Southern Annular Mode

The Southern Annular Mode (abbreviated SAM, and also known as the Antarctic Oscillation, AAO) is the principal mode of extratropical atmospheric circulation variability in the Southern Hemisphere, accounting for ~25% to 35% of the monthly variance in sea-level pressure and geopotential height from the surface into the lower stratosphere. Because the Southern Hemisphere between 40°S and 65°S is an almost uninterrupted oceanic channel (the Southern Ocean, broken only by the southern tip of South America and the Antarctic Peninsula), the SAM exhibits far greater zonal symmetry than its Northern Hemisphere counterpart, the North Atlantic Oscillation / Northern Annular Mode. Physically, the SAM represents a ring-like meridional redistribution of atmospheric mass between the mid-latitude subtropical high-pressure belt (~40°S–45°S) and the subpolar circum-Antarctic low-pressure trough (~60°S–65°S), directly governing the strength and latitudinal position of the Southern Hemisphere eddy-driven westerly wind belt (the Roaring Forties and Furious Fifties), Southern Ocean Ekman transport upwelling, Antarctic ice-shelf basal melting, and global oceanic CO₂ uptake.

Climatologists quantify the phase and amplitude of the Southern Annular Mode using two primary indices:

1. Zonal Pressure Difference Index (Gong & Wang, 1999; Marshall, 2003): Defined as the difference between normalized zonal-mean sea-level pressure (P^*) at 40°S and 65°S:

SAM Index = P^*_norm(40°S) − P^*_norm(65°S)

Because gridded meteorological reanalyses (such as NCEP/NCAR) exhibited spurious high-latitude pressure biases prior to the satellite era (pre-1979), British Antarctic Survey climatologist Gareth J. Marshall (2003) constructed the standard Marshall SAM Index using 12 continuous coastal and island surface weather stations (6 near 40°S in New Zealand, Australia, South America, and the southern Indian/Atlantic oceans, and 6 coastal Antarctic stations near 65°S) extending reliably back to 1957 (the International Geophysical Year).

1. Leading Empirical Orthogonal Function (EOF 1): Defined by David W. J. Thompson and John M. Wallace (2000) as the first principal component of monthly 850-hPa or 700-hPa geopotential height anomalies south of 20°S.

Variations in the SAM shift the Southern Hemisphere eddy-driven westerly jet stream and its associated baroclinic storm track north and south by 2° to 5° of latitude, producing contrasting weather and oceanographic regimes across Antarctica, South America, Australia, New Zealand, and South Africa:

Through wind-stress curl and Ekman dynamics across the Drake Passage latitude band (55°S–63°S, where no meridional continental barriers exist to support a western boundary current), the SAM directly drives the Southern Ocean Overturning Circulation and modulates both global sea-level rise and the oceanic carbon cycle:

1. Ekman Transport and Circumpolar Deep Water (CDW) Upwelling: Because the Coriolis parameter f is negative in the Southern Hemisphere, eastward westerly winds blowing around Antarctica drive a northward surface Ekman transport (M_E = τ × k̂ / f) away from the Antarctic continent (forming the northward-flowing surface Deacon Cell). By mass conservation, divergence of surface water south of the westerly wind maximum (~60°S) pulls old, warm (+1.0 to +2.0 °C above the in situ freezing point), nutrient- and carbon-rich Circumpolar Deep Water (CDW) upward from 1,500–3,000 meters depth along steeply tilted isopycnals to the surface around Antarctica.
2. West Antarctic Ice Sheet Basal Melting: During a positive SAM+ state, poleward-intensified westerlies pump this warm Circumpolar Deep Water up onto the continental shelf of the Amundsen and Bellingshausen Seas. Once on the shelf, warm CDW flows down submarine glacial troughs directly into the grounding-line cavities beneath Thwaites Glacier and Pine Island Glacier, accelerating basal ice-shelf melting (reaching 30–100 m yr⁻¹) and triggering Marine Ice Sheet Instability.
3. Outgassing of Natural Pre-Industrial CO₂ from the Southern Ocean: The Southern Ocean accounts for ~40% of total global oceanic uptake of anthropogenic CO₂. However, because ancient Circumpolar Deep Water has been isolated from the atmosphere for centuries, it is supersaturated in dissolved inorganic carbon (DIC) from remineralized organic matter. Corinne Le Quéré, Nicolas Gruber, and Nicole S. Lovenduski (2007) demonstrated that when the SAM shifts into a strong positive SAM+ phase, enhanced Ekman upwelling brings DIC-rich Circumpolar Deep Water to the surface, increasing natural CO₂ outgassing and temporarily weakening the net Southern Ocean carbon sink by 0.1 to 0.3 Pg C yr⁻¹.

Since the late 1960s, the Marshall SAM index has exhibited a statistically robust positive upward trend—particularly during austral summer (December–February, DJF)—bringing the summer westerlies to their most poleward, positive state in at least the last 1,000 years (confirmed by South American and New Zealand tree-ring and ice-core reconstructions by Nerilie J. Abram et al., 2014). Atmospheric general circulation modeling attributes this multi-decade shift to two synergistic anthropogenic forcings:

• Antarctic Stratospheric Ozone Depletion ("The Ozone Hole", 1979–2000): During austral spring (September–November), catalytic destruction of lower-stratospheric ozone (O₃) by chlorofluorocarbons (CFCs) eliminates ultraviolet shortwave solar heating over Antarctica, cooling the lower polar stratosphere by −6 to −10 K. By thermal-wind balance, this enhanced meridional temperature gradient strengthens the stratospheric polar night jet; over 4 to 8 weeks, this positive zonal-wind anomaly propagates downward to the surface during December–February, driving roughly 60% to 70% of the observed austral summer SAM+ trend over 1979–2000 (Thompson & Solomon, 2002).
• Rising Greenhouse Gas (CO₂) Concentrations: Increasing atmospheric CO₂ warms the tropical and subtropical upper troposphere via moist adiabatic lapse-rate amplification while radiatively cooling the polar stratosphere. This deepens the meridional upper-level temperature gradient across all four seasons, pushing the eddy-driven jet poleward (SAM+). Since the Montreal Protocol stabilized and began healing Antarctic stratospheric ozone after ~2000 (Banerjee et al., 2020), ozone recovery and rising greenhouse gases have acted in opposing directions during austral summer—causing the summer SAM trend to pause between 2000 and 2020—whereas in autumn and winter (when the ozone hole is absent), rising greenhouse gases continue to drive the SAM toward positive SAM+ values.

In 2020, formal detection-and-attribution analyses of meteorological reanalyses and Chemistry-Climate Model Initiative simulations proved that the healing of the Antarctic stratospheric ozone layer under the Montreal Protocol has paused the multi-decade poleward shift of the austral summer Southern Annular Mode since the year 2000 (Banerjee et al., 2020).

In 2020, compilation of high-resolution Southern Hemisphere paleoclimate proxy networks demonstrated that the late-twentieth-century positive excursion of the Southern Annular Mode exceeded the range of natural centennial variability across the past millennium and drove synchronous hydroclimatic drying across Patagonia and western Tasmania (Fogt & Marshall, 2020).

In 2023, coupled ocean–sea-ice analyses of the record-low austral summer 2022–2023 Antarctic sea-ice minimum showed that persistent positive Southern Annular Mode wind anomalies and Amundsen Sea Low deepening accelerated poleward Ekman heat transport and subsurface Circumpolar Deep Water shoaling prior to the sea-ice retreat (Purich & Doddridge, 2023).

1. Thompson, D. W. J., & Wallace, J. M. (2000). Annular modes in the extratropical circulation. Part I: Month-to-month variability013<1000:AMITEC>2.0.CO;2). Journal of Climate, 13(5), 1000–1016.
2. Thompson, D. W. J., & Solomon, S. (2002). Interpretation of recent Southern Hemisphere climate change. Science, 296(5569), 895–899.
3. Marshall, G. J. (2003). Trends in the Southern Annular Mode from observations and reanalyses016<4134:TITSAM>2.0.CO;2). Journal of Climate, 16(24), 4134–4143.
4. Lovenduski, N. S., Gruber, N., Doney, S. C., & Lima, I. D. (2007). Enhanced CO₂ outgassing in the Southern Ocean from a positive phase of the Southern Annular Mode. Global Biogeochemical Cycles, 21(2), GB2026.
5. Banerjee, A., Fyfe, J. C., Polvani, L. M., Waugh, D., & Chang, K.-L. (2020). A pause in Southern Hemisphere circulation trends due to the Montreal Protocol. Nature, 579(7800), 544–548.
6. Fogt, R. L., & Marshall, G. J. (2020). The Southern Annular Mode: Variability, trends, and climate impacts across the Southern Hemisphere. WIREs Climate Change, 11(4), e652.
7. Purich, A., & Doddridge, E. W. (2023). Record low Antarctic sea ice coverage indicates a new sea ice state. Communications Earth & Environment, 4(1), 314.