Browsing by Author "Sime, L"

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    Holocene dynamics of the Southern Hemisphere westerly winds and possible links to CO2 outgassing
    (Springer Nature, 2018-07-23) Saunders, KM; Roberts, SJ; Perren, B; Butz, C; Sime, L; Davies, S; van Nieuwenhuyze, W; Grosjean, M; Hodgson, DA
    The Southern Hemisphere westerly winds (SHW) play an important role in regulating the capacity of the Southern Ocean carbon sink. They modulate upwelling of carbon-rich deep water and, with sea ice, determine the ocean surface area available for air–sea gas exchange. Some models indicate that the current strengthening and poleward shift of these winds will weaken the carbon sink. If correct, centennial- to millennial-scale reconstructions of the SHW intensity should be linked with past changes in atmospheric CO2, temperature and sea ice. Here we present a 12,300-year reconstruction of wind strength based on three independent proxies that track inputs of sea-salt aerosols and minerogenic particles accumulating in lake sediments on sub-Antarctic Macquarie Island. Between about 12.1 thousand years ago (ka) and 11.2 ka, and since about 7 ka, the wind intensities were above their long-term mean and corresponded with increasing atmospheric CO2. Conversely, from about 11.2 to 7.2 ka, the wind intensities were below their long-term mean and corresponded with decreasing atmospheric CO2. These observations are consistent with model inferences of enhanced SHW contributing to the long-term outgassing of CO2 from the Southern Ocean. © 2021 Springer Nature Limited
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    Late quaternary changes in the westerly winds over the Southern Ocean – a progress report
    (VII Southern Connection Congress 2016, 2016-01-18) Hodgson, DA; Roberts, S; Perren, B; Saunders, KM; Verleyen, E; van Nieuwenhuyze, W; Vyverman, W; Sime, L
    The Southern Hemisphere westerly winds (SHW) are the strongest time-averaged oceanic winds. They drive the circulation of the Southern Ocean and changes in their strength and position are thought to modify the upwelling of carbon rich deep water, exerting significant control on the ocean-atmosphere balance of CO2. Thus changes in the SHW, such as the recently observed intensification, could influence whether the Southern Ocean acts as a net source or sink of atmospheric CO2, with major implications for global climate. At present the relationships between wind strength, CO2 and climate are poorly understood and there are very few studies within the core belt of the SHW in the sub-Antarctic zone c.46 to 60 deg South. We have been attempting to address this by producing centennial to decadal reconstructions of changes in SHW strength at sub-Antarctic islands in each of the major sectors of the Southern Ocean. In this talk we will show how lake sediments and peat deposits on the west coasts of these islands can yield proxy-based reconstructions of past changes in the SHW. We will review the statistical performance of our inference models, their application down selected sediment cores, and compare them with complimentary proxies of changes in wind strength based on precipitation and minerogenic inputs. The next phase of the project will use GCM simulations to help understand the patterns seen in the observational data and identify the drivers of past changes in the SHW.
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    Westerly wind variability at sub-Antarctic Macquarie Island: links to the Southern Annular Mode and Southern Hemisphere rainfall and temperature
    (International Union for Quaternary Research (INQUA), 2019-07-30) Saunders, KM; Roberts, SJ; Griffiths, AD; Meredith, KT; Dätwyler, C; Hernandez-Almedia, I; Butz, C; Sime, L; Neukom, R; Grosjean, M; Hodgson, DA
    The position and strength of the Southern Hemisphere westerly winds is important for temperature and rainfall variability from the mid- to high-latitudes of the Southern Hemisphere. They also influence Southern Ocean circulation and sea ice extent around Antarctica and are closely linked to changes in the Southern Annular Mode (SAM). While observations available since the 1950s show the winds have strengthened and shifted southwards, this period is too short to understand their natural variability, especially as stratospheric ozone depletion and rising greenhouse gases from anthropogenic activities are considered to be driving these changes. Sub-Antarctic islands, such as Macquarie Island (54°S, 158°E), are ideally situated to reconstruct changes in the westerly winds as they lie within the latitudes where the winds are strongest. Here, we reconstruct changes in westerly wind strength of the last ca. 1800 years using lake sediment records from Macquarie Island. The reconstruction involves the application of a diatom-sea spray inference model (transfer function) supported by geochemical, minerogenic and sedimentological analyses. The inference model was used to assess changes in sea spray inputs to a small, exposed lake on the western edge of the Macquarie Island plateau, where the amount of sea spray is directly related to the strength of the westerlies. The reconstruction shows close agreement with the southern South America temperature (Past Global Changes) and SAM reconstructions for much of the last millennium, with the main feature being a decrease in wind strength ca. AD 1450 that coincides with a decrease in temperature at many sites around the Southern Hemisphere and transition to a more negative SAM phase. The combination of a modern climatological framework for understanding Macquarie Island’s current climate together with modelling and palaeoclimatological reconstructions of the westerlies, demonstrates that changes recorded at Macquarie Island are representative of wind, rainfall and temperature across the mid- to high-latitudes of the Southern Hemisphere. © The Authors.