The southeast Australian foehn is a westerly foehn wind and a rain shadow effect that occurs on the coastal plain of eastern New South Wales, and as well as in southeastern Victoria and eastern Tasmania, on the eastern side of the Great Dividing Range.Rain Shadows by Don White. Australian Weather News. Willy Weather. Retrieved 24 May 2021.
Ranging from cool to hot (depending on the season), the effect occurs when westerly winds descend steeply from the Great Dividing Range onto the coastal slopes, causing major adiabatic compression (the rate at which temperature decreases with altitude) and a substantial loss of moisture.Where has the rain gone in Sydney? by Ben Domensino from Weatherzone. 22 June 2022. Retrieved 23 June 2022 The effect is known by other names, such as the Australian chinook, the Great Dividing wind, the Great Dividing foehn or simply westerly foehn.
Typically occurring from late autumn to spring, though not completely unheard of in the summer (particularly in eastern Tasmania), the foehn effect mainly occurs when a westerly or south-westerly frontal system (which brings rainy and windy weather to southern capitals like Melbourne, Perth and Adelaide) passes over the Great Dividing Range and thereby results in clear to partly cloudy, relatively warmer conditions on the lee.Rain one side, heat the other in NSW by Joel Pippard. Weatherzone. 16 April 2020. Retrieved 6 October 2021
Formation and characteristics
This foehn effect on the east coast of Australia is linked with the passage of a deep low pressure system across the Great Australian Bight and Bass Strait that cause strong winds to reorient virtually perpendicular to some parts of the Great Dividing Range, predominantly between late autumn into winter and spring (being most common during a negative AAO phase). Their occurrence is owed to the incomplete orographic blocking of comparatively moist low-level air and the subsidence of drier upper-level air in the lee of the mountains.
Foehn occurrence on the east coast can also occur when hot, north-westerly winds blow from the interior (even when there is little moisture on the windward side), because the air heats up faster as it descends onto the plains than it cooled as it ascended the ranges.
Typically between to , sometimes they may be brought on by a large polar air mass from the south-west of the continent in the Southern Ocean which moves east or north-eastward across Victoria towards the east coast.Wilder winds, less rain, as Roaring Forties become Furious Fifties By Peter Hannam and Environment Editor, Sydney Morning Herald, 11 May 2014. Retrieved 6 August 2020 Moreover, temperatures on the lee of the Great Dividing Range tend to rise substantially (due to a katabatic effect)The climate of Sydney, Australia The Department of Atmospheric Science. University of Wyoming. E. Linacre and B. Geerts, November 1998 when cold fronts push warm and dry air from the desert across the country's eastern states and over the Range (this is generally followed by a southerly buster).Early taste of spring in eastern Australia Ben Domensino from Weatherzone. Thursday August 19, 2021
As such, the Great Dividing foehn is one the few reasons why Sydney, among other places on the coastal plain, registers high temperatures in the warm season but seldom attains cold maximum temperatures in the winter.Weather Glossary - F Farmonline WeatherWas Penrith the hottest place on Earth on Sunday? by Ben Domensino, 8 January 2018. Retrieved 7 October 2021.Local climate processes in the Illawarra by Edward A. Bryant, Department of Geography, University of Wollongong, 1982 Furthermore, when the warm season north-westerly winds strike (such as the Brickfielder), the hottest and driest areas of southeastern Australia will generally be located along the southern coastal region of NSW in the lee of the Great Dividing range and coastal escarpment due to the foehn effect. Much lower relative humidity figures would also observed in these leeward stations.Urban Heat Island Mitigation Technologies. Edited by Rohinton Emmanuel. Glasgow Caledonian University. 2021.
Mechanism
The southeast Australian foehn is characterised by three principal features: surface winds blowing from the direction of the mountains, a marked increase in air temperature on the leeward side of the ranges, and an accompanying decrease in atmospheric moisture. As moist air ascends the windward slopes of the ranges, it cools and condenses, enhancing orographic lifting. Precipitation removes moisture from the air mass, while the release of latent heat during condensation contributes to warming. As the now drier air descends the lee slopes towards the coastal plain, it undergoes adiabatic compression and warms further.Sharples, J.J., McRae, R.H.D., Weber, R.O., Mills, G.A. (2009) Foehn-like winds and fire danger anomalies in southeastern Australia. Proceedings of the 18th IMACS World Congress and MODSIM09. 13–17 July, Cairns.
During these conditions, an orographic cloud band known as the Föhn wall develops along the ridgelines of the southeastern highlands as moisture condenses on the windward slopes. Downwind of the mountains, a Föhn arch, consisting of a broad layer of altostratus cloud, forms within the ascending branch of a standing lee mountain wave. On weather maps, a band of clear air known as the Föhn gap can often be observed between the cloud wall and the arched cloud cover on the lee side of the Great Dividing Range. This type of foehn event is considered thermodynamically driven.
Studies have identified topographically induced atmospheric waves associated with foehn development. These form as upper-level air descends over the mountain crest and propagates into the lee as large-scale, vertically propagating gravity waves. Model simulations indicate that strong downslope motion and enhanced wind shear contribute to increased turbulence near the surface, consistent with the gusty conditions frequently observed at lee-side stations. In addition to generating foehn winds, the prevailing westerlies can suppress the development of cooling sea breezes from the northeast, preventing them from penetrating inland across parts of the eastern seaboard.Why is Sydney warmer after a cold front? Joel Pippard from Weatherzone. November 20, 2022. Retrieved November 20, 2022.
Descending motion over the coastal escarpment is typically stronger than over the main range and is associated with more pronounced wind shear. As a result, downslope winds are often strongest near the surface on the lee side of the escarpment. Smaller-scale trapped lee waves may also develop, and their occurrence, combined with strong wind shear, can generate substantial turbulence throughout the boundary layer, contributing to the intense gustiness observed at the surface. At night, the foehn effect generally weakens as a mountain breeze develops. During this process, cooler, denser air flows downslope and accumulates on the lee side of the ranges, promoting significant night-time cooling and a large diurnal temperature range.
Distribution and regional effects
The Great Dividing Range foehn is most commonly observed in southeastern New South Wales, east of the Great Dividing Range, including the Sydney metropolitan area (particularly the Cumberland Plain), the Illawarra, parts of the Southern Highlands, the Monaro region, and the South Coast. It may also occur farther north in the Central Coast, Hunter Valley, and parts of the Mid North Coast. Foehn conditions are also observed in the East Gippsland region of Victoria and in eastern Tasmania.
Foehn winds may also affect areas east of the Great Dividing Range in southeastern Queensland and northern New South Wales. However, the phenomenon becomes progressively less frequent north of the Central Coast. Its influence is strongest along the South Coast, which lies directly within the path of the prevailing westerlies, whose influence diminishes markedly north of approximately 35° S. In these leeward regions, precipitation is derived primarily from the Tasman Sea to the east, as the Great Dividing Range blocks many frontal systems approaching from the Southern Ocean. Consequently, winters tend to be relatively dry and summers comparatively wet, in contrast to windward regions, which generally experience drier summers and wetter winters.
In southeastern Queensland, foehn winds are associated with pre-frontal and pre-trough northwesterly flow, post-frontal west to southwesterly winds linked to anticyclonic ridging over southern Australia, and, less commonly, east coast lows in the southeast. They are most frequent during August and September in the corridor between Toowoomba and Gatton. The transect between Applethorpe and Archerfield Airport records approximately 20 foehn events annually, while the Toowoomba–Archerfield transect averages around 19 events per year. The phenomenon becomes less common farther south towards Warwick.Subtropical Foehn Winds, Southeast Queensland, Australia by Leon Wiesner, Hamish McGowan, Andrew Sturman and Tony Dale. Wiley Research DE&I Statement and Publishing Policies. July 2, 2024. Retrieved December 3, 2024.
Areas west of the Great Dividing Range are situated on the windward side during westerly airflow and therefore do not experience a foehn effect under such conditions, instead often experiencing increased cloud cover and precipitation. Conversely, when southerly or south-easterly systems originating over the southern Tasman Sea or eastern Bass Strait are forced to rise over the ranges, foehn-like conditions may develop on the western side of the divide.Foehn winds and fire danger anomalies over S.E. AUSTR Fire Note, Bushfire Cooperative Research Centre (Bushfire CRC) and the Australasian Fire and Emergency Service Authorities Council (AFAC). June 2010. Retrieved 5 June 2022.
Regional characteristics Leeward zones
From north to south, the westerly foehn strongly affects areas east of the Great Dividing Range (the south-eastern coastal plain or eastern seaboard), including Newcastle, Gosford, Sydney, Wollongong, Nowra, Ulladulla, Moruya, Batemans Bay, Narooma, Bega and Merimbula. The eastern portion of the Blue Mountains lies within the leeward zone, with localities from Lawson to Springwood generally experiencing a foehn effect. A strong foehn effect is observed in the Monaro region to the south, particularly in places such as Bombala, Nimmitabel and Cooma. During vigorous cold fronts affecting New South Wales, foehn winds may also occur in the Hunter Valley and along parts of the Mid North Coast, including Taree, Port Macquarie and Coffs Harbour.NSW SES warns communities to brace for more damaging wind by Maitland Mercury, 31 May 2022.
Transitional zones
The western portion of the Blue Mountains is transitional, including Leura, Katoomba and areas further west. Further inland, Mount Boyce, Lithgow, Bathurst, Goulburn, Bowral, Taralga, Braidwood and Canberra in the Australian Capital Territory occasionally experience foehn winds, although they are at times exposed to isolated frontal showers from the west-southwest. When these areas experience foehn winds, they tend to have greater cloud cover (including wave clouds) than locations on the coastal plain to the east. In the East Gippsland region of Victoria, transitional areas include Omeo, Bendoc, Bairnsdale, Orbost, Mallacoota and Sale. These locations are highly susceptible to south-westerly systems and may, at times, experience notable cloud cover even during true westerly flows. Owing to their more southerly latitude and westerly position, cloud cover is generally greater than in comparable locations in New South Wales.