THE CLIMATE KELPIE BLOG: SAM goes up, SAM goes down— southern Australia’s climate gets turned all around

Posted by BCG on 12th February 2020

While the climate drivers El Niño–Southern Oscillation (ENSO) and the Indian Ocean Dipole are fast becoming household names, the Southern Annular Mode (SAM) is lesser-known, but can still have a substantial, short-term impacts on southern Australia’s climate.

“From late October to late December 2019, a negative SAM contributed to the lack of rainfall over parts of eastern Australia and the higher than average temperatures in the south and east” said Jonathan Pollock, Climatologist at the Bureau of Meteorology.

The Southern Annular Mode, or SAM as it is often called, refers to the non-seasonal northward or southward shift of the belt of westerly winds over the Southern Ocean. This belt lies on the southern side of the subtropical ridge — a band of high pressure that moves north and south with the seasons and is associated with drier and more stable conditions.

For most of the colder half of the year (May to October) the subtropical ridge sits over Australia and during the warmer half (November to April) it is generally located to the south of the mainland. The changing position of the subtropical ridge is the reason why cold fronts and troughs can penetrate well into southern Australia during winter, but during summer they are far less common.

Figure 1. Explainer video: Understanding the Southern Annular Mode (SAM). Source: Bureau of Meteorology.

SAM in summer

SAM varies between three phases: positive, neutral and negative. A neutral phase of SAM means the belt of westerly winds is located near its average position for that time of year.

But when SAM is in a positive or negative phase the position of these westerly winds moves south or north (Figure 2).

A positive SAM in summer means the westerlies are even further south than usual, allowing tropical onshore easterlies to bring more warm moist air into eastern Australia, often resulting in above average rainfall over parts of New South Wales, Victoria, Tasmania and Queensland.

But a negative SAM in summer means less moist onshore flow from the east and typically less rainfall than usual over eastern Australia. Instead, dry continental air from inland Australia is pushed into this region, bring warmer and drier conditions and often increased fire risk.

Figure 2. Summer impacts of the Southern Annular Mode (SAM). During a positive phase of the Southern Annular Mode (SAM), westerlies shift further south allowing moist onshore flow across much of eastern Australia. In a negative phase, eastern Australia tends to see more dry continental airflow. Source: Bureau of Meteorology.

SAM in winter

SAM’s impact can be quite different in winter because of the position of the subtropical ridge. The westerly winds are further north at that time of year and have more influence on our climate (Figure 3).

During neutral SAM in winter, westerly winds typically blow along Australia’s southern coastline bringing cold fronts and rainfall across southern Australia.

When SAM is in a positive phase during winter the westerly winds (and cold fronts) move further south than usual, meaning that places like southwest Western Australia, western Tasmania and parts of South Australia, Victoria and southern New South Wales typically miss out on some of their usual winter rainfall.

Conversely, when SAM is in a negative phase during winter, southern Australia can receive more cold fronts and storm systems, and typically more rainfall.

Figure 3. Winter impacts of the Southern Annular Mode (SAM). In its positive phase westerly winds shift further south than normal bringing wetter conditions to eastern Australia and drier to the south. In its negative phase the winds shift further north than usual, and these affects are reversed. Source: Bureau of Meteorology.

What else do we know about SAM?

The rainfall impacts from SAM are more pronounced during winter and summer compared with autumn and spring. In autumn, Australia’s climate typically sees very little effect from SAM, while in spring, the effect on rainfall resembles a weak summer pattern. But autumn and spring tend to be the times of the year when SAM has greatest influence on extreme heat.

“In the second half of spring and early summer 2019, SAM spent more time than usual in its negative phase. The belt of westerlies was further north than usual. That meant reduced rainfall in parts of the east because of decreased onshore flow” said Mr. Pollock. “At the same time, some southern parts of Victoria, or west-facing southern parts, recorded near average rainfall, and parts of western Tasmania above average rainfall despite the drying influence of one of the strongest positive Indian Ocean Dipole events on record.”

Negative SAM was one factor influencing the fire weather conditions in some parts of Australia towards the end of 2019. When the mode is in a prolonged negative phase, fire weather conditions in Australia are typically worse, particularly in New South Wales. A negative SAM is often associated with an early start to the bushfire season as it was in 2013 and 2018. It isn’t the only culprit though with climate drivers El Niño and the positive Indian Ocean Dipole also playing a role.

Most positive or negative SAM events last for around ten days to two weeks, though longer periods may also occur. Late 2019’s persistent negative SAM lasted around two months, only dipping back into the neutral range for a few days in November.

El Niño–Southern Oscillation events tend to favour particular phases of SAM during mid-spring and early summer. That wasn’t the case in late 2019 when ENSO was neutral—neither El Niño or La Niña. El Niño tends to favour negative SAM, while La Niña tends to favour positive SAM. Negative SAM events in summer generally reinforce El Niño’s drying influence over eastern Australia, while positive SAM events in summer generally reinforce La Niña’s wetting influence.

April to October rainfall across southeastern and southwestern Australia has declined in recent decades. The drying trend is linked with a trend towards more positive SAM (Figure 4); a known response to global warming.

Figure 4 Seasonal values of the observation-based SAM index. The smooth black curve shows decadal variations. Source: The Climate Data Guide: Marshall Southern Annular Mode (SAM) Index (Station-based).

Like our other climate drivers, SAM can influence weather patterns over parts of Australia. But unlike El Niño–Southern Oscillation and Indian Ocean Dipole events which persist for months at a time, SAM events generally only last for around ten days to two weeks. The Bureau of Meteorology’s Climate Outlooks provide guidance on rainfall and temperature patterns for the weeks, months and seasons ahead. The influence of SAM (and our other climate drivers) is explained in the Climate Outlooks summary information.