Understanding the Southern Annular Mode (SAM): Australia's Westerly Switch

13 May

The Southern Annular Mode (SAM), also called the Antarctic Oscillation (AAO), is one of the most important climate drivers influencing short-term weather variability across the Southern Hemisphere. If you're trying to understand why a week is warm and dry in southern Australia — or cold, wet, and stormy — SAM is often a key part of the answer.

What Is SAM?

SAM describes the north-south shift of the powerful westerly wind belt that circles Antarctica — known as the **mid-latitude jet stream**. These westerlies are responsible for driving cold fronts, low pressure systems, and the majority of rain-bearing weather systems across southern Australia.

SAM isn’t about how strong the westerlies are — it's about where they are positioned. When they shift further north or south than normal, we call that a **negative** or **positive** SAM event.

Positive vs Negative SAM

Positive SAM: The belt of westerly winds contracts towards Antarctica. This tends to block cold fronts from reaching southern Australia. It’s usually associated with **high pressure over the continent**, leading to **drier and more stable conditions** — especially in winter and spring.
Negative SAM: The westerlies expand northwards, allowing cold fronts and low pressure systems to shift further up into southern Australia. This phase typically results in **cooler, wetter conditions**, particularly for **Tasmania, Victoria, southern SA, and southern WA**.

The effects are **season-dependent**. In summer, a positive SAM may enhance moist easterly flow into eastern NSW and QLD, increasing rainfall and humidity. But in winter, it usually means a dry spell for the southern states.

How Is SAM Measured?

SAM is calculated using differences in atmospheric pressure between mid-latitudes (~40°S) and high latitudes (~65°S). The most common index is based on 700 hPa geopotential height anomalies — but others use sea-level pressure (SLP) or 500 hPa levels. The calculation involves:

Creating an average pressure profile over the Southern Hemisphere (the “climatology”).
Subtracting this average from the current pressure profile to get an anomaly pattern.
Using empirical orthogonal functions (EOFs) to extract the leading mode of variability — that’s the SAM.

A positive SAM index indicates stronger polar vortex pressure anomalies and a tighter, more poleward jet. A negative index indicates a weaker vortex and a more equatorward jet.

SAM and Australian Weather

SAM is one of the most important **short-term (weekly to monthly) climate drivers** for Australia — especially during the winter-spring months. Here’s how each phase plays out across regions:

Tasmania, southern VIC, southern SA, southwest WA: Rely on frontal rainfall. These areas tend to get **wetter** during **negative SAM** and **drier** during **positive SAM**.
NSW and southeast QLD (especially coast & ranges): Can benefit from positive SAM in summer via increased easterly moisture and rainfall.
Interior Australia: SAM effects are weaker but can influence broader air mass patterns and frontal penetration.

What Causes SAM to Shift?

SAM is influenced by tropical-extratropical interactions, stratospheric polar vortex variability, and even climate change. Notably:

Sudden Stratospheric Warming events (SSWs): Can induce a strong **negative SAM** lasting several weeks.
El Niño and La Niña: Can nudge the SAM toward positive or negative phases depending on season.
Antarctic ozone recovery: Linked to a **trend toward more positive SAM** over recent decades.

Why Is SAM Hard to Predict Beyond a Week or Two?

Unlike large ocean-atmosphere systems like El Niño or the Indian Ocean Dipole, which evolve over months, SAM is largely driven by internal atmospheric variability — particularly in the **mid- to high-latitudes**. This makes it more chaotic and less predictable at long lead times.

Even high-resolution climate models struggle to forecast SAM more than **10–20 days out** with skill. While stratospheric signals or large-scale tropical forcing can hint at upcoming trends, much of SAM’s week-to-week behaviour depends on:

Transient eddies and storms: Small-scale weather systems interacting with the jet stream influence SAM phase shifts — and these are inherently difficult to model beyond a few days.
Stratosphere-troposphere coupling: Sudden stratospheric warming events can shift SAM negative — but the exact timing and intensity of that influence varies.
Lack of ocean memory: SAM is not strongly tied to slow-moving sea surface temperatures (unlike ENSO), meaning there’s less persistence to build skillful seasonal forecasts.

The result? Most SAM forecasts are **only reliable 7–14 days out**, though ensemble guidance may hint at broader trends up to 3 weeks. Beyond that, it's best treated probabilistically — not deterministically.

Where to Track It

SAM forecasts are available from multiple sources, including:

Final Word

SAM may sound abstract, but it’s one of the most useful tools for understanding short-term shifts in Australia's climate. Whether you're watching for a break in a southern dry spell, gauging alpine snow potential, or assessing rainfall outlooks for eastern NSW — knowing what SAM is doing can tilt the odds in your favour.

Daniel Lester