While El Niño, La Niña and the Indian Ocean Dipole are the primary drivers of year-to-year climate variability in Australia; there's a set of secondary drivers that can also play a key role. One of these is the SAM or the Southern Annular Mode.

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.

Understanding the Southern Annular Mode (SAM)

The SAM is a driver of Australia's climate. Learn how it can influence the weather systems that bring rain to southern Australia.

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