What tropical cyclone Glenda helped to reveal about the climatic causes for the Great Western Woodlands

@tonyrebelo @jeremygilmore @ellurasanctuary @benjamin_walton @adriaan_grobler @wynand_uys @dnicolle @slowplants @alan_dandie @reiner @porcoespinho15 @iancastle @eremophila @hillsflora @terra_australis @arthur_chapman @guillaume_papuga

Western Australia is unique on Earth in having extensive woodlands under a semi-arid, temperate climate (https://www.youtube.com/watch?v=-racshg-_u8 and https://www.researchgate.net/publication/236335929_The_Extraordinary_Nature_of_the_Great_Western_Woodlands and https://cdn.wilderness.org.au/archive/files/the-great-western-woodlands-report.pdf).

Please see

I have hypothesised (https://www.jstor.org/stable/2844553 and https://www.jstor.org/stable/2844553) that part of the reason for this anomaly is a climatic pattern in which tropical cyclones degenerate southeastwards from the northwestern coast of Australia, becoming rain-bearing systems that episodically drench otherwise arid southeastern parts of Western Australia (please see second map in http://www.bom.gov.au/cyclone/climatology/wa.shtml).

This explanation might have seemed far-fetched, because of the remoteness of the hypothesised 'cause'. The distance from Exmouth (https://en.wikipedia.org/wiki/Exmouth,_Western_Australia) to Kalgoorlie (https://en.wikipedia.org/wiki/Kalgoorlie) is about 1200 km (https://upload.wikimedia.org/wikipedia/commons/0/06/WAHighways.png).

What I learned from the case of tropical cyclone Glenda (http://www.bom.gov.au/cyclone/history/pdf/glenda.pdf and https://en.wikipedia.org/wiki/Cyclone_Glenda) is:
It is not just a degenerating cyclone, happening to travel southeastwards, that brings rain to the Great Western Woodlands and the mulga region of Western Australia.

Instead, this rain can set in before the cyclone enters the continent, and seems to take the pattern of extending southeastwards, regardless of the happenstance trajectory of the cyclone itself.

This directionality seems intrinsic/inherent in the pattern. The rain can occur additionally to the cyclone, as part of a far more broadly-defined, manifold weather-event - extending over a week and 1500 km - for which we have no name.

Thus, we had a situation, 29-30 March 2006, in which the Bureau of Meteorology (http://www.bom.gov.au/wa/) was still forecasting risk to towns in the Pilbara (https://en.wikipedia.org/wiki/Pilbara), from a cyclone that was well offshore in the ocean north of the Pilbara and of uncertain trajectory, while already reporting the start of rain over a wide area of the mulga region (https://en.wikipedia.org/wiki/Western_Australian_mulga_shrublands#/media/File:Ecoregion_AA1310.svg).

What was particularly interesting is that

The following is a more detailed account of the events.

Tropical cyclone Glenda affected Western Australia at the end of March 2006.

It intensified in the sea off the Pilbara (https://earth.esa.int/web/earth-watching/natural-disasters/cyclones/cyclone-events/-/asset_publisher/4Lfz/content/cyclone-glenda-australia-march-2006/ and http://www.bom.gov.au/cyclone/history/glenda.shtml), then turned south to approach the Pilbara coast, as expected.

Alarm was sounded for coastal towns (https://en.wikinews.org/wiki/Cyclone_Glenda_closes_in_on_Western_Australia) - which, as it transpired, were fortunately spared.

Tropical cyclone Glenda struck the coast fast and hard, but then collapsed.

The intense system degenerated immediately after crossing the coast, into a system of low atmospheric pressure that posed no further risk beyond flooding in e.g. the Gascoyne region (https://en.wikipedia.org/wiki/Gascoyne).

Despite the fact that the cyclone degenerated in the northern half of Western Australia - and indeed before it deeply penetrated even the Gascoyne region - it produced widespread, soaking rain over a wide area. This included the Eastern Goldfields, where the Great Western Woodlands occur.

These events show that the rain-effect may not be merely a consequence of the spent cyclone travelling southeastwards - which is a typical pattern (http://www.bom.gov.au/cyclone/history/steve.shtml and https://upload.wikimedia.org/wikipedia/commons/a/a2/Steve_2000_track.png and https://en.wikipedia.org/wiki/Cyclone_Steve and https://www.ausstormscience.com/tropical-cyclones/historic-tropical-cyclones/ and http://www.bom.gov.au/cyclone/climatology/wa.shtml).

Instead, the rain may be an inherent part of a larger-scale system. This is perhaps partly owing to the effect of the Coriolis force (https://en.wikipedia.org/wiki/Coriolis_force) on the tropical warm, moist air, and the interaction of the latitudinal and longitudinal 'heat toughs' in northern and western Australia.

Rain already reached the Eastern Goldfields well before the cyclone actually entered the Australian landmass, and continued well after the cyclone degenerated not far from where it entered. For this reason, the normally semiarid Eastern Goldfields were bestowed with several days of rain.

This shows that copious rain in the Eastern Goldfields can occur despite the remoteness and short duration of a tropical cyclone itself.

The extension of the rain to the Eastern Goldfields in this way was despite the steep gradient up which the system had to climb, as it were, from minimal atmospheric pressure off the Pilbara to maximal atmospheric pressure in the Great Australian Bight (https://en.wikipedia.org/wiki/Great_Australian_Bight).

Because of the northwest-southeast band of cloud resulting from the Coriolis force, rain affected the eastern part of southern Western Australia two days before it affected Perth (https://en.wikipedia.org/wiki/Perth). It affected the northern Eastern Goldfields one day before it crossed the coast, but affected Perth one day after it crossed the coast.

This was the third such episode of rain to occur in southern Western Australia during the warm season of 2005-2006. The first one thoroughly wetted the Lake Grace area (https://en.wikipedia.org/wiki/Lake_Grace,_Western_Australia).

This was then added to by the next two systems. Thus, trees would have had an excellent opportunity to regenerate germinatively, just north of Fitzgerald River National Park (https://en.wikipedia.org/wiki/Fitzgerald_River_National_Park).

Much of the water sustaining the Great Western Woodlands may originate in the Timor Sea (https://en.wikipedia.org/wiki/Timor_Sea), more than 2000 km to the north - if not in the Pacific Ocean (https://en.wikipedia.org/wiki/Pacific_Ocean), even farther away.

This evaporative origin follows the 'heat trough' of the northern Australian coast, westwards, recharges itself over the sea off the Pilbara, and turns southwards and then southeastwards, because of a 'corner' that it meets.

This corner lies at the intersection of the band of low atmospheric pressure, running west-east in tropical northern Australia, with the coastal heat trough of the west coast of Australia (http://www.bom.gov.au/climate/about/?bookmark=westtrough).

The season of tropical cyclones in Western Australia is in autumn, rather than summer. This is exemplified by the fact that, in 2006, the last major cyclone threatening Darwin occurred at the same time as the first proper cold-frontal winter rain in Perth, i.e. on Anzac Day (https://en.wikipedia.org/wiki/Anzac_Day) in late April 2006.

My overall comment:
I stand by the basic conceptual framework of my climatic hypothesis, more than 40 years after publishing my explanation for the growth of trees in semi-arid Western Australia.

However, since than I have been able to track several tropical cyclones over the decades. Of these cyclonic events, Glenda was perhaps the most chronologically revealing, in showing the larger nature of the meteorological systems involved.

At first glance, there seems to be nothing special about the location of the Great Western Woodlands, which merely occupy part of the vast low-lying plain of Australia (https://upload.wikimedia.org/wikipedia/commons/5/52/Great_Western_Woodlands_location_within_Australia.jpg).

However, this location happens to be at the receiving end of climatic systems determined by the coastal configuration of the whole western section of the continent, from the tropics to the temperate zone.

The climatic factors promoting trees cannot be understood in a regional context alone. They must be framed in systems that are unusually extensive, owing to the lack of topographical barriers in Western Australia.

Posted on 28 December, 2022 07:25 by milewski milewski


Please scroll through the images in http://www.bom.gov.au/cyclone/history/yasi-mslp.shtml

Posted by milewski over 1 year ago

The following is a relevant excerpt from my diary of 2006:

(For useful context, please see https://en.wikipedia.org/wiki/2005%E2%80%9306_Australian_region_cyclone_season#:~:text=The%202005%E2%80%9306%20Australian%20region,2005%20to%2030%20June%202006.)

This summer, of early 2006, has seen damaging tropical cyclones in both northeastern Queensland and the Pilbara region. This has resulted in flooding around Katherine (https://en.wikipedia.org/wiki/Katherine,_Northern_Territory), big waves (up to 4m or even 8m high) at Sydney, widespread rain the region of Gascoyne-Murchison-Eastern Goldfields-eastern Wheatbelt (particularly Lake Grace), rain in the northeast of South Australia, cyclone-related summer-rain on several occasions in Perth, and a few other false-alarms of tropical cyclones in northwestern Western Australia.

This cyclone swept past Pannawonica (https://en.wikipedia.org/wiki/Pannawonica,_Western_Australia), downsizing to category 1, and headed towards the Gascoyne region.

During this period, I experienced unseasonal rain (about 2.5 cm) in Perth, followed by a similar rain event about two weeks later...and then another similar amount (about 2.5 cm) a week later again, from a thunderstorm at night, associated with a 'heat trough' down the western coast of southwestern Australia.

On 15-16 February 2006, soft drizzle fell in Perth and Fremantle (https://en.wikipedia.org/wiki/Fremantle), during a cool change immediately after the hottest day so far this summer (38.5 degrees C). I take this to be the result of a combination of a) the 'heat trough' moving eastwards from the Swan coastal plain (https://en.wikipedia.org/wiki/Swan_Coastal_Plain), drawing air inland from the Indian Ocean, and b) a 'cold front' brushing the extreme southwest of Western Australia.

This shows how tropical moist air and winter-type moist air combine to provide the occasional even at the height of the mediterranean-type summer in Perth.

As a result, we went from a maximum of 38.5 degrees C on one day to a maximum of ?22 degrees C on the following two days, plus overcast, cool conditions for 1.5 days, and even a bit of rain.

In January 2006, tropical cyclone Clare (https://en.wikipedia.org/wiki/Cyclone_Clare), classified as of category 3, affected Western Australia. On 10 January 2006, wind-gusts up to 200 km/h occurred at Dampier (https://en.wikipedia.org/wiki/Dampier,_Western_Australia), Karratha (https://en.wikipedia.org/wiki/Karratha,_Western_Australia, Roebourne (https://en.wikipedia.org/wiki/Roebourne,_Western_Australia), and Wickham (https://en.wikipedia.org/wiki/Wickham,_Western_Australia). The cyclone crossed the coast of the Pilbara region at 1 am on this same day. About 20 cm of rain fell in the worst-affected areas on the night of 10 January 2006, lasting to the morning of the next day, and producing floodwaters.

Another tropical cyclone affected Western Australia about two weeks after Clare.

Posted by milewski over 1 year ago
Posted by slowplants over 1 year ago

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