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 shifts 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 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 drier 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 and 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 Niña 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 set 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. For more information visit www.bom.gov.au/climate/sam