclone Debbie.
The 2019–20 Australian region cyclone season was a below average tropical cyclone season for the waters surrounding Australia between longitudes 90°E and 160°E. The season officially began on 1 November 2019 and ended on 30 April 2020; however, tropical cyclones can form at any time of year, as evidenced by Tropical Cyclone Mangga during May 2020. As such, any system existing between 1 July 2019 and 30 June 2020 would count towards the season total. The season featured the region's second-latest start on record, with the formation of the first tropical low only occurring on 4 January 2020. A total of eight tropical cyclones formed during the season, which represents the region's least active season since the 2016–17 season. Three systems intensified further into severe tropical cyclones, and three systems made landfall within the region at tropical cyclone intensity. A total of 28 fatalities were caused, either directly or indirectly, as a result of impacts from the season's systems. Cyclone Ferdinand was the strongest of the season reaching Category 4 in late February 2020. However, it was the second-strongest storm, Cyclone Damien, that was the most damaging. Damien was the strongest tropical cyclone to strike Western Australia's Pilbara Region since Cyclone Christine in 2013, making landfall directly over the town of Dampier.
This glossary of meteorology is a list of terms and concepts relevant to meteorology and atmospheric science, their sub-disciplines, and related fields.
A black nor'easter is a persistent and potentially violent north-easterly storm that occurs on the east coast of Australia, particularly from southeastern Queensland to southern New South Wales, usually between late spring and early autumn, about two days a year.
The southeast Australian foehn is a westerly foehn wind and a rain shadow effect that usually occurs on the coastal plain of southern New South Wales, and as well as in southeastern Victoria and eastern Tasmania, on the leeward side of the Great Dividing Range.
Effects of the El Niño–Southern Oscillation in Australia are present across most of Australia, particularly the north and the east, and are one of the main climate drivers of the country. Associated with seasonal abnormality in many areas in the world, Australia is one of the continents most affected and experiences extensive droughts alongside considerable wet periods that cause major floods. There exist three phases — El Niño, La Niña, and Neutral, which help to account for the different states of ENSO. Since 1900, there have been 28 El Niño and 19 La Niña events in Australia including the current 2023 El Niño event, which was declared on 17th of September in 2023. The events usually last for 9 to 12 months, but some can persist for two years, though the ENSO cycle generally operates over a time period from one to eight years.
The Australian High, also known as the Australian subtropical ridge, is a large, semi-permanent high pressure area or subtropical anticyclone that seasonally vacillates between the Great Australian Bight in the south to the Northern Territory in the north. It is generally located between 25 and 40 degrees of south latitude, depending on the season.