Southern Annular Mode or Antarctic Oscillation
The Southern Annular Mode (SAM) is a climate driver. It can affect rainfall and temperature in southern Australia. SAM is also called the Antarctic Oscillation (AAO).
Strong westerly winds blow almost continuously around the globe in the mid to high latitudes of both hemispheres. The southern hemisphere winds are sometimes called the Roaring 40s and Furious 50s.
SAM describes the north–south movement of this wind belt away from its normal location. This can disrupt weather systems that affect Australia, especially in the south.
The impact can be felt all year round. It also can affect the rain from storms and cold fronts that move from west to east along the wind belt.
Video: Understanding the SAM
So what is the SAM? Well in the southern hemisphere a belt of high pressure extends right around the globe in the subtropics. We call this the subtropical ridge. On the southern side of this ridge there are strong westerly winds – the 'Roaring Forties' and the 'Furious 50's'. SAM refers to the unusual north or south shift of these winds.
This shift in the winds away from their normal position can disrupt weather systems around the southern hemisphere, including those that bring rainfall to southern Australia.
SAM varies between three phases. Positive, neutral and negative. These phases last around two weeks but can sometimes hang around even longer.
SAM's influence on Australian weather patterns varies during the year. Winter is the time that has the greatest impact across southern Australia. Typically during winter the top of this belt of westerly winds is located over southern Australia. This means cold fronts are drawn up from the south and can cross the Australian coast bringing normal winter rains. But if these winds tip towards Antarctica, the rain-bearing systems also head south. This means fewer fronts and drier conditions for southern Australia.
This shift south in the weather systems is called a positive SAM. But a positive SAM can also mean parts of the east coast get wet, as it draws more tropical air southwards and allows more onshore easterly winds than normal.
The opposite is a negative SAM, which occurs when the westerly winds are further north than usual. This allows more weather than cold weather to be drawn up from the south and a negative SAM winter can also help bring good snowfall to the alps. But if it moves too far north the westerlies blow right across the continent, meaning the air is wet in the west but dry by the time it reaches the east coast.
The impact of SAM is opposite in summer when the subtropical ridge is at its southernmost position and the influence of the westerly winds on our rainfall decreases. This is because they are further away from our coastline. But if the winds do shift north in summer, a negative SAM more dry air from the interior blows over eastern Australia, resulting in below-average rainfall.
During a positive phase of SAM in summer the ridge shifts even further south. This expands the area receiving tropical air from the north and allows more moist onshore winds along the east coast. More available moisture means more chance of rain.
In spring SAM's effect on our rainfall can resemble a weak summer pattern. For many parts of southern Australia a negative phase can increase the odds of a heatwave. SAM has little impact on rainfall during autumn, though a negative phase can increase the odds of extreme heat in northern and central Australia.
The southern annular mode is often related to ENSO events. In El Niños it tends to be more negative and in La Niñas it's generally more positive. This can reinforce some of the ENSO impacts particularly in the summer months.
In recent decades there has been a trend towards more positive SAM events; and our weather systems have shifted southwards.
The southern annular mode plays an important role in driving Australia's climate. As with all our climate drivers, no two SAM events and no two sets of impacts are exactly the same.
Understanding this key Australian climate driver can help you better understand our seasonal outlooks and make smarter climate decisions.
Phases and duration
SAM has 3 phases:
- positive – the wind belt is south of its normal position
- neutral – the wind belt is in its normal position
- negative – the wind belt is north of its normal position.
Each positive or negative phase tends to last for about 1–2 weeks. At times the phases can last longer. The drivers for this are:
- winds blowing around Antarctica in the stratosphere about 10–60 km above Earth's surface (the Antarctic vortex)
- changes in the tropical Pacific Ocean – El Niño and La Niña.
Drivers and trends
Antarctic vortex effect on SAM
The Antarctic vortex is a wind pattern in the stratosphere. It goes around the southern hemisphere high latitudes from west to east.
The speed of the winds peaks in winter and slows and breaks down in late spring or early summer. This affects the amount of ozone, so changes the size of the Antarctic ozone hole.
Together, the vortex and amount of ozone can push SAM to be positive or negative:
- When the vortex weakens faster and earlier, there is more ozone over the Antarctic region. SAM is more negative.
- When the vortex lasts longer and breaks down later, this depletes the Antarctic ozone. SAM is more positive.
El Niño and La Niña relationship
SAM can reinforce some effects of the El Niño–Southern Oscillation during October to December:
- During El Niño, SAM tends to be more negative.
- In La Niña, it's generally more positive.
Trends
In the past several decades there has been a shift towards more positive SAM events from December to May.
In summer, this was primarily due to an expanding Antarctic ozone hole in the 1980s–90s. Other factors may be:
- climate change due to human activities (anthropogenic climate change)
- natural changes that happen over spans of 10 to 30 years (naturally occurring decadal climate variability).
How SAM affects rainfall
The wind belt shifts north and south with the seasons, due to the tilt of the Earth on its axis.
Given its seasonal nature, SAM's influence on rainfall varies greatly depending on the time of year and region.
Winter
In winter, SAM affects rainfall in Australia's south. This can include:
- northern Tasmania
- Victoria
- south-west Western Australia.
Spring and summer
In spring and early summer, SAM influences rainfall over larger areas of:
- New South Wales
- Victoria
- Tasmania.
Autumn
SAM has little impact on rainfall during autumn.
Positive SAM – seasonal impact
In a positive SAM phase, the belt of westerly winds shifts south. It moves away from Australia and towards Antarctica. This has the greatest impact across southern Australia in winter.
Winter
In winter, the usual position of the wind belt is close to Australia. Cold fronts are drawn up from the south. They can cross the Australian coast, bringing normal winter rains.
But when the wind belt moves towards Antarctica, the rain systems also head south. There are:
- weaker than normal westerly winds
- higher pressures over southern Australia.
Cold fronts find it harder to move inland. This typically means more stable conditions, and drier weather for southern Australia.
When the wind belt moves south, it typically draws in onshore winds over eastern Australia. Flowing from the Tasman and Coral seas, this onshore wind is moist and can bring more rainfall to the east coast.
Summer
In summer, the wind belt is usually further south than in winter. In a positive SAM phase, it moves even further towards Antarctica.
When the wind belt moves south:
- it draws more moist onshore winds to eastern Australia, including eastern Tasmania, which generally brings more rainfall
- western Tasmania, where the westerly winds generally bring rainfall, may get less rain
- the subtropical dry zone also moves south, which may mean more rainfall for subtropical regions.
SAM's influence on rainfall is similar in spring and summer. It may have more impact in eastern Australia's summer, as it enhances the average higher rainfall.
Negative SAM – seasonal impact
In a negative SAM phase, the belt of westerly winds expands and moves north towards the equator and Australia.
Winter
As the wind belt moves north, southern Australia can experience:
- stronger than normal westerly winds
- lower atmospheric pressure
- more cold fronts and storm systems
- more rain events.
In subtropical eastern Australia, there may be less rainfall. As the westerly wind belt moves north, it pushes away the moist onshore winds.
A negative SAM phase in winter can mean more storms in southern Australia and less rainfall in the east
Spring and summer
There can be less rainfall over eastern Australia as the wind belt moves north. It blows dry air from inland areas and pushes away the moist onshore breezes that can bring rain.
The subtropical dry zone shifts north with the wind belt. This can make warm seasons drier in subtropical regions.
For Tasmania, westerly winds can get stronger, bringing more rain in the state's west.
For many parts of southern Australia, a negative SAM phase can make heatwaves more likely.
Autumn
For northern and central Australia, a negative SAM can increase the chances of extreme heat in autumn.
A negative SAM phase in spring and summer can increase the chance of heatwaves in southern Australia