ANSWER 1

Written response:

Scientists are naturally a cautious and conservative bunch. We like to have lots of evidence before coming to conclusions. Unfortunately, determining the precise effect of something specific like a positive Southern Annular Mode (SAM) on the harmful algal bloom affecting South Australia is a difficult scientific question that will almost certainly be the focus of significant research in coming years. Until that work is done, it is impossible to say with confidence what the effect is, we can however speculate.

The Southern Annular Mode (SAM) is an index that describes the position and strength of the westerly winds commonly known as the “Roaring Forties” and “Furious Fifties”. The Bureau of Meteorology has a nice summary on their webpage here. The SAM has been mostly positive between March and August 2025, but was fairly neutral from September 2024 to March 2025 (see attached graph from http://www.bom.gov.au/climate/enso/?ninoIndex=nino3.4&index=nino34&period=weekly#tabs=Southern-Ocean), so it is natural to wonder if the positive SAM has contributed to the harmful algal bloom.

When the SAM is positive, these winds are a bit stronger and have moved south towards Antarctica. For the coast of South Australia, this means on average that the westerly winds are a bit weaker, there are fewer storms, and there is less rain. This could potentially exacerbate and extend the harmful algal bloom by helping keep the algae near the surface of the ocean and isolating the upper ocean from the deeper parts of the ocean. As an example, recent research has shown that weaker than average winds had a big effect on the 2023 marine heat wave in the North Atlantic Ocean, making it much stronger than it would otherwise have been (https://www.nature.com/articles/s41586-025-08903-5).

In contrast, when the SAM is negative, the westerly winds are a bit weaker overall, and move north towards Australia. For those of us in southern Australia, the northward movement is more important than the slight weakening and we actually get stronger westerly winds from a negative SAM. Stronger westerly winds would help to mix the upper few hundred metres of the ocean and might make the harmful algal bloom dissipate a bit faster.

Climate indices like the Southern Annular Mode are a good way of describing the average climate over a period of time, and can help us to build up a picture of what sort of weather we might expect. But, our world is full of surprises, and the weather can do all sorts of things regardless of whether the SAM is positive or negative. Think of it as loading the dice – a particular outcome becomes more likely, but it’s not guaranteed.

There have been many periods of positive SAM in the past, but they have not caused harmful algal blooms. Untangling the factors that came together to cause this one is an important research topic, but one that we haven’t solved yet.

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