BOM defends summer dry forecast in spite of wet spell

BOM defends summer dry forecast in spite of wet spell

Grains
Good late summer rain does not distract from the fact it has been overall drier and hotter this summer according to the BOM. Map source: BOM.

Good late summer rain does not distract from the fact it has been overall drier and hotter this summer according to the BOM. Map source: BOM.

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The wet weather across parts of Australia to start 2020 is not necessarily part of an ongoing pattern warns the BOM.

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THE BUREAU of Meteorology (BOM) said the breakdown of the strong Indian Ocean Dipole (IOD) positive event allowed some good rainfall across the nation in January and February but warned the rain bands were not part of a sustained wetter pattern.

Andrew Watkins, BOM head of climate predictions, said there was no clear sign of macro-climatic drivers such as an IOD or El Nino event on the horizon.

"To put it simply the rain has been part of more chaotic, short-term weather patterns rather than an ongoing climate factor that could mean a wetter than average period," Dr Watkins said.

"There wasn't that big driver there and that allowed the shorter and more localised impacts on the weather to have more of an influence and that is a lot harder to predict."

The BOM came under criticism for its summer climate update issued in December, where it predicted it would be markedly hotter and drier.

Dr Watkins defended the accuracy of the report.

There has been flooding in parts of Australia but the Bureau of Meteorology warns it is not necessarily part of a longer term wetter than average period.

There has been flooding in parts of Australia but the Bureau of Meteorology warns it is not necessarily part of a longer term wetter than average period.

"We based a lot of the forecast on the IOD positive and through December it was certainly the case that it was drier and hotter, a lot of places broke records for both temperature and low rainfall.

"What happened after that we had the IOD positive and the other big factor keeping it dry, the negative Southern Annular Mode (SAM) both break down just that little bit earlier than anticipated in January.

"That set the season up for the monsoon to develop slightly earlier and that in turn saw those better than average rainfalls we saw in some parts of the country.

However, Dr Watkins cautioned that while the rain captured the headlines it was far from a general soaking.

"We saw those falls down the east coast and they were truly impressive totals, how a dam the size of Warragamba near Sydney can go from 40 per cent to 70pc in a matter of days is mind-boggling, but it did not get everywhere, a lot of Central West NSW for example remained dry while Darwin has also experienced well below average rainfall for the year."

Overall, he said it was still one of the warmest summers in record, driven by the extreme December heat.

He said the outlook was currently neutral in both the Indian and Pacific Oceans which meant other drivers of weather would continue to influence what happened over the coming months.

"In the Pacific it is a little bit contradictory, we've got warm water around northern Australia, which is normally a sign of more moisture, but equally there is also warm water in the western Pacific which often means less rainfall for us, so it is not a typical pattern.

"At present we don't estimate the Pacific Ocean to be a big influence one way or the other over the upcoming three months."

In the Indian Ocean he said it was also likely to remain neutral, although if anything there is more chance of an IOD positive, correlated with dry conditions than a negative.

"It is still firmly in the neutral range but the models do suggest if anything emerges it will be a positive, rather than negative event."

Dr Watkins said the forecasting skill for autumn, traditionally difficult to predict, was improving, but said with neutral macro-climatic factors it would mean an increased reliance on shorter-term forecasting.

"The more localised factors will mean tools like the seven day forecast will play a big role in assessing what is coming up."

He cited the current example where two tropical cyclones are building and threatening to dump heavy rain as a case.

"It looks as though there will be heavy rain in some parts but with the dynamic nature of the events it is really difficult to pinpoint exactly where it will fall until closer to the event."

The story BOM defends summer dry forecast in spite of wet spell first appeared on Farm Online.

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