Cooper Alan

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Cooper
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Alan
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  • Article
    Obliquity-driven expansion of North Atlantic sea ice during the last glacial
    (John Wiley & Sons, 2015-12-10) Turney, Christian S. M. ; Thomas, Zoë ; Hutchinson, David K. ; Bradshaw, Corey J. A. ; Brook, Barry W. ; England, Matthew H. ; Fogwill, Christopher J. ; Jones, Richard T. ; Palmer, Jonathan G. ; Hughen, Konrad A. ; Cooper, Alan
    North Atlantic late Pleistocene climate (60,000 to 11,650 years ago) was characterized by abrupt and extreme millennial duration oscillations known as Dansgaard-Oeschger (D-O) events. However, during the Last Glacial Maximum (LGM) 23,000 to 19,000 cal years ago (23 to 19 ka), no D-O events are observed in the Greenland ice cores. Our new analysis of the Greenland δ18O record reveals a switch in the stability of the climate system around 30 ka, suggesting that a critical threshold was passed. Climate system modeling suggests that low axial obliquity at this time caused vastly expanded sea ice in the Labrador Sea, shifting Northern Hemisphere westerly winds south and reducing the strength of meridional overturning circulation. The results suggest that these feedbacks tipped the climate system into full glacial conditions, leading to maximum continental ice growth during the LGM.
  • Article
    Rapid global ocean-atmosphere response to Southern Ocean freshening during the last glacial
    (Nature Publishing Group, 2017-09-12) Turney, Christian S. M. ; Jones, Richard ; Phipps, Steven J. ; Thomas, Zoë ; Hogg, Alan ; Kershaw, Peter ; Fogwill, Christopher J. ; Palmer, Jonathan G. ; Bronk Ramsey, Christopher ; Adolphi, Florian ; Muscheler, Raimund ; Hughen, Konrad A. ; Staff, Richard A. ; Grosvenor, Mark ; Golledge, Nicholas ; Rasmussen, Sune O. ; Hutchinson, David K. ; Haberle, Simon ; Lorrey, Andrew ; Boswijk, Gretel ; Cooper, Alan
    Contrasting Greenland and Antarctic temperatures during the last glacial period (115,000 to 11,650 years ago) are thought to have been driven by imbalances in the rates of formation of North Atlantic and Antarctic Deep Water (the ‘bipolar seesaw’). Here we exploit a bidecadally resolved 14C data set obtained from New Zealand kauri (Agathis australis) to undertake high-precision alignment of key climate data sets spanning iceberg-rafted debris event Heinrich 3 and Greenland Interstadial (GI) 5.1 in the North Atlantic (~30,400 to 28,400 years ago). We observe no divergence between the kauri and Atlantic marine sediment 14C data sets, implying limited changes in deep water formation. However, a Southern Ocean (Atlantic-sector) iceberg rafted debris event appears to have occurred synchronously with GI-5.1 warming and decreased precipitation over the western equatorial Pacific and Atlantic. An ensemble of transient meltwater simulations shows that Antarctic-sourced salinity anomalies can generate climate changes that are propagated globally via an atmospheric Rossby wave train.