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Why Is There So Much Rain Mum?

Why is there so much rain in Brisbane, Queensland, Australia

I can remember a time, only around 5 years ago, when the site of rain and the sound of a storm would invoke such wonder in my young children.  At that time, Australia was in the middle of one of the worst droughts on record, and the number of times the children had actually seen rain could be counted on one hand!  What a difference a few years makes…now there is so much water!  Brisbane (our nearest major city) has had 449mm (almost 18 inches) for December; this is the wettest month for 14yrs and one of the wettest Decembers on record.

So why is it so rainy?  Time for a quick lesson in Meteorology!  Australia’s weather is influenced by a particular air pattern called the Walker Circulation in the tropical pacific (see Fig 1).  Trade winds bring moisture over northern Australia, this air rises over the warm coastal waters and this leads to rainfall.  The air then travels east and sinks over the cooler waters off South America. At this stage, the air is dry and rainfall is scare (this cooler water borders the Atacama desert, one of the driest places on Earth).

Typical Walker Circulation Pattern Weather

Fig 1. Source

The next piece of the puzzle is the Southern Oscillation index (SOI).  Put simply, the SOI is a monthly measure the average air pressure difference between Tahiti and Darwin.  When the SOI is persistently positive, then this tends to indicate a strengthening of the Walker Circulation and increased rainfall over the Australian continent.  This weather pattern is called La Niña  (Lah Nee-Nyah), which is Spanish for “the girl-child”, and yes, there is another associated weather pattern called El Niño  (el NEEN-yo) which is Spanish for “the boy-child”.  Strong La Niña events have in the past caused severe floods (Brisbane 1974, inland Australia 1989) and are associated with below average temperatures in northern and eastern Australia, and increased tropical cyclone formation in the Coral Sea.  Conversely, in an El Niño event, the opposite tends to occur, with a weakening in the Walker Circulation and a persistently negative SOI, leading to dryer conditions over eastern Australia, and if severe, drought.

southern Oscillation Index SOI Weather

Fig 2.  Source

 southern Oscillation Index SOI Weather 1969 to 1976

Fig 3.  Source

At present the 30 day SOI value is 22: this is very high (in the top 5% of historical observations), and the SOI has been positive for 8-9 months (Fig 2).  Compare this to the SOI graph (Fig 3) which includes 1974, the wettest year ever recorded in Australia, and there are similarities.  Long-term forecasts expect this La Niña to continue for the first quarter of 2011, but the intensity will start to decrease.

So now when your stir crazy child(ren) asks why is it so rainy you can introduce them to ‘the girl’ and ‘the boy’ who influence Australia’s weather.

The Bureau of Meteorology (www.bom.gov.au) website is a fantastic resource for all things weather related (Australia).  It has great education section which can be found here — great for school projects.

Images: La Niña vs El Niño (the girl-child and the boy-child)

ElNinoLaNina_Weather

References:

http://www.bom.gov.au/info/leaflets/nino-nina.pdf

http://www.bom.gov.au/climate/enso/

http://www.bom.gov.au/announcements/media_releases/qld/20101004.shtml

http://www.weatherzone.com.au/news/a-wet-wet-christmas-for-brisbane-and-gold-coast/15734

http://www.weatherzone.com.au/qld/brisbane/brisbane

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7 Comments

  • Reply
    Nicole
    December 28, 2010 at 10:26 am

    I just wish the shops would stock gumboots again. Zoe out grew hers!! and I can not find them anywhere. We have raincoasts and umbrellas though. so they still bounce on the trampoline and play on their new waterslide. waterplay in summer is still fun. Days are still hot enough, togs are made for it. Oh and would your advanced surgical trainee like to take my gallbladder out? Im on the waiting list but he can do it now if he likes!! Hair dressers bring their own models, how about surgeons bringing their own patients.

  • Reply
    Bonnie
    December 28, 2010 at 11:49 am

    Very interesting stuff Matt!! I like the line … “time for a lesion in Meterology.” 🙂

  • Reply
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    December 28, 2010 at 6:56 pm

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  • Reply
    Sharron Peacock
    December 28, 2010 at 8:40 pm

    Thanks so much Matt…..great info and a great read!!! It is interesting for sure….Hamish might be writing something to this affect in about…..well…..a year maybe…..LOL!! 🙂

    • Reply
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      February 18, 2015 at 1:33 am

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