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Bureau of Meteorology declares spring 2019 the driest on record

Kate Doyle, Monday December 2, 2019 - 17:45 EDT
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Spring was a continuation of dry conditions for many parts of Australia. - ABC licensed

Spring 2019 was officially the driest of the Bureau of Meteorology's 120 years of rainfall records.



It comes on top of previous dry seasons, with the year-to-date rainfall now the second lowest on record.

Western Australia was particularly dry, as well as hot, recording its lowest spring rainfall, highest average temperature and highest mean daytime temperature.

It was the fifth-warmest spring on record for the country as a whole — mean maximum temperatures were 2.41C warmer than average, second only to 2014.

How do you calculate driest spring?



BOM senior climatologist Blair Trewin said the first thing they did was set up hypothetical grids over the country, with the sides of each grid measuring approximately 25 kilometres.

"We average the stations that are closest to each grid point to get an estimated value at that grid point — then we average across the grid points," Dr Trewin said.

"What that does is it means we have a fairly consistent picture regardless of which individual sites are reporting."

Heavy rainfall in one region can affect the average, but Dr Trewin said spring was generally a season with relatively few variations.

"Tasmania is by some margin the wettest part of the country in spring, but Tasmania is only fairly small," he said.

"Probably the real standout with this spring is it's been dry almost everywhere."

Why so dry?

This year has seen one of the strongest positive Indian Ocean Dipoles (IOD) on record.

The climate driver works to bring wet conditions to eastern Africa but dry conditions to Australia, with warm waters in the western Indian Ocean and cool waters near Australia.

It hasn't just been dry in Australia — there have also been record-low rainfalls in places like Singapore, according to Dr Trewin.



The other major climate driver at play has been the , bringing strong westerly winds over the continent.

Dr Trewin said while the westerlies could bring increased rain to Tasmania and southern Victoria, they tended to bring about a dry pattern for much of eastern Australia, particularly New South Wales and Queensland.

"When we look at the map of rainfall for spring, the most prominent area of lowest on record was north-eastern New South Wales and south-eastern Queensland," he said.



Hot days but less extreme nights

Daytime temperatures were particularly high this spring — the second highest on record nationally.

Queensland, New South Wales, South Australia, Western Australia and the Northern Territory all recorded mean maximum temperatures in the top 10 from 110 years of records.

Overnight temperatures were less hot, bringing down the average.

Dr Trewin said dry conditions were often associated with warmer days and cooler nights for two reasons:
cloud cover acts as a blanket, blocking the sun and keeping daytime temperatures down while keeping the warm air in overnight; without this cloud blanket, nights are cooler and days are warmer.
soil moisture acts as a moderating factor on temperatures; without that moderator, temperatures fluctuate.

How important these factors are depends on where you are.

Water stores down

The , and while coastal capital city water storages are at less dire levels, they are all .



State and territory water storage capacities range from South Australia at 94.4 per cent to New South Wales at 27.2 per cent.

Australia's total accessible water volume is 47.7 per cent, down from 55.6 per cent last year; this value has been on a general decline since the big wet of 2016.

There is little relief in sight

The BOM's outlook for summer, released late last week, suggests there is .

There is, however, a sliver of hope that the IOD and SAM will return to more neutral conditions late in summer.

While that is not a promise of widespread drought-breaking rain, it is at least potentially a break from extra dry conditions for parts of the country.



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- ABC

© ABC 2019

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