Warm and dry — that has been the story of the season, and now there is a good chance that is set to continue for south-east Australia.

The Bureau of Meteorology has just declared an El Nino 'watch', which means that while we are still in the neutral phase of the El Nino Southern Oscillation (ENSO) there is an increased chance of El Nino in the months ahead.

El Nino is typically associated with lower than usual rainfall for large parts of eastern Australia in spring, according to senior climatologist at the Bureau, Robyn Duell.

"We can also see warmer days in the southern part of the country during spring in El Nino," she said.

Sea surface temperatures (SST) and Pacific trade winds. ( Supplied: Bureau of Meteorology )

What does this graph mean? Trade winds are looking around average for most of the Pacific along and around the equator

Trade winds are looking around average for most of the Pacific along and around the equator They have, however, been slightly below average over the central equatorial Pacific

They have, however, been slightly below average over the central equatorial Pacific When there is an El Nino the easterly trade winds are significantly weakened or even reversed Supplied: BOM

Autumn was warm and dry, winter is shaping up much the same, and now there is twice the normal likelihood we'll get a spring El Nino.

"When we go to a watch, we're looking at around a 50 per cent chance," Ms Duell said.

"So for every 10 times we've issued to watch before, five times we haven't actually gone on to see El Nino conditions. But the other half of the times we have."

So it is still not a certainty, but it is more of a chance than normal.

"In a normal year we would only be saying there's about a 25 per cent chance, so it is an elevated risk. But it's still a 50/50 risk at this point," she said.

How does El Nino affect Australia?

If this El Nino does develop it would be a bit unusual.

What El Nino, La Nina actually means for weather Terms such as El Nino, La Nina and ENSO are thrown around a lot in relation to Australian weather, but what do they actually mean? Read more Read more

"If El Nino does develop this year, it'll be a little later than we typically see an El Nino develop which can be an indication that it's likely to be weak," Ms Duell said.

But this does not necessarily mean less of an impact on rainfall.

"In the past we've had some weak El Ninos which has had very big impacts associated with them and widespread drought across Australia," she said.

"We've also had some very strong El Ninos, by many measures, which have had quite a modest impact.

"So it's really a question of whether we get El Nino or not, not so much the strength."

How do they know so far in advance?

There are many indicators that climate scientists use to determine if an El Nino, or its sister La Nina, are happening or likely to happen, such as:

upper and lower Pacific Ocean temperatures;

upper and lower Pacific Ocean temperatures; strength of the trade winds;

strength of the trade winds; cloudiness; and

cloudiness; and the Southern Oscillation Index (SOI).

The Southern Oscillation Index has been in neutral since late April. Values above +7 typically indicate La Nina conditions while below -7 indicates El Nino. ( Supplied: Bureau of Meteorology )

But to tell what is likely to happen beyond the immediate future they use computer models.

"The bureau runs a model on a super computer and there are many other countries which also run these complex computer models and we look at what they are predicting," Ms Duell said.

"So at the moment we survey eight different models, including the bureau's model, and five of eight of those models are indicating that El Nino is possible in spring."

But, a little ironically, it is the Bureau's model that is the outlier for the future ENSO outlook.

The following diagram shows that the Australian Bureau of Meteorology's model has the lowest likelihood of reaching El Nino sea surface temperatures in November, dragging down the mean.

Five of the eight models suggest there will be an El Nino in November, an exception being BOM's. ( Supplied: Bureau of Meteorology )

"All of the models that we look at are giving a realistic possible scenario for the future," Ms Duell said.

"When you're forecasting a long way ahead, up to months and seasons, the difficulty is there's a lot of chaos in the environment.

"Very small, tiny changes to what you feed into the model at the very beginning can have quite a different outcome in the future."

The chance of an El Nino this spring is 50 per cent, that's double the normal likelihood. ( Supplied: Bureau of Meteorology )

Ms Duell said that they deal with all the different possible scenarios from the tiny, little changes by running the model multiple times, making little tweaks to the initial conditions.

"We run 33 different scenarios, called an 'ensemble', and all of the models that we look at do a similar sort of thing," she said.

"Every model that we look at is giving a realistic possibility of something that could happen.

"Because it's so difficult to forecast that far in advance we also like to see some early signs in the observations as well.

"So even though the models are quite gung-ho for El Nino at the moment, we're not seeing as many indicators in the actual observations to elevate the risk above a 50 per cent chance at the moment."

Should we be worried?

With the second driest autumn on record for the south of Australia and a dry outlook for the south-east over the next few months, the chance of a dry spring is the last thing many farmers will want to hear.

Its not looking good for rain over the next few months if you're in the brown or orange sections of this map. ( Supplied: Bureau of Meteorology )

Ms Duell said that according to the outlook "it is quite likely that we will see less rainfall than we typically will in the remaining winter months".

"I think the takeaway message here is that it's time to definitely factor into your plans that there is a possibility and a real risk that it will be a drier than usual season in eastern Australia for winter and spring.

"It's really a risk management-type situation where this needs to be factored into plans if you have a sensitivity to rainfall."

Loading...