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, 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].
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.
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. With less accumulation (snowfall) and warmer temperatures, the Patagonian Ice Sheet started to retreat.
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. 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. 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.
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