The westerly winds and the Patagonian Ice Sheet

The moisture-bearing Southern Westerly Winds

The Patagonian Ice Sheet, which formed during the Last Glacial Maximum (LGM) around 21,000 years ago, was strongly influenced by the Southern Westerly Winds. These winds blow around the Southern Hemisphere in the mid-latitudes (see map below) and deliver snow and rain to the western coast of southern South America[1], sustaining glaciers. These strong winds also control the location of major ocean fronts (the boundary between water masses of different temperature) in the Southern Ocean and, as a result, the temperature of waters at the ocean surface[2,3].

Windswept Nothofagus antarctica tree, Ushuaia, Tierra del Fuego, Patagonia, Argentina. By Leonardo Pallotta, Wikimedia Commons.

Reconstructing past changes in the southern westerlies

As Patagonia covers a large latitudinal transect, extending from 40°S to 56°S, it is a critical region for investigating how the Southern Westerly Winds, and other climate systems, have changed over the last 21,000 years, and how these changes affected Patagonian glaciers.

Map of the Southern Hemisphere showing the Southern Westerly Wind belt (SWW) and Subtropical Front (STF) in the present day. The westerlies bring rain and snowfall to the west coast of Patagonia. The Subtropical Front sits at the northern limit of the westerly wind belt. Figure copyright Jacob Bendle.

Southward wind shifts driving glacier recession

Following the end of the Last Glacial Maximum (LGM) the Southern Westerly Winds abruptly shifted southward towards Antarctica[4,5], and pulled the warm Subtropical Front with them[2,3] (see left-hand side of diagram below). Records of former glacier extent show that the Patagonian Ice Sheet began to rapidly retreat and thin at about the same time (~18,000 years ago[6,7,8]), suggesting that as the winds moved south, the amount of snowfall feeding the ice sheet decreased. The wind-driven shift of the Subtropical Front caused the coastal waters around Patagonia to warm[2]. With less accumulation (snowfall) and warmer temperatures, the Patagonian Ice Sheet started to retreat.

Diagram showing how the location of the Southern Westerly Winds (SWW) and Subtropical Front (STF) impact the mid-latitudes. Left: When the westerly winds and Subtropical Front contract (move south) cool, moist air stops flowing over Patagonia, and warm waters enter the mid-latitude oceans. This favours glacier retreat. Right: When the westerly winds and Subtropical Front expand (move north) strong winds bring precipitation to Patagonia, and cold Southern Ocean waters cool the mid-latitude oceans. This favours glacier advance. Figure copyright Jacob Bendle.

Northward wind shifts driving glacier advance

Whereas the southward shift of the Southern Westerly Winds triggered Patagonian Ice Sheet retreat at ~18,000 years ago, a northward wind shift between ~14,500 and 12,800 years ago, in the Antarctic Cold Reversal (a cool period recorded in Antarctic ice cores), revived glacier activity[9,10,11,12].

As the westerly winds moved north over Patagonia (see right-hand side of diagram above), increased snowfall led to glacier growth. Because the winds also pulled cool Southern Ocean waters into the mid-latitudes, ocean and air temperatures around Patagonia cooled, leading to less ice sheet melting. The combination of increased accumulation (snowfall) and decreased ablation (melting) led to glacier readvance.

Hemisphere-wide glacier response

Glaciers in the Southern Alps of New Zealand also readvanced in the Antarctic Cold Reversal, at the same time as glaciers in Patagonia[13]. This suggests that the shift in the position of the the westerly winds and ocean fronts were a major driver of climate and ice sheet behaviour across the entire mid-latitude belt below ~40°S.

SWW controls on climate

The Southern Westerly Wind system controls the climate of the Southern Hemisphere in other ways, and these are important for modern and past glaciers.

For example, when the westerlies move towards Antarctica, the warm waters they drag southwards causes the sea-ice around Antarctica to break up and retreat[14]. This causes the ocean around Antarctica to warm, and releases heat to the atmosphere.

Also, when the westerly winds are positioned over the Southern Ocean, they cause relatively warm water that is trapped at depth to rise to the ocean surface. This releases heat and CO2 from the ocean, and causes atmospheric warming[15].

References

[1] Garreaud, R.D., Vuille, M., Compagnucci, R. & Marengo, J. 2009. Present-day South American climate. Palaeogeography, Palaeoclimatology, Palaeoecology, 281, 180–195.

[2] Lamy, F., Kaiser, J., Arz, H.W., Hebbeln, D., Ninnemann, U., Timm, O., Timmermann, A. & Toggweiler, J.R. 2007. Modulation of the bipolar seesaw in the Southeast Pacific during Termination 1. Earth and Planetary Science Letters, 259, 400–413.

[3] Barker, S., Diz, P., Vautravers, M.J., Pike, J., Knorr, G., Hall, I.R. & Broecker, W.S. 2009. Interhemispheric Atlantic seesaw response during the last deglaciation. Nature, 457, 1097–1102.

[4] Denton, G.H., Anderson, R.F., Toggweiler, J.R., Edwards, R.L., Schaefer, J.M. & Putnam, A.E. 2010. The last glacial termination. Science, 328, 1652–1656.

[5] Moreno, P.I., Villa-Martínez, R., Cárdenas, M.L.& Sagredo, E.A. 2012. Deglacial changes of the southern margin of the southern westerly winds revealed by terrestrial records from SW Patagonia (52°S). Quaternary Science Reviews, 41, 1–21.

[6] Boex, J., Fogwill, C., Harrison, S., Glasser, N., Hein, A., Schnabel, C. & Xu, S. 2013. Rapid thinning of the Late Pleistocene Patagonian Ice Sheet followed migration of the Southern Westerlies. Scientific Reports 3, 2118.

[7] Moreno, P.I., Denton, G.H., Moreno, H., Lowell, T.V., Putnam, A.E. & Kaplan, M.R. 2015. Radiocarbon chronology of the last glacial maximum and its termination in northwestern Patagonia. Quaternary Science Reviews, 122, 233–249.

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