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Southern Annular Mode: The climate 'influencer' you may not have heard of

Kate Doyle, Tuesday August 14, 2018 - 16:53 EST
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The tilt and rotation of the Earth relative to the sun cause huge global circulations. - ABC

In the world of climate drivers El Nino is the big name. The Bureau of Meteorology has a whole section on its website dedicated to monitoring it, and updates are hotly anticipated.

Today's update said El Nino was staying at "watch", so there is a 50 per cent chance of an El Nino this season, twice the normal likelihood.

El Nino is monitored closely because it is associated with dry conditions in eastern Australia, but it is not the only climate driver to be keeping an eye on this year.

The Southern Annular Mode (SAM) is the teenager of climate drivers — it is temperamental, misunderstood and increasing in strength.

Earlier this year SAM had a hand in keeping things dry, and now it is tangled up in the westerlies that have been battering the south over the past week.

It even has the potential to team up with El Nino this summer.

How does SAM work?

One of the reasons SAM has been flying under the radar is that it is mixed up with the seasons.

Because of the tilt of the Earth relative to the sun, during a normal summer the subtropical ridge — a band of descending air that goes around the globe — moves south to sit over southern Australia, bringing lots of warm, dry weather to the south of the continent.



During a normal winter, the subtropical ridge moves further north, bringing dry weather to northern Australia and allowing westerly winds — associated with a band of low pressure that goes around the globe under the subtropical ridge — to push the fronts that carry wet and stormy conditions to the south.

This is why southern Australia gets wet winters but up north the wet season is in summer.



According to Bureau of Meteorology climatologist Felicity Gamble, SAM is when this seasonal movement is disrupted.
A positive SAM is when the westerly winds move further south than normal for the time of year.
A negative SAM is when the westerlies move further north than normal for the time of year.

Which may sound simple, but it all gets a bit confusing when the impact of SAM is mixed in with the normal impacts of the seasonal movement.

SAM in winter

Normally in winter, the subtropical ridge sits just far enough north that rain-bearing fronts and low pressure systems regularly get far enough north to bring rain to southern Australia.



A positive SAM in winter means the high pressure ridge sits further south than normal, which keeps the south dry.

So a positive SAM during winter can be devastating for southern farmers who rely on winter rainfall.

But a positive SAM does not mean it is dry everywhere.



A positive SAM in winter can bring more rain for northern New South Wales and into Queensland because of moist onshore winds.

Ms Gamble said there also was some evidence that suggested a high incidence of east coast lows during positive phases of SAM.

A negative SAM in winter is almost the opposite — extra rain for the south.

But in northern New South Wales, there tends to be drier than average winter conditions because the moist onshore winds from the east are replaced by dry air from the west.

"The westerlies that come into New South Wales have come across the land, so they bring a much drier, continental airmass," Ms Gamble said.

SAM in summer

True to its inconsistent, teenage behaviour, SAM in summer is totally different to SAM in winter.

Usually in summer, the subtropical ridge and its high pressure systems sit over southern Australia keeping things nice and dry, with just the occasional burst of moisture from onshore easterlies or tropical moisture.



A positive SAM in summer tends to bring above-average rainfall for the south, specifically the southern and eastern half of Victoria, the eastern half of Tasmania, and much of New South Wales.

This rain is caused by moist onshore winds and tropical influences making their way further south than normal.

"We see a much stronger easterly flow that brings increased chance of rainfall to areas that are on the eastern edge of [southern] Australia," Ms Gamble said.

"With the highs further south, tropical moisture becomes avaliable to be pulled into rain systems."



A negative SAM in summer has the opposite effect. It brings dry conditions to southern Australia because of enhanced westerly flow and high pressure systems.

SAM vs the big guns



Ms Gamble said SAM played a dominant role in the high and mid-latitude climate of the southern hemisphere.

"It can explain up to around 15 per cent of the weekly rainfall variation in southern Australia, which is comparable to what ENSO [El Nino Southern Oscillation] can impact during the winter and spring months," she said.

But SAM has a short attention span — it does not last for as long as the other climate drivers.

ENSO can last for years and the Indian Ocean Dipole for months, but SAM generally only remains in one phase for a week or two.

Positive SAM on the rise



SAM may be flighty and inconsistent, but all those pulls are starting to add up.

There is a scientifically tested trend towards positive SAM during the southern hemisphere summer and autumn months.

This has meant wetter than average summers in south-east Australia and eastern Tasmania, and drier summers in western Tasmania.

Ms Gamble said there had been various studies that considered natural variability, but the trend could not be explained by natural causes alone.

"It's generally evident that changes in stratospheric ozone, so the loss in ozone from CFCs, as well as increasing greenhouse gases, have caused this positive trend in SAM," she said.



So what has been going on with SAM this year?

SAM has mainly sat in neutral this year, but it did tip into positive for a week in early June, which was reflected in rainfall patterns.

"During that time we saw drier than average conditions through Victoria, west-southwest WA and Tasmania … We saw a little bit more rainfall than usual [in NSW but] it was mainly coastal, unfortunately, didn't make it too far inland," Ms Gamble said.



Just to show how flippant it can be, in the past week SAM has done a 180 and gone negative, bringing rain to the south.

Ms Gamble said the continued dry conditions in NSW were also typical of negative SAM at this time of year.

What about the year to come?

If you are hoping for rain it would be best to look away now.



The current outlook suggests twice the normal likelihood of El Nino this summer, which already signals dry conditions for eastern Australia.

When you add SAM to the mix, the outlook just gets worse, because the two climate drivers are associated.

"During an El Nino, we tend to see more negative SAM events and during a La Nina, we tend to see more positive SAM events," Ms Gamble said.

Negative SAM in summer is further bad news because it is associated with dry conditions for south-east Australia.

Negative summer SAM and El Nino combined would be a recipe for dry weather in the south-east.


- ABC

© ABC 2018

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