Southern Annular Mode

Before I start this feature into the Southern Annular Mode, I am going to explain the lack of posts. I have had some issues in my weather data supply in the last week or two. As of today, the weather data supply has been restored. But I am in the middle of transferring my weather data systems, and hopefully better content can be distributed for the Australian and NZ snow season. Thank you for your patience. New blog posts are coming soon!

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So the Southern Annular Mode or SAM (or Antarctic Oscillation or AAO) is basically an index showing how far equatorward or poleward the sub tropical ridge is, and therefore the polar jetstream. What is the Sub Tropical Ridge? Here’s an example MSLP chart showing it’s position.

The polar jetstream is below this ridge of highs. In summer, Negative SAM (equatorward for the STRHs), the Sub Tropical Ridge sits over Southern Australia, limiting rainfall and increasing temperatures. When Positive SAM (poleward for the STRHs) occurs in summer, the Sub Tropical Ridge moves south of the mainland and sometimes Tasmania, allowing rainfall often associated with the tropics to fall and temperatures to cool.

In winter, it’s a different story. The Sub Tropical ridge and jetstream has moved equatorward with the change of seasons. So in a Negative SAM situation in winter, the Sub Tropical ridge is over Central Australia and allows cold fronts to come through over Southern Australia and give rainfall and colder temperatures. In a positive SAM situation in winter, the Sub Tropical ridge is over Southern Australia and limits cold fronts coming into Southern Australia.

Here is the rainfall anomalies associated with Positive SAM over the seasons. You can see drier conditions in winter for Southern Australia and wetter conditions in summer for Southern Australia (Composite relatively outdated though).

And how much does this impact Australia’s climate/weather story.

This chart shows how much of the rainfall in that period can be attributed to SAM. Overall, it is responsible for about 15% of the weekly rainfall variance, similar to ENSO. Same with snowfall too.

I do have to mention it, others explain SAM with tight jetstream and strong westerly winds(Positive SAM) and a loose jetstream and weaker westerly winds(Negative SAM). And some look at MSLP on the Antarctic continent itself to check SAM’s status.

The final thing I want to talk about is the relationship between SAM and ENSO. In austral summer, there is a correlation between Canonical El Niño and Negative SAM and a further correlation between Canonical La Niña and Positive SAM. In austral late Autumn-Early winter, Warm-pool El Niño promotes Negative SAM and Warm-pool La Niña promotes Positive SAM. More Here

Further reading: Full scientific explanation of SAM’s effect on the Australian continent.

The relationship between ENSO and SAM.

More on SAM precipitation impacts.

SAM impacts on the entire Southern Hemisphere.

And Forecasts…

GFS Ensemble Forecasts

Stay cool Snowy Hibbo

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