Winter weather

the pond in summer

I’ve been hibernating from blogging while writing my thesis, but I had to blog about winter’s late arrival. There was a dismal amount of rain in May, 46mm on only 4 days with almost half falling on one day. This is way below the monthly average of 87mm and means in the last five months Perth has had a severe rainfall deficiency (in the lowest 5% of historical totals). To complement this, Monday was the hottest June day on record: 26°C. The cold weather has supposedly arrived now with 6°C last night (even though sunny days are forecast until Wednesday).

Perth is not the only place experiencing low rainfall.

Rainfall was below to very much below average across most of the continent during May 2009. Another month of low rainfall for southern Australia exacerbated already dry conditions. Victoria has now experienced its third driest start to the year on record and southwestern WA its fifth driest since reliable records commenced in 1900. Short-term rainfall deficits are now evident over most of southeast and southwestern Australia.

Rainfall deficiencies for 5 months, 1 January to 31 May 2009

The low rainfall may be due to a sustained period of positive Indian Ocean Dipole (IOD) mode. IOD is an ocean and atmosphere phenomenon in the equatorial Indian Ocean that affects the climate of countries surrounding the Indian Ocean basin. (Similar to El Niño Southern Oscillation in the Pacific Ocean which affects eastern Australia.) During a positive IOD the tropical east Indian Ocean is cooler than normal water and the tropical west Indian Ocean is warmer, leading to lower than normal winter and spring rainfall over central and southern Australia. A positive IOD occurred during winter and spring of 2007 and 2008, causing lower rainfall during those times. In 2007 this was combined with La Niña in the Pacific Ocean. I blogged about Perth’s lack of rain in June 2007, although by the end of winter 2007 the average rainfall was recorded.

It’s not only the Indian Ocean that’s causing less rain.

Very long-term rainfall deficiencies outside of the usual drought periods persist across parts of southern and eastern Australia. Most notably, rainfall has been below average across much of southwest and southeast Australia since 1997.

My rainwater tank filled from last November’s rain and because I was only using it to refill the pond over summer and some watering of the garden, it wasn’t quite empty before May’s rain. After that downpour it was almost half full.

The pond in my garden overflowed from May’s rain, but in the dry days since that deluge quite a bit has evaporated and June’s rain so far has not led to overflowing. I’ve just planted two bog plants in the area where the overflow runs, so I hope there will be a bit more winter rain so they can enjoy this winter wet area and maybe even attract some frogs!


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