Southern Annular Mode Weather Cycle

A guide to the Southern Annular Mode, a regional weather system affecting southern Australia, Antarctica and the Southern Ocean marine ecosystem.
Drought-like conditions in SE Australia caused by a positive phase of the Southern Annular Mode
Drought-like conditions in New South Wales, influenced by a negative Southern Annular Mode. Photo: VirtualSteve © CC 3.0

The Southern Annular Mode (SAM), also known as the Antarctic oscillation (AAO), is a variable weather system in the subtropical southern hemisphere.

SAM is closely associated with a band of strong westerly winds – known as the “Roaring Forties” and the “Furious Fifties” – that encircle the globe between 40 and 50 degrees south of the equator. Although these winds follow a regular latitudinal orbit, they can shift their orbit northwards towards the equator, or southwards towards the pole. These shifts trigger different weather patterns along their route. (See also: What’s the Difference Between Climate and Weather?)

The meteorological effects of the Southern Annular Mode are felt most strongly in Australia and Antarctica, although they also impact on New Zealand and parts of South America.

SAM also drives the Antarctic Circumpolar Current (ACC) – the largest ocean current in the world – as it flows clockwise around the continent of Antarctica. The ACC keeps warmer ocean waters (from the Pacific, Indian and Atlantic Oceans) away from the polar continent, helping to maintain the huge Antarctic Ice Sheet which is the keystone of Earth’s climate system in the southern hemisphere.

SAM is just one of several important regional weather cycles active in the southern hemisphere. Others include the El Niño-Southern Oscillation (ENSO), the Indian Ocean Dipole (IOD) and the travelling Madden-Julian Oscillation (MJO) in the Indo-Pacific.

Map of Subtropical Ridge lying across Australia during the southern hemisphere winter.
Image Credit: Bureau of Meteorology. Australia. (CC BY-SA 3.0)

How Does the Southern Annular Mode Work?

SAM’s westerly winds vary their orbit according to the season. This is because they are linked to a band of high pressure known as the subtropical ridge which moves south in the summer and north in the winter. In addition, during each of these two periods, SAM has three phases: neutral, positive and negative, during which the winds also shift their position. 1

SAM Over Australia in Winter (May to October)

During the cooler half of the year, a neutral/normal Southern Annular Mode occurs when the belt of westerly winds blows over the southern edge of Australia. This allows low pressure and cold fronts to move up from Antarctica bringing normal winter rains to the southern areas of the country.

Effects of the Southern Annular Mode weather cycle in winter.
Effects of the Southern Annular Mode weather cycle in winter. During a positive SAM phase in winter, westerlies shift further south than usual, bringing more rainfall to eastern Australia but leaving the south much drier. In a negative SAM phase in winter, the winds shift further north than usual, and these effects are reversed. Image Credit: Bureau of Meteorology. Australia. (CC BY-SA 3.0)

A positive SAM phase occurs when winds shift further south than usual. This causes the rain-bearing weather fronts to follow suit. This leads to the formation of higher pressure over southern Australia, which means less cloud and rain and more sunshine, with many areas receiving below average rainfall. However, a positive SAM can also trigger extra rainfall on the east coast, with a risk of floods, as its withdrawal to the south allows more tropical water vapor to enter Australia’s eastern airspace from the Pacific.

Conversely, during a negative SAM phase, southern Australia receives more cold fronts and typically more rainfall, or snowfall on the alps. But if the system moves too far north the westerlies blow right across the continent, so by the time the moist air reaches the east coast, it’s completely dry.

In this connection, it’s worth noting that the south east corner of Australia – by far the most populated and industrialized part of the country – relies heavily on westerly winds as its main source of rain.

Map of Subtropical Ridge, which lies south of Australia during the Summer
Image Credit: Bureau of Meteorology. Australia. (CC BY-SA 3.0)

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SAM Over Australia in Summer (November to April)

During the warmer half of the year the subtropical ridge and the belt of westerlies are normally well to the south of the mainland. When this is the case, a neutral SAM has little influence on Australian rainfall.

A positive SAM phase in summer, is in play when the westerly winds shift even further south than normal. This allows even more tropical weather to move into eastern areas from the Pacific bringing higher than normal rainfall over parts of Queensland, NSW, Victoria and Tasmania.

Conversely, during a negative SAM phase, the winds shift north pushing dry air from the Australian interior into eastern areas. This leads to drier conditions with an increased risk of drought and bushfires.

Effects of the Southern Annular Mode weather cycle in summer.
Effects of the Southern Annular Mode weather cycle in summer. During a positive SAM phase in summer, westerly winds move further south allowing a moist onshore air flow across a large part of eastern Australia. In a negative SAM phase, eastern Australia tends to experience more dry airflow from the continental interior. Image Credit: Bureau of Meteorology. Australia. (CC BY-SA 3.0)

How Long Does a SAM Phase Last?

SAM’s positive or negative phases last for about 7-14 days, although their duration can be longer. The time lapse between positive and negative events can be anything between a week to a few months.

But over the past 40 years or so, a positive SAM has predominated during the cooler half of the year, and has been a major contributor to the prolonged period of dry weather in southern Australia during the period 1997-2010. 2

Positive and Negative SAMs Over New Zealand

In New Zealand, a positive Southern Annular Mode is linked to higher than normal air pressure, resulting in light winds and settled weather conditions. Meantime, wind and storm activity typically increases in the Southern Ocean.

A negative SAM usually results in lower than normal air pressure, with more unsettled weather conditions. The weather is calmer in the Southern Ocean, with higher pressure and slacker than normal winds.

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Positive and Negative SAMs Over Antarctica

In the Antarctic, a negative Southern Annular Mode occurs when the belt of westerlies move northwards towards the equator. The disappearance of these winds leads to reduced CDW upwelling on the continental shelf.

During a positive SAM, the winds strengthen and move closer to the continent. As a result, increased circumpolar deep water (CDW) upwells onto the continental shelf. Because it’s warmer than near-surface water, it leads to the melting of glaciers and their ice shelves, especially in West Antarctica.

WORTH KNOWING: What is Thermohaline Circulation?

A positive SAM also results in higher phytoplankton productivity in the seasonal ice zone (SIZ) of the Southern Ocean, due to greater upwelling of nutrient-rich water, changes in ocean mixing, and sea ice characteristics. This has beneficial ramifications for the marine food web – notably the availability of krill for higher trophic levels. 3

These microbial changes also increase the efficiency of the ocean’s biological pump – see How Do Oceans Influence Climate Change? – as fatter phytoplankton sink more rapidly than regular phytoplankton, while increased feeding by krill converts phytoplankton biomass into feces that sink more rapidly into the depths. All this acts as a negative climate feedback for global warming.

Yes. The Southern Annular Mode often correlates with ENSO events. In El Niños, for instance, SAM tends to be more negative and, in La Niña, it is more positive. This can boost ENSO impacts, notably in summer. It can also reinforce a positive phase of the Indian Ocean Dipole. The Australian Bushfires of 2019 were the result of a combination of an El Niño, a positive Indian Ocean Dipole (IOD); and a negative Southern Annular Mode (SAM). 4

What Causes the Southern Annular Mode? Is it Climate Change?

Scientists still don’t understand exactly why SAM enters a positive or negative phase. However, according to climate models, there may be a correlation between global warming and the recent predominance of a positive SAM.

According to ice cores analyzed by scientists, the Southern Annular Mode’s westerly winds are further south than at any time over the past 1,000 years or more. This trend is attributed to increasing levels of greenhouse gases and, more recently, a reduction in the ozone level in the stratosphere. 5

Ozone is a greenhouse gas, so its depletion (the ozone hole) has kept Antarctica cooler. This has pulled the band of westerly winds that circles the continent closer to Antarctica, which has increased the speed of the wind, making Antarctica even cooler and drier. It has also pulled other weather systems, like the Southern Annular Mode, closer to Antarctica – hence the predominant positive SAM mode. 6

In the absence of a global climate change mitigation plan, continuing increases in CO2 emissions are expected to cause a further positive increase in the Southern Annular Mode in all seasons. 7 This is despite the anticipated full recovery of ozone concentrations by around 2065. 8

References

  1. “Southern Annular Mode and the Australian climate.” []
  2. “Bureau of Meteorology.” []
  3. “The Southern Annular Mode (SAM) influences phytoplankton communities in the seasonal ice zone of the Southern Ocean.” Greaves, B. L. et al. Biogeosciences, 17, 3815–3835 []
  4. “What role did climate change play in the Australian megafires?” Paul Read. Monash University. []
  5. “Evolution of the Southern Annular Mode during the past millennium. ” Abram, N., Mulvaney, R., Vimeux, F. et al. Nature Clim Change 4, 564–569 (2014). []
  6. “Ozone depletion, ultraviolet radiation, climate change and prospects for a sustainable future.” Nature Sustainability. []
  7. “Annular mode changes in the CMIP5 simulations.” Gillett, N. P. and Fyfe, J. C.: Geophys. Res. Lett., 40, 1189–1193, 2013. []
  8. “Emergence of healing in the Antarctic ozone layer.” Solomon, S. et al. Science, 353, 269–274. 2016. []
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