Seasonal outlook contains some concerning signals: AgVic agronomist

Agriculture Victoria seasonal risk agronomist Dale Grey was speaking at the autumn seasonal climate outlook webinar.

"The Bureau of Meteorology has announced the El Nino is dead, we have finally got below that 0.8 degree threshold, to 0.64 degrees," Mr Grey said.

"That was the last thing we were looking at to make sure the El Nino is dead - all the other indicators we have been looking at have been dead for a little while.

There was the potential for a La Nina to redevelop later in the year but that was far from certain yet, he said.

"Until we start to see some cold outbreaks along the Equator, we won't be seeing any surface La Nina activity, although we are seeing a pre-La Nina signal, in the undersea."

Forecasters would need to see cold water welling up at the surface, but none of that was happening.

"Maybe some of this is very early signs," he said.

"There is a possible La Nina, but it's early days - I wouldn't be getting too excited, as yet."

But he said a bigger challenge was the potential development of a positive Indian Ocean Dipole.

A positive IOD generally means there is less moisture than normal in the atmosphere, to the northwest of Australia, changing the path of weather systems coming from the west.

That could result in less rainfall and higher than normal temperatures over parts of Australia during winter and spring.

Mr Grey told the webinar the whole of the Indian Ocean was very warm, particularly off the coast of east Africa.

"Despite all the talk about a La Nina, it's only the ocean to depth that is remotely interested in that environment, the pressure, wind and clouds couldn't give a hang," Mr Grey said.

"The positive IOD in the Indian Ocean is basically saying 'hold my beer'.

"Its sea surface is not positive IOD like but gee, there's a lot of warmth off the coast of Africa, which could be driving things.

"There is no shortage of models that are interested in a positive IOD."

The other climate driver, the Southern Annular Mode (SAM) had "spent most of summer in a positive phase", which had moderated temperatures and helped increase rainfall.

"That was unexpected, in terms that we wouldn't expect an El Nino year to have positive SAM, we'd expect a negative summer SAM," he said.

For autumn, the climate models were "sitting on the fence" at neutral, for rainfall.

"That's code for 'plan anything'," Mr Grey said.

Two of the climate models had a drier trend, across the state, eight had drier conditions for the south-west.

"That concerns me, if it's true," he said.

In a positive SAM, westerly winds contract towards Antarctica, restricting the passage of cold fronts inland.

Modelling "really does reek of that pressure pattern and SAM going positive into winter," he said.

"The majority of the world's models are bringing in a La Nina in that late winter, the vast majority are keeping the positive IOD," he said.

"What is interesting is the atmosphere does seem to be quite primed to be a positive IOD.

"There are lots of neutral forecasts for rainfall, except in the south-west, and likely warmer temperatures."