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= Southern Annular Mode = The Southern Annular Mode (SAM), also known as the Antarctic Oscillation, is a quasi-regular oscillation in atmospheric pressures observed over Antarctica and the southern mid-latitudes. It is the primary mode of climate variability in the southern hemisphere, influencing atmospheric circulation between 40oS latitude and the South Pole.

The SAM has two phases, positive and negative. The positive phase is associated with a contraction and intensification of the Southern Hemisphere westerlies towards Antarctica. The negative phase is associated with an expansion and weakening of the Southern Hemisphere westerlies towards the southern mid-latitudes. Changes in atmospheric circulation caused by the SAM result in changes to oceanic circulation, sea-ice dynamics, ecological systems, and southern-hemisphere socioeconomic systems.

The SAM was first identified by Roger and Loon (1982), coining it as the Antarctic Oscillation (AAO). It was originally named the Antarctic Oscillation as a counterpart to the Arctic Oscillation. In contemporary research, the AAO has fallen out of use in favor of the SAM. While both terms are valid, meteorological agencies in regions impacted by this atmospheric mode, such as the Australian Bureau of Meteorology, use the Southern Annular Mode. Agencies outside of these regions, such as the United States' National Oceanic and Atmospheric Administration, utilize both the Southern Annular Mode and Antarctic Oscillation.

Numerical indices
The SAM represents the exchange of atmospheric mass between the mid-latitudes and Antarctica via changes in zonal sea-level pressure. The SAM appears as the first Empirical Orthogonal Function (EOF) of monthly-mean sea-level pressure between 40° S and 65° S. It accounts for an average of 22% of annual sea-level pressure variance in this region. A numerical definition of the SAM is calculated as the difference between normalized zonal mean sea-level pressure between 40° S and 65° S. Several other indices have since been developed to better characterize regional impacts. They differ in data sources, comparison regions, time period of analysis, and analysis method.

Positive
During a positive phase of the SAM, there are anomalously low pressures around Antarctica and anomalously high pressures at southern mid-latitudes. This intensifies and contracts the westerly wind belt that drives the Antarctic Circumpolar Current toward Antarctica as a result of anomalously-low pressure over the South Pole and anomalously-high pressure over the Southern Ocean. A positive phase of the SAM is associated with an increase in rainfall (including East coast lows) and reduced heat in southeastern Australia in the summer. This is due to higher onshore flows from the Pacific Ocean and, in winter, decreased snow in the alpine areas and rainfall in the far south and southwest. Recent decades show a trend toward increased positive SAM index values, thought to be driven by stratospheric ozone depletion. Subsequent recovery of stratospheric ozone supported by the Montreal Protocol has limited the growth of the SAM, limiting the protective effect of a strong SAM on Antarctic sea-ice.

Negative
During a negative phase of the SAM, there are anomalously high pressures around Antarctica and anomalously low pressures at southern mid-latitudes. This leads to a weakening of the southern hemisphere westerlies around Antarctica, but an expansion of the westerlies towards the mid-latitudes. In its negative phase, geopotential height anomalies over the South Pole and Southern Ocean are reversed from the positive phase, and the westerly wind belt is weakened and expands toward the equator. A negative phase of the SAM is associated with decreasing rainfall in the southeast of Australia in the summer as well as increasing the possibility of spring heatwaves. Moreover, winters will usually be wetter than normal in the south and southwest with more snowfall in the alpine areas, but drier in the east coast due to less moist onshore flows from the east.

Atmospheric interactions
The SAM has a pronounced effect on atmospheric circulation patterns in the Southern Hemisphere, with the largest impact occurring on intra-seasonal and inter-annual timescales. It affects many components of the coupled air-sea system of the Southern Hemisphere, including sea surface temperature, oceanic circulation patterns, and synoptic-scale weather patterns. The positive mode of the SAM is associated with cooling temperatures over Antarctica and a large portion of Australia, warming over Argentina and Tasmania, anomalously dry conditions in South America and parts of Tasmania, and anomalously wet conditions over Australia and South Africa.

Precipitation in South Africa is affected by the SAM shifting the subtropical jet meridionally: during the positive phase of SAM, the jet is shifted toward the poles, storms are weakened at mid-latitudes, and South Africa has dry winters; during the negative phase of SAM, the jet is shifted toward the equator, storms are strengthened at mid-latitudes, and South Africa has wet winters. Additional regional atmospheric conditions correlated with the positive phase include strengthening of the storm track over the Southern Ocean, colder surface temperatures over much of Antarctica, warm and dry conditions over New Zealand, Tasmania, and South Australia, and cool and wet conditions over Eastern Australia. Globally, the positive phase of the SAM is linked to increased tropical cyclone activity in the Western North Pacific. Propagation of the Madden-Julian Oscillation is also associated with the SAM, which results in a warming of the Indian Ocean and anomalous rainfall patterns across the monsoon tracks.

The SAM also affects precipitation in southeast South America, a region that includes extremely productive agricultural zones and one of the largest hydrological basins in the world, the Río de la Plata. During positive modes of the SAM in the spring, there is decreased precipitation due to weakened atmospheric moisture convergence and intensified anti-cyclonic anomalies; during negative modes in the spring, precipitation is increased because of strengthened atmospheric moisture convergence and increased cyclonic anomalies.

Interactions between the Southern Annular Mode and the El Niño Southern Oscillation
One of the large-scale atmospheric impacts of the SAM is through its connection to the El Niño-Southern Oscillation (ENSO). When an El Niño phase of ENSO occurs in conjunction with a negative phase of SAM, the two phenomena amplify one another through momentum flux interactions which increases anomalous atmospheric circulation in mid-latitudes. Similar amplification of anomalous mid-latitude atmospheric circulation occurs when a La Niña phase of ENSO occurs concurrently with the positive phase of SAM. In contrast, momentum flux interactions oppose and weaken one another during an El Niño phase and a positive SAM phase, and during a La Niña phase and a negative SAM phase, which decreases circulation anomalies at mid-latitudes. Model analysis of warming sea surface temperatures and SAM has shown that, as sea surface temperatures rise, El Niño is weakened, and the connection between the SAM and El Niño is also weakened.

During an El Niño event, a chain of atmospheric and ocean teleconnections leads to the modulation of the Antarctic Dipole. As an El Niño event develops, tropical Pacific-sourced Pacific Rossby waves increase pressures over the Amundsen Sea. Contrasting sea-surface temperature anomalies in the Pacific and Atlantic strengthen the Hadley cell in the South Pacific, while weakening it in the Atlantic. This leads to a shift in the subtropical-jet stream, strengthening the Ferrel cell in the Pacific and weakening it in the Atlantic. These changes in circulation lead to increased poleward heat transport in the Pacific and weakening poleward heat transport in the Atlantic.

Oceanic interactions
Oceanic conditions associated with the SAM include changes to coastal upwelling along the Antarctic perimeter and changes to the depth of the mixed layer at the northern boundary of the Antarctic Circumpolar Current. Subantarctic Mode Water (SAMW) forms in the Southern Ocean and its formation and properties are correlated with SAM and ENSO. The thickness of the central pool is strongly negatively correlated with SAM and ENSO, while the thickness of the eastern pool is strongly positively correlated with SAM and ENSO. The positive phase of the SAM is associated with an increase in the mixed-layer depth (MLD) in the eastern SAMW formation area due to increased surface cooling from the enhanced westerly wind speeds; this effect can persist for 6 months or longer if it is a particularly strong positive phase.

Southern Ocean uptake and outgassing of CO2
Positive phases of the SAM are associated with increased coastal upwelling due to the tightening and strengthening of the westerly winds around Antarctica. Deep water is upwelled due to surface waters being forced equatorward via Ekman Transport caused by the enhanced westerly winds which have moved closer to the Antarctic coast. This increased upwelling has been linked to accelerated Totten Ice Shelf melting and increased outgassing of carbon dioxide gas to the atmosphere caused by increased partial pressure of carbon dioxide from the upwelled carbon-rich deep water. The Southern Ocean typically acts as a carbon sink of atmospheric carbon dioxide, and its role in outgassing during periods of increased upwelling is an area of ongoing research. The enhanced upwelling brings nutrients including iron, a limiting micronutrient for primary productivity, to the surface, resulting in increased production in surface waters during SAM-positive periods which occur during the austral summer. This enhanced production somewhat mitigates the outgassing effect of the increased upwelling. Alternatively, a SAM-positive period occurring during the austral winter will result in no additional primary productivity, and the upwelled water equilibrates by releasing carbon dioxide to the atmosphere. With higher concentrations of atmospheric carbon dioxide in the future, SAM-positive events may be associated with greater uptake of carbon dioxide by the Southern Ocean. In contrast to the positive phase, the negative phase of the SAM is associated with reduced Antarctic coastal upwelling and a shoaling of mixed layer depth from reduced westerly strength. Deep water eddies are an area of active research because of their compensating effect on enhanced outgassing through redistribution of carbon-rich water beneath the mixed layer depth.

Sea ice interactions
Changes in the amount of sea-ice around Antarctica reflect complex and dynamic interactions between oceanic and atmospheric processes. For example, meridional (northward and southward) winds can bring warm, tropical air poleward, or cold, polar air equatorward. Increases in cold equatorward air will enhance sea-ice production. Additionally, the northward direction of flow will enhance the equatorward transport of ice, increasing sea-ice extent. Changes in coastal upwelling or downwelling drives a fast-response in sea-surface temperatures around Antarctica, modifying sea-ice formation conditions through oceanic forcing.

Regional
Observations of sea-ice extent indicate different trends in the seas around Antarctica. These different seas are referred to as Antarctic sectors, providing geographical boundaries for comparison. Observational records indicate that some sectors have increases in sea-ice, while some are showing decline. Recent studies have investigated the causes of the differing trends in sea-ice extent for each sector. The timing of retreat and advance is also different for each region, although austral summer typically shows the greatest change. When averaged across the whole of Antarctica, this shows a general expansion in sea-ice extent. These different signals are the result of interactions between equatorial tropical climate-modes, Southern Hemisphere climate modes, and the coupled oceanic-atmospheric system for a given region. The SAM is often investigated as a driver for these changes, as it represents the average atmospheric circulation around Antarctica.

Observations and modelling studies suggest that the SAM influences Antarctic sea-ice in a zonally asymmetric fashion. Positive SAM years have statistically significant increases in sea-ice and decreases in sea-surface temperature anomalies in the Ross and Amundsen Seas, with significant decreases in sea-ice and increases in sea-surface temperatures in the Weddell Sea sector. This asymmetric response in sea-ice extent and sea-surface temperature is referred to as the Antarctic Dipole (ADP). The ADP is generated through circulation anomalies produced by the Amundsen Sea Low (ASL). During positive SAM events, the ASL is deepened. This leads to enhanced cyclonic circulation in the Amundsen-Bellingshausen Sea region, with a strong southerly component in the Ross Sea and corresponding northerly component along the western coast of the Antarctic Peninsula. These components lead to cold air advection in the Ross Sea creating positive sea ice anomalies. Along the western coast of the Antarctic Peninsula, this enhanced southward atmospheric flow leads to warm air advection, reducing sea-ice extent and increasing surface temperatures.

Seasonal
The season in which a SAM event occurs is an important factor in determining the Antarctic sea-ice response. For a positive austral summertime SAM event, the southern hemisphere westerlies will be strengthened and located closer to the Antarctic Coast. This circulation change brings cooler waters to the surface around the coast and transports them equatorward, resulting in a cooling around the coast and warming farther north. These cooler waters around Antarctica will promote the growth of sea-ice in the following austral Autumn. The opposite pattern occurs for a negative summertime SAM event, in which reduced transport leads to a local warming. This warming then inhibits the growth of sea-ice in the following austral Autumn. The record low Antarctic sea-ice extent in 2017 coincided with a record negative SAM event, which can be attributed to this mechanism.

Impacts from anthropogenic climate change
Attributions of changes in sea-ice extent around Antarctica to anthropogenic climate change is an ongoing area of research. These studies are complicated by the relatively short observational periods in the region and the high amount of variability in Antarctic sea-ice. Paleoclimate studies suggest that the SAM is in its most positive phase as compared to the past 1000 years. Studies of sea-ice extent from ice cores can also attribute changes in sea-ice extent in the Amundsen/Bellingshausen Sea and the Ross Sea to changes in atmospheric circulation around Antarctica during the Holocene. While these studies indicate that contemporary changes in sea-ice are anomalous, they are not outside of the natural variability of the observational record.

An additional hurdle to attribution is the differing response of the sea-ice system in observations and climate models to a more positive SAM. Climate model s indicate that a more positive SAM should increase sea-surface temperatures around Antarctica, inhibiting the growth of sea-ice. With the general trend towards a more positive SAM state due to stratospheric ozone depletion, this should result in decreased sea-ice extent around Antarctica. However, observations indicate an expansion of sea-ice and cooling of surface ocean waters. The mechanisms controlling these different responses is currently debated.

Recent research has hypothesized a two-timescale response of sea-surface temperatures to changes in atmospheric circulation. On interannual timescales, the ‘fast’ response will drive surface cooling around Antarctica by the advection of cold surface water towards the equator. On decadal timescales, the ‘slow’ response will bring warmer deep waters to the surface via Ekman pumping and increased ocean surface mixing. This mechanisms suggests that the sea-ice and surface ocean response to a more positive SAM is a short-term cooling and sea-ice expansion, followed by widespread warming.

SAM Reconstructions
In 2014, Nerilie Abram used a network of temperature-sensitive ice core and tree growth records to reconstruct a 1000-year history of the Southern Annular Mode. This work suggests that the SAM is currently in its most extreme positive phase over at least the last 1000 years, and that recent positive trends in the SAM are attributed to increasing greenhouse gas levels and later stratospheric ozone depletion.

Recent research has suggested that atmospheric controls on modern SAM records are not applicable to paleo-reconstructions. However, reconstructions indicate that the recent increase in the SAM is anomalous compared to the past 500 years. This is in agreement with previous reconstructions, suggesting changes in the SAM driven by greenhouse gases and changes in solar irradiance.

CO2 Ventilation during Ice Ages
It is hypothesized that during the last ice age, the southern hemisphere westerlies were in a more equatorward position. This allows for the accumulation of carbon dioxide in the Southern Ocean, accounting for the reduction in atmospheric carbon dioxide observed during glacial conditions. While this mechanism over millennial timescales is not attributable to the SAM, it is analogous to observed variability in carbon dioxide ventilation and uptake on interannual timescales (see the Oceanic Interactions section).

With more equatorward westerlies, upwelling around the Antarctic coast is limited. Additionally, reduced wind stress leads to increased stratification of the water column. This reduction in upwelling and increased stratification leads to an increase in the residence time of deep waters, creating a sink of atmospheric carbon dioxide. As global climate transitioned to an interglacial climate, the westerlies contracted around Antarctica. The contraction of the westerlies drives increased upwelling and oceanic mixing, allowing for the outgassing of carbon dioxide.

Additional studies have investigated the validity of this mechanism. Proxy and model comparison projects highlight disagreement between observations and reconstructions of the strength and position of the westerlies during glacial conditions. Additional studies suggest a need for more advanced biogeochemical and sea ice representations within models. Recent work hypothesizes that this mechanism is controlled by the Earth’s radiative budget through orbital cycles, as described in Milankovitch Theory.

Ecological impacts
The varying atmospheric modes of climate variability, including the SAM, have important implications for abundance, distribution, and catch of marine life in the Southern Ocean. Intensified positive values of SAM are linked with enhanced westerly winds in the Southern Ocean and consequently warmer sea surface temperatures, impacting the variability of biological productivity and keystone species biomass. Thus, the phases of SAM have magnified impacts throughout the Antarctic food web.

Food web interactions
Spatial variability of several environmental properties, including photosynthetically active radiation (PAR), mixed layer depth (MLD) and the extent of nutrient-rich Circumpolar Deep Water (CDW), have been significantly linked to variations in the SAM in the northern Antarctic Peninsula (NAP) region. A SAM positive phase has been linked to increased intensity of visible light reaching surface waters, increased CDW inputs, and enhanced vertical mixing, all which support increased levels of primary productivity.

The western Antarctic Peninsula (WAP) region shows similar relationships between environmental properties and changes in SAM phase, specifically related to temporal variations in sea-ice advance and retreat. For the WAP, there are conflicting studies on the impact of negative or positive SAM phases to phytoplankton abundance and spatial variability, which may be due to time-scale differences (i.e. decadal vs. yearly) in those studies. In several multi-decadal studies, elongated sea-ice coverage is linked to increased water column stability in the WAP, providing overall positive conditions for phytoplankton. Effects of global warming, such as warming waters, exacerbate positive SAM values and cause further reduced sea-ice coverage in the region, consequently reducing phytoplankton biomass and negatively impacting apex predators further up the food web. Certain phytoplankton species are shown to be highly sensitive to abrupt changes in ice conditions and temperature fluctuations during the winter months, indicating highly variable spatial distribution during the SAM negative phase. In general, early or late sea-ice retreat affects the timing of phytoplankton blooms in the WAP region and overall phytoplankton biomass, introducing trickling effects to higher trophic levels in the food web.

In regards to planktonic populations in the WAP region, such as the keystone species Antarctic krill (Euphausia superba), negative SAM values have been shown to compound a decrease in population size and distribution over time, especially when combined with increased krill fishery catch rates. Krill fishery stocks in the Southern Ocean have declined dramatically in recent years due to climate change-induced intensification of the negative SAM phase, reducing the ice coverage impact E. superba distribution, size, and recruitment.

Further along the food web, organisms such as whales (Southern right whale, Eubalaena australis) and seabirds (Adélie penguin, Pygoscelis adeliae, chinstrap penguin, P. antarcticus, and gentoo penguin, P. papua) rely on krill for food or reproductive timing and are affected by decreases in krill population and distribution. Variation in krill population size and distribution has been shown to negatively impact penguin growth rates and population size, although environmental variability may have a greater impact to both krill and penguin populations. Abundant krill populations have been correlated with increased observations of Southern right whale calves as they provide the nutrients required for whales to maintain the energy reserves for reproduction. Therefore, a decrease in krill population causes reproductive suppression due to nutritional stress of the whales.

Socio-economic impacts
The phases of SAM can have severe implications to humans living in the southern hemisphere, specifically those affected by variations in rainfall patterns, fires, and impacts to the krill fishery.

Human interactions
Due to alterations in rainfall patterns that are linked to the SAM, human populations in the southern hemisphere are exposed to increased rain in some areas and droughts in others. Additionally, there is a strong correlation between fire activity in Australia and positive SAM phases. Anthropogenic climate change is likely to increase the severity of these events that are tied to the phases of SAM. Changes in monsoon intensity in India and the South China Sea have been linked to increased risk to populations in those areas. Australian wildfires are becoming additionally severe, specially with the massive fires that ravaged the area during the 2019-2020 bush fire season. Important fisheries can also be disrupted by SAM fluctuations, which ultimately affects local economies and diets.

Variation in rainfall patterns is likely to become more pronounced under future climate projections, and differences between summer and winter rainfall totals are projected to increase. Climate change projections, particularly the Representative Concentration Pathway 8.5 (RCP8.5), show drastic increases in severe rainfall totals in the southern hemisphere. Different locations experience the rainfall from SAM in various ways. Rainfall has been shown to increase in South Africa, southern Brazil, and Australia during the summer months that correlate with a positive SAM, while in winter, the rainfall continues for eastern Australia and southern Brazil. East Africa short rains have also been linked to the SAM. These short rain seasons are crucial to the agriculture of the region and can result in severe flooding events. Rainfall totals in some areas of the world are tied to unique rainy seasons, especially monsoon events. Monsoons are affected by the phases of the SAM; a positive phase has been shown to increase the intensity of monsoons in India. These monsoons are a common occurrence in the region during the rainy season, but increasing severity of the SAM is projected to not only increase the intensity but the frequency of the storms. Locations around the world experience the phases of the SAM differently, but all affect the local human populations and agriculture.

In addition to increased rainfall, droughts and fire-favorable conditions are linked to the SAM phases. Fires in the Patagonia have been linked to variations in the SAM, and decadal time scales, fires in the Tasmania region of Australia are closely linked to positive phase of the SAM. Australian bush fires are part of the ecosystem but there is a trend for increasing severity of these events. The North Atlantic Oscillation similarly affects ecosystems in the northern hemisphere in regards to fire activity. Climate change is likely to increase the severity of these events all around the world.

Fisheries
There has been increasing interest in the Antarctic region as a new fisheries source, and Japan, Chile, Russia, Poland, and Korea are harvesting krill in the Antarctic. This fishery is closely tied to sea-ice and climate change. The krill fishery can be used as a food source and have been shown to be useful nutritional supplements. With decreasing sea-ice, the region is becoming easier to traverse and thus allowing more boats to go in and harvest the krill. With the projected increase in fishing, there is an increasing pressure to establish stock assessments and fishery regulations in the region. This has lead scientists to advocate for a review of the fishery and implications that harvesting could have on the amount of haul and larger ecological impacts for the Antarctic region.