La Niña delivers record deluge

Climate: La Niña

At a glance

• The La Niña phase of the El Niño Southern Oscillation looks to be coming to an end.
• Last year was the second wettest on record for Australia and one of the most extreme La Niñas ever seen.
• Farmers urged to check the Southern Oscillation Index in winter and make timing and management decisions accordingly.
• Visit ClimateDogs online to understand more about Victoria's weather patterns.

When the rain started last September, most Victorian farmers welcomed the deluge. Three months later, many were less impressed.

After a ten year drought, the rainfall signalled the arrival of La Niña and the break-down of El Niño. We turn to DPI's ClimateDogs, Enso, Indy, Sam and Ridgey and the Bureau of Meteorology to work out exactly what happened and why now is the best time for predicting spring rainfall.

So what is La Niña?

ClimateDog Enso rounds up tropical moist air in the Equatorial Pacific Ocean.

DPI's Chris Sounness explains that it all comes back to our oceans. The Pacific and Indian Oceans are largely responsible for the moisture that leads to our rainfall, while the Southern Ocean provides the fronts and lows.

The El Niño Southern Index ENSO —ClimateDog Enso — goes through three phases: El Niño, La Niña or neutral. The index refers to the location of warm and cool water, wind strength and atmospheric pressures in the Equatorial Pacific Ocean region.

The Southern Oscillation Index (SOI) can predict which phase ENSO is in. If atmospheric pressure at Tahiti is lower than Darwin we're in El Niño, if it's higher, we're in La Niña.

Key indicators of La Niña include:

• Cooler than average sea surface temperatures in the central and eastern Pacific Ocean along the equator and cooler sea temperatures at depth in the same region.
• Strong easterly trade winds along the equator.
• Below average cloudiness near the equator.

Warm sea surface temperatures, weak winds and above average cloudiness, meanwhile, are indicators of El Niño. Neutral indicators such as average sea surface temperatures, winds and cloudiness — mean just that — we're in a neutral phase.

Why does La Niña mean more rain?

ClimateDog Indy delivers water from the Indian Ocean.

According to Mr Sounness, it's all basic physics. "In the central Pacific when it's colder than normal on the surface, warm water tends to accumulate along the Queensland coastline, so we have warmer water off the Queensland coast.

And rain it did. According to Bureau of Meteorology climatologist Belinda Campbell, 2010 was the second wettest year on record for Australia and one of the most extreme La Niña events ever observed. The difference in rainfall in the past six months as compared to 2009 is illustrated clearly in Figures 1 and 2.

Figure 1: Victorian rainfall deciles, October 1, 2010 to March 31, 2011

Figure 2: Victorian rainfall deciles, October 1, 2009 to March 31, 2010

A negative Indian Ocean Dipole — ClimateDog Indy — contributed to increased rainfall over parts of southern Australia through spring and into summer, Ms Campbell explained.

Indy is associated with warmer than normal water in the tropical eastern Indian Ocean and cooler than normal water in the tropical western Indian Ocean.

"In spring 2010, we had very warm sea surface temperatures off the northwest coast of Australia," she explained.

This warms the air above the warm pool of water leading to convection, with warm rising air forming cloud.

"Through spring we saw several north-west cloud bands, which travelled from north west Western Australia, through central Australia (parts of the Northern Territory, South Australia and New South Wales) to northern Victoria," Ms Campbell said.

She said while a frontal system would affect southern Victoria, when the northwest cloud band comes from the north, the alpine regions "act as a blocker" for rainfall in southern parts.

The very strong La Niña continued after spring, with Victoria recording its wettest summer since records began.

Figure 3: Victorian annual rainfall, 1900–2010

What happens next?

ClimateDog Ridgey and high pressure systems can block rainfall in Victoria.

ClimateDog Sam influences the strength and frequency of cold fronts over Victoria.

La Niña has now ended, according to the Bureau's latest ENSO wrap.

"It's likely conditions in the Pacific Ocean will tend towards neutral conditions as we go into winter and early spring," Ms Campbell said.

She said prediction was made more difficult by Australia's continuing climate variability, pointing to the regular peaks and troughs of Australian rainfall over the last 100 years.

But Ms Campbell explained that although historically rainfall follows a pattern of dry years followed by wet years as a result of climate drivers such as Enso and Indy, ultimately it was very hard to predict specifics as "no two events are ever the same".

She said the sub-tropical ridge — ClimateDog Ridgey — could also affect the passage of cold fronts across southern Australia and block rain coming south.

In winter, Ms Campbell said analysis would focus on frontal activity, with the strength and position of fronts around southern Australia influenced by the Southern Annual Mode — ClimateDog Sam.

"For Victoria, the rainfall is typically related to frontal systems through autumn, winter and spring," she said.

"Essentially, when you have a positive Southern Annual Mode you're more likely to have below average rainfall."

Last winter, south east Australia experienced one of the longest running stretches of positive Southern Annual Mode indices in history.

"It was a significant event," Ms Campbell said. "In the case of last winter, we had a positive Southern Annual Mode and as a result southern Victoria in particular recorded below average rainfall in May, June and July 2010."

What should farmers do now?

Mr Sounness spends a lot of his time communicating seasonal climate information to farmers and said using that information effectively on-farm came down to working out what you can and can't influence.

"The reality is that stuff you can influence is far more important to focus on rather than stuff outside your control," he said.

"You should be aware of that, but let's focus on timing and management."

Mr Sounness said farmers needed to accept the fact that seasonal climate information was accurate, but not precise, nor would it be any time soon.

"It's realising where it fits in the scheme of things."

"Winter is the best time to use the tools to see what the probability for spring rainfall is. The Southern Oscillation Index is most effective in June and July, so that's the time to look at it.

"If the Southern Oscillation Index is positive, which it was last year, there's an increased chance of spring rainfall. If it's negative, there's a decreased chance.

"The same happens in reverse where the Southern Oscillation Index is negative, there's a greatly increased chance of below average rainfall."

In June, the Southern Oscillation Index was showing a marked atmospheric pressure drop off between Tahiti and Darwin, falling from a high of about 30 in early April to 3.6.

Mr Sounness said farmers already had vast amounts of weather knowledge about their regions, but finding out about the drivers of climate gave them the ability to then understand why weather is the way it is and act accordingly.

What's the outlook for climate science?

Ms Campbell said climate science had come a long way in recent years.

"There's been a lot of improvement in the observations and in the predictions.

"We currently use statistical models for our seasonal climate outlook, but we also use dynamical models.

"The dynamical models are experimental at the moment, but the Bureau is looking at developing those for public use in coming years."

Ms Campbell said the dynamical models used a larger quantity of up-to-date data to make its predictions, adding they didn't have as long a lag time.

The Bureau also has access to satellite imagery back to the 1970s, which gave it a solid base to make comparisons between the atmospheric conditions during the 2010–11 La Niña and previous events, Ms Campbell said.