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Natural variability in our climate

Large-scale climate processes, such as the El Niño – Southern Oscillation (ENSO), the Indian Ocean Dipole (IOD) and the Southern Annular Mode (SAM), affect Australia’s climate in cycles. These processes change Australia’s climate by moving warm water and warm, moist air to different regions.

Rainfall near Balaklava, South Australia. Credit CSIRO John Coppi

El Niño – Southern Oscillation

ENSO has 3 phases: El Niño, La Niña and the neutral phase. During the neutral phase, the trade winds of the Pacific Ocean blow warm, moist air and warm waters towards north-eastern Australia. The warmer waters cause air to rise, creating rainfall. During La Niña, these winds get even stronger, increasing the amount of warmer water near Australia, increasing the rainfall and causing floods. During El Niño, these winds weaken, or even reverse, moving warm water away from Australia and towards South America. With less warm water, rainfall is less likely to form and hence Australia often experiences drought.[1]

Indian Ocean Dipole

During the positive phase of the IOD, the eastern Indian Ocean experiences lower than normal sea-surface temperatures compared with the western Indian Ocean. Warmer waters in the west cause more rain over Africa and there is less rainfall in Australia and parts of southern Asia. During the negative phase of the IOD, these conditions are reversed and Australia is cooler and wetter.[2]

Southern Annular Mode

SAM refers to north-south changes in Antarctica’s strong westerly wind belt. SAM affects rainfall in different regions of Australia depending on the phase (positive, neutral or negative) and the season.[3] During the negative phase, the westerly winds move closer to Australia. This brings more low-pressure systems to southern Australia, and areas such as Victoria experience more rain and storms. During the positive phase, the westerly winds move further south, so high-pressure systems form over southern Australia, leading to drier, more stable weather.[4] SAM phases are usually quite short, lasting from a week to a few months.

Into the future

Under a changing climate, SAM is predicted to spend more time in the positive phase, bringing drier weather to parts of southern Australia during winters, and wetter conditions during summer in south-eastern Australia and eastern Tasmania.[5]

Increasing surface temperatures mean that a typical La Niña year now is warmer than El Niño years were back in the 1980s.[6] ENSO events are likely to increase in amplitude and frequency due to climate change, having stronger effects and occurring more often.[7] [8] This may result in more flooding in some Pacific Island nations and more drought elsewhere, such as mainland Australia.[9] Even if we keep global warming to 1.5 °C, we still expect extreme ENSO events to persist for another 100 years or so.[10]

Extremely positive IOD events are likely to increase as climate changes.[11] These events tend to correspond with El Niño events, bringing more drought to Australia.[12] Limiting global temperatures would have an immediate effect on halting the increased frequency of extreme positive IOD events.[13]

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