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Latest monsoon onset for Darwin on record is now a certainty despite recent rain

Kate Doyle, Tuesday January 28, 2020 - 19:29 EDT
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The wait for the monsoon continues despite torrential rain for Australia's north. - ABC

The deadline for the latest monsoon onset on record has come and gone and there is still no official monsoon for Darwin.

It is a bit of a big deal as Darwin would usually expect the monsoon onset around Christmas.

The records go back to the 1950s and the previous record for the latest-occurring monsoon was for January 25, back in 1973.

But the lack of an official monsoon onset hasn't stopped the rain, with big totals falling in Australia's tropical north over the last few weeks.

So how can there be all this rain but it still isn't technically a monsoon?

Well, Mosese Raico, duty forecaster for the Bureau of Meteorology (BOM) in Darwin, said there were a number of criteria which need to be met.

What are the criteria?

It is a bit of a complicated process but in a nutshell:
There needs to be a monsoon trough present at low levels
The winds need to be flowing from the northern hemisphere across into the southern hemisphere and feeding into that trough
Then these lower-level conditions need to be supported by upper-atmospheric conditions
Finally, this all needs to happen for an extended period of time

It's been tantalisingly close

The first close call came in January when a low pressure system, that would go on to become Cyclone Claudia, scooted through south of Darwin then went out to the Timor Sea.

The problem was it moved too quickly — the correct winds were not in place long enough to officially classify as the monsoon onset in Darwin, according to Mr Raico.

Then just this weekend things were looking good, but again it didn't quite make it.

"We did have some definitely good winds and very moist westerly flow across particularly north-western parts of the Top End," Mr Raico said.

"Unfortunately we didn't have other criteria met."

Mr Raico said there was some cross-equatorial flow but it wasn't sufficient enough or long enough to actually be classified as a monsoon onset and the upper-atmospheric line-up was also lacking.

Beyond the more recent conditions, it doesn't come as a huge surprise that the monsoon onset is so late.

The monsoons in both hemispheres are the result of the north/south shift in tropical rains in response to the seasonal tilt of the Earth.

What affects one often affects the other and the monsoon was

So it follows that ours would also be late.

The blame for this tardy withdrawal and late onset has been laid at the feet of the strongest positive Indian Ocean Dipole on record, which blocked flow to the eastern Indian Ocean and slowed the monsoon moving south.

It hasn't stopped the rain

After a dry start to the wet season this year, the rain has been making up for lost time since the Indian Ocean Dipole broke down at the very end of last year.

around Darwin, with "monsoon-like" conditions, despite the lack of official monsoon.

Between January 21 and 27 Gunn Point recorded just under 300 millimetres, the Tiwi Islands had 282mm and Darwin Airport recorded 211mm, according to Mr Raico.

"Look, it is quite significant in terms of rainfall," he said.

Up until last weekend Darwin was tracking as the driest wet season on record, but due to the rainfall over the last week that is no longer the case.

"We're below average but we are not the lowest on record any longer," Mr Raico said.

It comes as a relief after a poor wet season for the Top End last year.

Then there has been all the rain that over

Rainfall is expected to continue in the Gulf region for the next few days.

So when will the finnicky criteria be met?

Mr Raico said at this stage it's a bit hard to say when the official monsoon will come but in the short-term it isn't looking promising.

"With regards to longer-term, like seven days or so, it's quite varied in terms of our guidance that we have at the moment," he said.

"We can't give a definite answer on that one yet. We are hoping we do get the onset at some stage soon."

Regardless of the technical monsoon criteria, what really matters is the rain.


© ABC 2020

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