BOM announces short, weak La Nina probability for summer lacking rainfall but packing heatwaves

BOM announces short, weak La Nina probability for summer lacking rainfall but packing heatwaves

BOM announces short, weak La Nina probability for summer lacking rainfall but packing heatwaves

Updated 23 November 2017, 10:35 AEDT

There is a 70 per cent chance there will be a La Nina this summer, about three times its normal likelihood, but the rainfall typically associated with the conditions are not expected due to competing climate drivers.

The Bureau of Meteorology's latest climate wrap has tilted the warning level over to a La Nina alert, the highest level below an official La Nina.

A La Nina is now highly likely this summer, but the rainfall typically associated with the conditions is not expected due to competing climate drivers.

This La Nina could be more associated with heatwaves than rainfall.

"There are two things that have happened," said Dr Blair Trewin, a senior BOM climatologist.

"The sea surface temperatures in the tropical Pacific have cooled a bit further, so in the key areas of the central and east-central Pacific we are now looking at sea surface temperatures of 0.5-0.6 of a degree below average.

"The other thing that has changed is the projections of the seasonal climate prediction models firming up around the likelihood that we will see a La Nina of sorts — probably a fairly weak and short lived one in these next few months."

What can we expect?

While it looks like there is 70 per cent chance of a La Nina this summer, models suggest it will be short lived and will not bring about the widespread rainfall typically associated with La Nina.

The latest outlook, released last week, suggests it could be warm and dry for parts of Australia over the next three months despite the looming La Nina.

"The interesting thing about this one is that the conditions around the Australian continent itself are not those typical of a La Nina," Dr Trewin said.

"Normally when you have a La Nina in the eastern-central equatorial Pacific you will have warm waters around Australia and you will have warm temperatures in the Indian Ocean. And we don't have those at the moment."

Dr Trewin said that this meant the usual sources of moisture that a La Nina brings to the continent would be largely missing.

"The climate outlook for this summer is, if anything, leaning slightly towards the dry side despite the likelihood of a weak La Nina," he said.

"Whereas there are other parts of the world which we would expect to see more typical La Nina signals."

What about extreme events?

Dr Trewin said the change from a 'Watch' to an 'Alert' will not affect bushfire or cyclone risk too much.

"It has been incorporated into the outlooks for a couple of months now. One way or the other we were probably going to be borderline La Nina," he said.

According to Dr Trewin there is a natural disaster which a weak La Nina could have a big impact on.

"It is worth noting that we have seen some very significant extended heatwaves during weak La Nina summers, probably the most directly comparable event in recent times is 2008-2009," he said.

"I don't want to frighten the horses too much by saying this summer is going to be another 2009. There have been other weak La Nina summers where nothing of the sort has happened.

"But it does illustrate that you can get significant heatwaves in these types of summers, particularly in Victoria and Tasmania."

Which may be confusing because typically it is El Nino that is associated with hot temperatures in the east of Australia.

"El Nino brings you more individual extreme hot days, but La Nina tends to give you more prolonged heat waves," he said.

Scientists have been intrigued by the link between these extended heatwaves and weak La Ninas.

Dr Trewin said it is still a bit of an open science question, but "it most likely relates to the characteristics of movements of weather systems — things tend to move more slowly in a La Nina year."

Slow moving highs would allow the temperatures to build up over a number of days.

What are the thresholds for an official La Nina?

Sea surface temperatures are now the main method of defining a La Nina rather than the traditional difference in pressure between Darwin and Tahiti — the Southern Oscillation Index (SOI).

"These days we rely mainly on the sea surface temperatures," Dr Trewin said.

"What we are looking at is how sea surface temperatures in the key regions of equatorial-central Pacific to be more than 0.8 of a degree below average, sustained over a period of at least a couple of months."

While most look at sea surface temperatures, different organisations have different methods of defining a La Nina.

"The Americans (NOAA) have a slightly weaker threshold, they use 0.5 of a degree rather than 0.8, but they require an event to be sustained for longer," he said.

"Usually, once we do our assessment of events, they are similar."

But the SOI has not been completely abandoned. It is still a factor to be considered.

"The SOI is also behaving in a rather La Nina-like position. It has been generally positive over the past four or five months, although not at the levels that would be consistent with a strong La Nina," Dr Trewin said earlier this year.

"Normally, a strong La Nina will have SOI values of above plus-10 for most of the period of several months. This time round so far we have only gone above plus-10 very briefly.

"So again, that is pretty consistent with a marginal event."