The Southern Annular Mode (SAM) has just reached its highest value since 2023. But what exactly does this strong positive SAM mean for Australia’s weather in the immediate future?

What is the Southern Annular Mode?

The SAM is one example of what meteorologists call "climate drivers". These are broad-scale processes that drive the movement of heat, wind and moisture across the oceans and atmosphere.

We've all heard of El Niño (which was declared last week) and its cousin La Niña. These climate drivers are associated with shifts in sea surface temperatures and wind and cloud across the equatorial Pacific Ocean.

The SAM is a climate driver which influences the north-south displacement of the belt of powerful winds which flow year-round from west to east over the Southern Ocean, relative to where the belt would normally be positioned at a given time of year.

The position of the belt of westerlies over the Southern Ocean has strong impacts on Australia’s weather, especially in southern and eastern parts of the country.

In winter, the belt of westerlies is typically located closer to Australian latitudes, while in summer, it contracts southwards towards Antarctica. But a positive or negative SAM can shift its location.

During a negative SAM in winter the band of westerlies is located closer to Australia, making cold fronts and low pressure systems more active than usual across Australia's southern states. This can enhance wind, rain and snow in southern Australia.

A positive SAM in winter (see above chart) tends to have the opposite effect, forcing the westerly winds, cold fronts and low pressure systems further south than usual for this time of year, reducing their influence on Australia’s weather.

While a positive SAM in winter tends to reduce the frequency of moisture-laden Southern Ocean systems pushing towards southern Australia, it does increase the chances of wet weather pushing onshore to Australia’s east coast from the Tasman Sea.

The green patch in the image above illustrates this. And right on cue, a rainy spell is forecast for parts of NSW and Queensland starting later this week.

What does the current strongly positive SAM mean for our weather?

As stated, the SAM has reached a three-year high this week. It is now very strongly positive.

That means that mean sea level pressure is currently trending higher than normal near Australia's latitudes, and the westerly wind belt that flows between Australia and Antarctica is located further south than usual for this time of year.

This has been evident in the sort of weather we’ve seen lately across southeastern Australia, with fewer cold fronts, frequent blocking high pressure systems, and unseasonably warm temperatures.

The last time the SAM index reached 4 (or higher) was in May 2023, when it peaked at 5.5. The values in the index are a measure of standard deviation from the norm in terms of mean sea level pressure. In very basic terms, it means we’ve seen a lot more highs than lows.

Image: The trend of the SAM over the last 12 months, showing how this week’s value is the highest over that period. Source: BoM.

What a positive SAM typically means for snowfalls in Australia

Image: Minimal natural snow up high at Blue Cow ski area, part of Perisher resort. Thankfully, a string of cold nights is allowing snowmaking on many slopes at Australian ski resorts, enabling at least some skiing and snowboarding for the forthcoming school holidays. Source: ski.com.au.

With the school holidays rapidly approaching and the ski slopes bare and grassy across most of the Australian Alps, a positive SAM is not good news for the snowfields.

We already know that there’s a strong correlation between El Niño years and low snowfall in Australia. Compounding that, snow-bearing cold fronts tend to visit Australia’s shores less frequently during a positive SAM in winter.

Thankfully, neither of these climate drivers prevents the occasional strong, cold, moisture-laden system from surging northwards towards Australia.

So while the big picture indicators provide little cause for optimism, the day-to-day variability of individual weather systems still offers hope. Please check the Weatherzone snow page for the latest forecasts, live snow cams and more.