The southeast Australian foehn is a westerlyfoehn 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.[1] 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, whereby causing major adiabatic compression (which is the rate at which temperature decreases with altitude) and a substantial loss of moisture.[2][3][4]
The effect is known by other names, such as the Australian chinook, the Great Dividing wind, the Great Dividing foehn or simply westerly foehn. The southeast Australian foehn is distinguished by three criteria; surface winds which blow from the mountains' direction, a sharp rise in air temperature in the leeward side of the mountains, and an accompanying diminution in atmospheric moisture.[2]
Typically occurring from late autumn to spring, though not completely unheard of in the summer (particularly in eastern Tasmania),[a] the Great Dividing foehn mainly occurs when a westerly or southwesterly frontal system (including a general westerly flow) passes over the ranges, thereby providing clear to partly cloudy and relatively warmer conditions on the lee.[b][5]
The foehn effect on the coastal plains of southeastern Australia is mostly linked with the passage of a deep low pressure system or westerly cold fronts across the Great Australian Bight and southeastern Australia that cause strong winds to reorient virtually perpendicular to some parts of the Great Dividing Range, predominantly between late autumn into winter and spring, particularly during a negative SAM 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.[2] Foehn occurrence on the southeast coastal plains can also occur when hot, northwesterly winds blow from the interior (even when there is little moisture on the windward side), because the air heats up faster as it descends into the plains than it cooled as it ascended the ranges.[7]
Averaging between 60 km/h (37 mph) to 70 km/h (43 mph), sometimes they may be brought on by a large polar air mass from the southwest of the continent in the Southern Ocean which advances northwards across Victoria towards the east coast.[8] Moreover, temperatures on the lee of the Great Dividing Range tend to rise substantially (due to a katabatic effect)[9] when westerly cold fronts passing over southern Australia 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).[2][10]
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.[7][11][12] Furthermore, when the warm season northwesterly 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.[13]
As the moist air rises over the windward side of the ranges, it cools and it would condense, thereby creating precipitation on the upwind slopes. The precipitation then gets rid of the moisture from the air mass on the lee side of the ranges, and the condensation raises the air temperature as it descends the lee slopes towards the coastal plains because of the adiabatic compression.[14]
During these conditions, an orographic cloud band, or the Föhn wall, builds up along the ridgelines of the southeastern highlands due to condensation of moisture as the air ascends the windward slopes. Meanwhile, the Föhn arch, with its broad layer of altostratus cloud, shapes downwind of the mountains in the ascending component of a standing lee mountain wave. In weather maps, a band of clear air called the Föhn gap, which is over the downwind of the Great Dividing region, can be seen between the wall and arched cloud cover. This foehn wind can be referred to as thermodynamically driven.[1]
The existence of topographically induced atmospheric waves in connection with foehn occurrence has been indicated, which develop with the descent of upper-level air above of the ridgetop and pass into the lee of the ranges as broad-scale, vertically supporting gravity waves. The wind shears and the strength of the downslope motion manifested in the model examination also point that the onslaught of foehn conditions results in increased turbulence near the surface, evident in the gusty conditions observed at the lee stations.[2] In addition to the foehn winds, the same westerly winds also ward off the cooling sea breezes that arrive from the northeast, thereby preventing them from developing in the eastern seaboard.[15]
A vertically propagating gravity wave over the affected region exists. The descending motion over the coastal escarpment is stronger than that over the primary range and is connected with more powerful shear. The downslope winds tend to be strong, particularly near the lee's surface of the coastal escarpment. Smaller-scale, trapped lee waves over the affected region exist, and their incidence, together with the strong wind shears, signal significant turbulence throughout the boundary layer, which is concordant with the heavy gusty surface winds registered on the leeside.[2] At nighttime, the foehn effect subsides due to a mountain breeze – This is when denser cool air flows down the mountain slopes to settle in the downwind side, thereby providing relatively cold conditions in the night and, consequently, a high diurnal range of temperature.
Foehn winds may also impact other parts of Australia, such as east of the Great Dividing Range in southeast Queensland and northern New South Wales.[2] The Great Dividing foehn does not heavily impact areas northward from the Central Coast. The effect is gradient; being more common and efficacious towards the South Coast (due to the latter region being in the track of prevailing westerlies, which exponentially falters north of 35° S).
With leeward areas, or areas that receive foehn winds, precipitation is predominantly derived from the Tasman Sea to the east, since the Great Dividing Range blocks frontal westerlies off the Southern Ocean (which are most frequent between May and October). Therefore, due to the foehn effect, winters in leeward zones are drier with the summers being relatively wet, unlike those on the windward side which, conversely, have drier summers and damp winters.[2]
Areas that lie to the west of the Great Dividing Range are windward and therefore never experience a foehn effect under a westerly stream, with persistent cloud cover. On the contrary, the Great Dividing Range also blocks frontal systems originating in the southern Tasman as well as the eastern Bass Strait. When south/southeasterly frontal systems lift over the coastal slopes, the western edge of the Range would, conversely, experience foehn-like winds.[d][16]
Leeward zones
Windward - Overcast, oftentimes foggy and/or snowy conditions resulting from uplift on the western slopes. Relative humidity largely in excess of 80% throughout the day. (near Orange, New South Wales)
Leeward - Meanwhile, dry and mostly sunny conditions occur on the coastal plain due to compression of cold air as it descends the Ranges' leeward side. Relative humidity no greater than 50%. (Sydney CBD)
The eastern portion of the Blue Mountains lies in the leeward zone, with places from Lawson to Springwood generally featuring a foehn effect. The western portion is transitional (Katoomba and westwards).
A strong foehn effect is observed in the Monaro region to the south, in places such as Bombala, Nimmitabel and particularly Cooma.
When a vigorous cold front consumes the state of New South Wales, foehn winds would occur in the Hunter Valley in the mid-north coast in places such as Taree, Port Macquarie and Coffs Harbour.[17]
In the East Gippsland region of Victoria, transitional areas include Omeo, Bendoc, Bairnsdale, Orbost, Mallacoota and Sale, as these are highly susceptible to southwesterly systems and would even experience notable cloud cover from true westerlies in some instances. Due to their south-facing location and western longitude, cloud cover is significantly greater than in their New South Wales counterparts.
In Tasmania, Hobart, New Norfolk, Scamander, Swansea and St Helens on the east coast; as well as Oatlands, Ouse and Bothwell in the Midlands, are downwind of the Central Highlands, thereby usually receiving foehn winds (particularly in the warm season, though sporadically throughout the year). However, due to their south-facing location, they are all susceptible to southwesterly systems and may occasionally experience some cloud cover from westerlies.
Effects
The Great Dividing wind can be particularly damaging to homes and would affect flights, in addition to being uncomfortable, as the wind chill factor would usually make the temperatures feel cooler than what they are.[18][19][20] The Australian foehn has also impacted international sporting events and as well as recreational aviation, such as in 2007, when a light aircraft crashed in the Central Highlands due to severe winds on a region that is prone to mountain-wind waves.[2] Much like the Santa Ana winds in California, they may elevate fire danger in the warmer months due to their dry, gusty nature.[21]
In September, when the foehn effect is usually strong, green pastures on the windward side (left, Central Tablelands) can be contrasted from the dry landscape on the leeward (right, Greater Western Sydney).
28 May 2000 was a striking example of the 'divided' weather between the western and eastern faces of the range. On the western face, Hunters Hill in Victoria registered a maximum temperature of just −0.7 °C (30.7 °F), whereas Cooma Airport on the eastern face reached 7.3 °C (45.1 °F). These stations are at altitudes of 981 metres (3,219 ft) and 930 metres (3,050 ft) respectively. Furthermore, Thredbo Village reached a maximum of −0.5 °C (31.1 °F); this is warmer than that recorded at Hunters Hill, despite being over 400 metres (1,300 ft) higher in altitude; whereas Cabramurra at a more similar altitude only topped at −3.0 °C (26.6 °F).
On 29 September 2000, a remarkable foehn event was recorded in the lee of the Blue Mountains region in Sydney, where maximum temperatures at Penrith, Badgerys Creek, Bankstown Airport, and Sydney Airport were around 10 °C (18 °F) above average. The elevated temperatures again coexisted with the inflow of significantly drier air. Simultaneously, the leeward stations in the southern New South Wales coast showed a sharp increase in temperature (9°C in 2 hours) and a decrease in relative humidity. Similar warming and drying were also observed further inland at Cooma, Braidwood, Canberra, and Bombala.[2]
On 29 May 2007, it was observed that the temperature at Sale (leeward side) was around 4–9 °C (7–16 °F) higher than the corresponding temperatures at Melbourne and Wangaratta (which lie on the upwind side). In this foehn event, Sale had a high above 24 °C (75 °F), whereas the latter cities struggled to reach higher than 12 °C (54 °F). Furthermore, the relative humidity was 31% at Sale and as high as 80%–90% at Melbourne and Wangaratta.[2] Unusually warm and dry conditions were also registered at other stations in the downwind side of the ranges – Bairnsdale, Orbost, Latrobe Valley, and Nowa Nowa, which recorded temperatures of 24 °C (75 °F), 24.2 °C (76 °F), 22.9 °C (73 °F), and 22.6 °C (73 °F), respectively, making this location in the lee of the ranges consistent with the position of the foehn gap and foehn arch.[2]
On 2 April 2008, maximum temperatures on the Gippsland coast coexisted with peak wind speeds from the northwest that gusted to 75 km/h. Temperatures at Bairnsdale, Latrobe Valley, and Nowa Nowa were 2°–4°C higher than average, with reductions in relative humidity also being observed.[2]
On 28 April 2008, predominant winds were mainly westerly with the hottest and driest areas of southeastern Australia located along the coastal fringe of southern New South Wales, in the lee of the Great Dividing Range. Temperatures on the windward side of the mountains reached at about 8°C below average, while in the lee temperature peaked at only about 1°–2°C below average, therefore indicating a positive anomaly of about 6°–7°C.[2]
On the evening of 18 September 2008, temperatures at Mount Nowa Nowa and Bairnsdale rose after sunset, while relative humidity displayed complemental behavior during the course of the night. On 19 September, the relatively warm and dry conditions prevailed along the Gippsland coast, in contrast to upwind conditions.
On 27 October 2008, foehn wind dynamics were observed over the Gippsland region to the southeast of the Australian Capital Territory on the lee of the ranges, associated with northwesterly winds over southern New South Wales. These downwind regions experienced lower humidity levels and higher than average temperatures. The temperature at Orbost reached 32 °C (90 °F); the temperature at Mount Nowa Nowa rose to 26 °C (79 °F); Bega reached 36 °C (97 °F), which is approximately 14 °C (25 °F) above the average maximum temperature for October–November. At Moruya, the temperature rose to a maximum of 35.4 °C (96 °F). Similar but less pronounced effects were also observed in Green Cape, Bombala, and Cooma. In contrast, Albury, which is on the windward side of the ranges, only reached a maximum of 27.7 °C (82 °F).[2]
On 23 August 2012, a foehn effect caused Sydney to record its 3rd warmest August day on record where it reached 29.0 °C (84 °F) at the CBD and 30.0 °C (86 °F) at Sydney Airport.[24]
On 18 July 2016, Mallacoota reached an unseasonable high of 23.5 °C (74 °F) due to the foehn effect, a record warm winter day for that region in Victoria.[25]
On 20 September 2023, during a heatwave in southeast Australia, Gabo Island, Ulladulla and Montague Island recorded highs of 32.1 °C (90 °F), 35.4 °C (96 °F) and 33.4 °C (92 °F), respectively, due to strong foehn winds on the leeward side of the mountains.[26]Western Sydney surpassed 35.0 °C (95 °F), and Sydney Airport recorded its highest September temperature at 35.9 °C (97 °F).[27]
^They occur throughout the year in Tasmania as the island sits in the path of the Roaring Forties and/or the prevailing westerlies.
^Temperatures on the coastal plain are relative and therefore variable, ranging from 15 °C (59 °F) at the coolest (which is usual during polar blasts) to as high as 45 °C (113 °F) – All depending on the conditions on the windward side.
^Victoria is mostly exposed to westerly fronts due to its south-facing location and western longitude. Therefore, Victoria's east can still be windward in some occasions, especially when westerly fronts are vigorous.
^When southwesterly frontal systems are powerful, their accompanying clouds and precipitation may occasionally 'spillover' the NSW coastal plain for a short period of time, although no more than 2 mm (0.079 in) of rain will be recorded.
References
^ abRain Shadows by Don White. Australian Weather News. Willy Weather. Retrieved 24 May 2021.
^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.
^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.
^Sharples, J.J. (2009) An overview of mountain meteorological effects relevant to fire behaviour and bushfire risk. International Journal of Wildland Fire, 18, 737-754.
^An Ill Wind: The Foehn in Leukerbad and Beyond Sarah Strauss. The Journal of the Royal Anthropological Institute Vol. 13, Wind, Life, Health: Anthropological and Historical Perspectives (2007)
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