Highlights
- What are El Niño and La Niña events? And what's the neutral phase?
- The El Niño-Southern Oscillation (ENSO) is a "coupled" ocean–atmosphere phenomenon
- The Indian Ocean Dipole and the Southern Annular Mode are also key drivers
El Niño and La Niña events are a natural part of the global climate system.
El Niño and La Niña are part of a natural cycle known as the El Niño-Southern Oscillation (ENSO), explains Professor Scott Power, Director of the Centre for Applied Climate Sciences at the University of Southern Queensland.
"The El Niño–Southern Oscillation (ENSO) is the term scientists give to a naturally occurring oscillation in the climate system that consists of three different phases," said Professor Power.
The two opposite phases, “El Niño” and “La Niña,” require certain changes in both the ocean and the atmosphere because ENSO is a coupled climate phenomenon.
“Neutral” is in the middle of the continuum.
Pacific Ocean – even in neutral state the Western Pacific is warm. Source: Reproduced with the permission of the Bureau of Meteorology.
El Niño and La Niña events
La Niña is typically associated with higher-than-average rainfall across much of Australia, particularly inland eastern and northern regions, sometimes causing floods.
El Niño is typically associated with reduced rainfall northern and eastern Australia.
They occur when the Pacific Ocean and the atmosphere above it change from their neutral 'normal' state for several seasons, says Professor Power, who is also an Adjunct Professor at the School of Earth, Atmosphere and Environment at Monash University.
During “El Niño”, the oceans tend to warm up in the central and eastern Pacific, and the ocean tends to cool off a little bit to the north of Australia and in the waters around Southeast Asia.
"Those changes in the ocean have a really profound impact on the overlying atmosphere, so the trade winds tend to get weaker and the convection and the rainfall tend to move away from Southeast Asia and north of Australia to the east — away — so northern Australia and Southeast Asia tend to dry out and parts further east tend to get wetter," he explained. 
La Niña is the opposite ENSO phase to El Niño.
During an El Niño event, trade winds weaken, allowing the area of warmer than normal water to move into the central and eastern tropical Pacific Ocean. Source: Reproduced with the permission of the Bureau of Meteorology.
"La Niña, which is what we are in at the moment, tends to do the opposite, so the waters north of Australia and in Southeast Asia tend to warm up and that attracts lots of rainfall, and so things tend to get wetter," explains Professor Power.
"And as those ocean waters warm up, the sea level actually starts to rise. So, during La Niña, you tend to see more rainfall, higher sea levels and more mild temperatures," he added.
“Neutral” is in the middle of the continuum, in which Equatorial Pacific Sea Surface Temperatures are generally close to average.
However, there are some instances when the ocean can look like it is in an El Niño or La Niña state, but the atmosphere is not reacting — or vice versa.
During a La Niña event, the Walker Circulation intensifies with greater convection over the western Pacific and stronger trade winds. Source: Reproduced with the permission of the Bureau of Meteorology
"A very irregular oscillation"
Professor Power says that the oscillation is a very irregular.
We might have an El Nino this year, and next year you might have La Nina or a neutral year where you might even have another La Nina.
So, the oscilation can change from one phase to another very irregularly, and each of those phases tend to last for about 12 monthson average.
It's only possible to have one of those three phases at a time.
"Around about half of all years are El Niño neutral years, when there is not much happening; then you can get around one quarter of years— La Niña,” and the other quarter— El Niño,", explains Professor Power.
ENSO indicators
The Bureau of Meteorology monitors and reports on a range of ENSO indicators, including:
- short-term bursts of tropical rainfall activity
- water temperatures at the sea surface and at depth
- ocean heat content – measuring the amount of energy stored in the ocean
- the
- atmospheric air pressure
- cloudiness – measuring the amount of cloud in tropical regions
- the strength of the trade winds and winds higher in the atmosphere
- ocean currents.
These climate indicators provide information about current ENSO conditions, and are inputs into climate models that are used to predict conditions for the months ahead.
Source: Getty Images/Andrew Merry
So, what is the Southern Oscillation Index?
According to Dr Lynette Bettio, a senior climatologist in the Climate Monitoring team at the Bureau of Meteorology, the Southern Oscillation Index measures the difference in surface air pressure between Tahiti and Darwin.
The Southern Oscillation Index is actually the atmospheric part of the El Nino and La Nina events.
"We've been actually able to see well that when the Southern Oscillation is either really strongly positive values or strongly negative values, that it actually goes with those phases of El Nino and La Nina and impacts Australian rainfall," she explained.
The Indian Ocean Dipole
During the bushfires that scorched much of Australia in 2019, another factor was at play: The Indian Ocean Dipole.
A negative Indian Ocean Dipole leads to more rain, explains Agus Santoso, a senior research associate at the UNSW Climate Change Research Centre, and a CSIRO adjunct science leader.
The Indian Ocean Dipole is a climate phenomenon that is similar to ENSO but it's occurring in the Indian ocean side.
"So, the positive face of the Indian Ocean Dipole is when the sea surface temperature in the eastern side of the Indian ocean, just to the west of Australia and in Indonesia, it becomes colder than normal, and when that happens it leads to dryer condition over south-east Australia," explained Mr Santoso.
The Southern Annular Mode
The Southern Annular Mode is another an important driver of rainfall variability in Australia — particularly in southern Australia.
It’s also known as the Antarctic Oscillation.
The Southern annular mode is an atmospheric condition which controls the location of the rest of the winds of the Southern Ocean, like storms over the Southern part of Australia like Tasmania.
"So, during the positive face of the Southern Annular Mode, the trade winds move south, so the Southern part of Australia tend to be drier, but when that happens it allows more wind coming from the Eastern part of Australia," explains Mr Santoso.
This brings moisture that can lead to rainfall.
When this is combined with La Nina, it can lead to more intense rain.
Climate change
Dr Power says climate change needs to be taken into consideration as well as an added factor to all these key drivers.
In order to understand what's happening you have to take into account not only the natural variability, but also what climate change is doing to all of those climate variables.
He says, in addition to natural variability like the El Nino Southern oscillation, human-forced climate change is also impacting these weather events resulting in long-term warming, long-term increases in sea levels across the globe, including around Australia.
