Southern Annular Mode

Today, glaciers in Patagonia and Antarctica are receding. In both cases, this has been attributed to changes in the Southern Westerly Winds and the Southern Annular Mode (SAM). The Southern Westerly Winds are an important wind belt that encircles the globe in the southern mid-latitudes1,2, with its core between 50-55°S1.

The Southern Annular Mode describes the north-south movement of this wind belt over a timescale of decades to centuries. The Southern Annular Mode is a key climatic component that will strongly affect how glaciers in the Southern Hemisphere respond to climate change. It explains the key drivers for glaciation in the Southern Hemisphere, and why glacier advances are asynchronous with those in the Northern Hemisphere3,4.

The Southern Annular Mode is currently in a positive phase5, and this is projected to continue due to increased greenhouse gas emissions. A positive phase of the Southern Annular Mode will continue to drive changes in the Southern Westerly Winds, causing warming and drying over Patagonia, and increased upwelling of warm Circumpolar Deep Water and glacier recession in western Antarctica and the Antarctic Peninsula.

The Southern Ocean climate system

The Southern Westerly Winds and Southern Ocean are an important coupled climate system that controls climate in the southern third of the world.  This system is closely connected to CO2 degassing from the deep ocean, and the position of the Intertropical Convergence Zone1.

Westerly Winds and ocean fronts around Antarctica

The Westerly Winds and Ocean Currents

The Southern Westerly Winds drive the Antarctic Circumpolar Current6. This current encircles Antarctica. The northern boundary of the Antarctic Circumpolar Current is the Subantarctic Front (SAF), and the southerly boundary is the Polar Front (PF).

The Antarctic Circumpolar Current (ACC) is the World’s strongest current. It is able to flow unimpeded through the Drake Passage and around the continent of Antarctica. The winds over the channel and the flow of the ACC are aligned for the length of the channel. The ACC is getting stronger as the Westerly Winds have contracted poleward during the current postive phase of the Southern Annular Mode, as the winds are more directly aligned with the current26.

Simplified schematic map of ocean currents of the Southern Ocean.

Beneath the cooler surface waters is a warmer layer of water. This is Circumpolar Deep Water (CDW), centered at a depth of ~500 m. It is dense and warm, with a high salt content27.

The Antarctic continental shelf is around 600 m deep, and deepens as it heads inland (a ‘retrograde’, or negative, slope). The ability of Circumpolar Deep Water to access the continental shelf is determined by the height of the CDW in the water column and the local bathymetry. The higher in the water column the CDW sits, the more it will be able to cross the continental shelf27.

Strengthening of the regional westerly winds has increased the circulation of CDW onto the continental shelf, where it can reach ice-shelf cavities and grounding lines28. If more CDW can penetrate onto the continental shelf, it may be able to reach the base of ice shelves on the Antarctic continent, melting them and risking ice-shelf collapse and marine ice sheet instability.

The Southern Annular Mode

The belt of the Southern Westerly Winds moves north and south over timescales of 10s to 100s of years. The Southern Annular Mode (SAM) (also known as the Antarctic Oscillation; AO) describes the north-south movement of these winds.

The Southern Annular Mode is usually defined as the difference in the zonal mean sea level pressure at 40°S (mid-latitudes) and 65°S (Antarctica)7,8. Changes in air pressure distribution causes changes in the strength and position of the westerly winds7. The SAM-index is effectively a measure of the strength of the Southern Westerly Winds, and as it increases, the westerlies have been moving south and increasing27.

Positive Southern Annular Mode

In a Positive phase of the Southern Annular Mode, there is lower anomalous air pressure over Antarctica, and higher anomalous air pressure over the mid-latitudes7.

In a Positive Southern Annular Mode (the situation today), the belt of strong westerly winds strengthens and contracts towards Antarctica. It weakens at the northern boundary in the mid-latitudes (40-50°S). It is drier over Patagonia, driving glacier recession. In Antarctica, increased Circumpolar Deep Water upwells onto the continental shelf, driving glacier and ice sheet recession9.

Negative Southern Annular Mode

In a Negative Southern Annular Mode, the belt of strong Southern Westerly Winds expands northwards towards the equator, bringing cold, wet weather to Patagonia and glacier advance, and decreased Circumpolar Deep Water upwelling on the Antarctic Continental Shelf. The winds are weaker in this phase.

This was the situation during Holocene neoglaciations in Patagonia and during the Last Glacial Maximum4,10–12.

Positive and Negative phases of the Southern Annular Mode

Changes in the Southern Annular Mode

We can track the past behaviour of the Southern Annular Mode in numerous proxies such as ice cores8, lake sediment cores in Patagonia13 and glacier behaviour10. The Southern Westerly Winds in Patagonia can be reconstructed through changes in precipitation.

Over recent decades, the Southern Annular Mode has been consistently positive8. There have been decreases in surface pressure over the Antarctic and corresponding increases in the mid-latitudes. This has resulted in an increase in the Westerlies over the Southern Ocean.

The positive trend in the Southern Annular Mode since ~AD 1940 has been attributed to rising greenhouse gas levels and ozone depletion14, and the long-term average SAM index is now at its highest level for the last 1000 years8.

Numerical models suggest that increased greenhouse gas emissions will drive further increases in the SAM-index over the next 100 years, driving further glacier recession in Patagonia and Antarctica8,15. Increasing CO2 leads to a poleward shift and a strengthening of the Southern Westerly Winds7.

Oceanographic implications of the increasing strength of the Westerlies ove rthe Southern Ocean include an intensification of the eddy field and a reduction in the efficiency of the Southern Ocean CO2 sink due to changes in upwelling and mixing23.

The Southern Westerly Winds in Patagonia

The Southern Westerly Winds surround the Antarctic continent. The moisture within these winds sustains the Patagonian icefields today, leading to large temperate ice masses16. At latitudes within the core of the wind belt, precipitation is high and temperature seasonality is low17.

The Andes mean that there is a strong precipitation gradient across Patagonia, with the western Andes receiving vastly more precipitation than the drier lands to the east.

Present-day climate in Patagonia (From Davies et al., 2020). A: mean annual air temperature. B: Mean annual precipitation. Note large precipitation gradient. C: Mean annual wind speed. D: Ocean fronts around the Southern Hemisphere.

In Patagonia, changes in the Southern Westerly Winds have meant that it is warmer and drier. This has driven rapid glacier recession in Patagonia. Past periods of rapid glacier change in Patagonia have also been attributed to changes in these winds. When the southern westerly wind belt moved north (for example, during the Antarctic Cold Reversal17), there was a strong precipitation increase in Patagonia, and the glaciers advanced. This has been related to negative phases of the Southern Annular Mode10,11.

Conversely, when the Southern Westerly Wind belt contracts, during positive phases of the Southern Annular Mode, it is warm and dry over Patagonia and glaciers shrink11. Over the last 10,000 years, several periods of glacier advance and retreat have been related to the changes in the Southern Annular Mode over a centennial timescale1.

The Southern Westerly Winds in Antarctica

A strengthening of the circumpolar vortex leads to a deepening of the Amundsen Sea Low, cooling most of Antarctica but warming the Antarctic Peninsula, with drier conditions over West Antarctica and the Ross Ice Shelf and Lambert Glacier23. The deepening of the Amundsen Sea Low, adjacent to Thwaites Glacier, has influenced the amount of CDW beneath Thwaites Ice Shelf27.

The positive SAM index is driving surface cooling over East Antarctica. Winter cooling over East Antarctica during periods with a high SAM index is due to the greater thermal isolation of Antarctica23. This is due to increased zonal flow, decreased meridional flow, and an intensified temperature inversion on the ice sheet due to weaker near-surface winds23.

West Antarctica and Circumpolar Deep Water

Mass loss from Antarctica is dominated by the Amundsen Sea/Bellingshausen Sea sectors. In West Antarctica, mass loss from West Antarctica has averaged 6.9 mm per decade from 1979-20179. This mass loss is largely driven by ocean melt.

Stronger westerly winds in the Bellingshausen Sea and northern Amundsen Sea have changed ocean circulation, leading to more Circumpolar Deep Water (CDW) intruding onto the continental shelf18, towards the grounding zones of Thwaites Glacier19. The enhanced polar westerlies are pushing more Circumpolar Deep Water onto the continental shelf where it melts the base of ice shelves9,20.  Continental shelf waters in the Amundsen Sea have warmed, causing ice shelf thinning and retreat of grounding lines21.

The mass loss in the Amundsen Sea, Wilkes, Western Peninsula and Bellingshausen Sea sectors of Antarctica has been increasing since the 1970s, and this is consistent with the polar contraction of the westerlies that force more CDW onto the continental shelf9. The Circumpolar Deep Water reaches glaciers through the deep bathymetric troughs on the sea floor that were carved by former ice streams. This water can then melt ice shelves and destabilise glaciers9.

Warm Circumpolar Deep Water is penetrating beneath the ice shelves of Pine Island Glacier and Thwaites Glacier.

Antarctic Peninsula

Around the Antarctic Peninsula, glacier advances and recessions have been related to a weakening or strengthening of the band of Southern Westerly Winds10. Around the Western Antarctic Peninsula, glaciers that terminate in warm Circumpolar Deep Waster have undergone considerable retreat 22.

The positive phase of the SAM also causes warming on the Antarctic Peninsula8. The greatest warming has been observed during the summer months, and is associated with the strengthening of the Southern Westerly Winds and the positive phase of the Southern Annular Mode. Stronger winds have resulted in more warm, maritime air masses crossing the peninsula and reaching the low-lying ice shelves. The spine of the Antarctic Peninsula mountains also causes the adiabatic descent and warming of the winds as they cross the topography23.

Increased sea ice and an equatorward expansion of the Westerlies favour larger glaciers in Patagonia and the eastern Antarctic Peninsula. Reduced glacier extents in Patagonia and the eastern Antarctic Peninsula occur when conditions resemble a persistent positive Southern Annular Mode10.

Ice cores on the Antarctic Peninsula, on the Landsat Image Mosaic of Antarctica.

Future projections

Under a limited warming scenario (~1.5°C), with the gradual repair of the ozone hole and stabilisation of greenhouse gas emissions, it is likely that the Southern Westerly Winds will return to values typical of the Twentieth Century24. Ozone recovery in particular will encourage a weakening of the circumpolar westerlies25.

Under a higher emissions scenario, increases in greenhouse gases would cause a continued shift in the Southern Westerly Winds poleward, with a strengthening in all seasons24. Wind-driven changes in ocean currents will increase ocean heat transport to the Amundsen Sea, entering the cavities beneath floating ice shelves and driving higher basal melt, and a reduction in backstress on grounded ice upstream24.

Further reading

References

1.           Moreno, P. I. et al. Onset and evolution of southern annular mode-like changes at centennial timescale. Sci. Rep. 8, 3458 (2018).

2.           Toggweiler, J. R. Shifting Westerlies. Science (80-. ). 323, 1434–1435 (2009).

3.           Darvill, C. M., Bentley, M. J., Stokes, C. R. & Shulmeister, J. The timing and cause of glacial advances in the southern mid-latitudes during the last glacial cycle based on a synthesis of exposure ages from Patagonia and New Zealand. Quat. Sci. Rev. 149, 200–214 (2016).

4.           Davies, B. J. et al. The evolution of the Patagonian Ice Sheet from 35 ka to the present day (PATICE). Earth-Science Rev. 204, 103152 (2020).

5.           Marshall, G. J. Trends in the Southern Annular Mode from observations and reanalyses. J. Clim. 16, 4134–4143 (2003).

6.           Carter, L., McCave, I. N. & Williams, M. J. M. Chapter 4 Circulation and Water Masses of the Southern Ocean: A Review. in Antarctic Climate Evolution (eds. Florindo, F. & Siegert, M. B. T.-D. in E. and E. S.) 8, 85–114 (Elsevier, 2008).

7.           Lee, D. Y., Petersen, M. R. & Lin, W. The Southern Annular Mode and Southern Ocean Surface Westerly Winds in E3SM. Earth Sp. Sci. 6, 2624–2643 (2019).

8.           Abram, N. J. et al. Evolution of the Southern Annular Mode during the past millennium. Nat. Clim. Chang. 4, 564–569 (2014).

9.           Rignot, E. et al. Four decades of Antarctic Ice Sheet mass balance from 1979–2017. Proc. Natl. Acad. Sci. 116, 1095–1103 (2019).

10.        Kaplan, M. R. et al. Holocene glacier behavior around the northern Antarctic Peninsula and possible causes. Earth Planet. Sci. Lett. 534, 116077 (2020).

11.        Reynhout, S. et al. Holocene glacier fluctuations in Patagonia are modulated by summer insolation intensity and paced by Southern Annular Mode-like variability. Quat. Sci. Rev. in press, (2019).

12.        Sagredo, E. A. et al. Trans-pacific glacial response to the Antarctic Cold Reversal in the southern mid-latitudes. Quat. Sci. Rev. 188, 160–166 (2018).

13.        Moreno, P. I. et al. Southern Annular Mode-like changes in southwestern Patagonia at centennial timescales over the last three millennia. Nat. Commun. 5, 1–7 (2014).

14.        Thompson, D. W. J. et al. Signatures of the Antarctic ozone hole in Southern Hemisphere surface climate change. Nat. Geosci. 4, 741–749 (2011).

15.        Bracegirdle, T. J. et al. Back to the Future: Using Long-Term Observational and Paleo-Proxy Reconstructions to Improve Model Projections of Antarctic Climate. Geosciences  9, (2019).

16.        Vilanova, I., Moreno, P. I., Miranda, C. G. & Villa-Martínez, R. P. The last glacial termination in the Coyhaique sector of central Patagonia. Quat. Sci. Rev. 224, 105976 (2019).

17.        Montade, V., Peyron, O., Favier, C., Francois, J. P. & Haberle, S. G. A pollen–climate calibration from western Patagonia for palaeoclimatic reconstructions. J. Quat. Sci. 34, 76–86 (2019).

18.        Steig, E. J., Ding, Q., Battisti, D. S. & Jenkins, A. Tropical forcing of Circumpolar Deep Water Inflow and outlet glacier thinning in the Amundsen Sea Embayment, West Antarctica. Ann. Glaciol. 53, 19–28 (2012).

19.        Scambos, T. A. et al. How much, how fast?: A science review and outlook for research on the instability of Antarctica’s Thwaites Glacier in the 21st century. Glob. Planet. Change 153, 16–34 (2017).

20.        team, I. et al. Mass balance of the Antarctic Ice Sheet from 1992 to 2017. Nature 558, 219–222 (2018).

21.        Schmidtko, S., Heywood, K. J., Thompson, A. F. & Aoki, S. Multidecadal warming of Antarctic waters. Science (80-. ). 346, 1227–1231 (2014).

22.        Cook, A. J. et al. Ocean forcing of glacier retreat in the western Antarctic Peninsula. Science (80-. ). 353, 283–286 (2016).

23.        Mayewski, P. A. et al. State of the Antarctic and Southern Ocean climate system. Rev. Geophys. 47, 1–38 (2009).

24.        Rintoul, S. R. et al. Choosing the future of Antarctica. Nature 558, 233–241 (2018).

25.        Siegert, M. et al. The Antarctic Peninsula Under a 1.5°C Global Warming Scenario. Front. Environ. Sci. 7, (2019).

26. Toggweiler, J. R., & Russell, J. (2008). Ocean circulation in a warming climate. Nature, 451(7176), 286–288.

27. Holland, D. M., Nicholls, K. W., & Basinski, A. (2020). The Southern Ocean and its interaction with the Antarctic Ice Sheet. Science, 367(6484), 1326 LP – 1330. https://doi.org/10.1126/science.aaz5491

28. Pattyn, F., & Morlighem, M. (2020). The uncertain future of the Antarctic Ice Sheet. Science, 367(6484), 1331–1335.

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