Ribergaard Mads H.

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Mads H.

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  • Preprint
    Surprising return of deep convection to the subpolar North Atlantic Ocean in winter 2007–2008
    ( 2008-11-07) Våge, Kjetil ; Pickart, Robert S. ; Thierry, Virginie ; Reverdin, Gilles ; Lee, Craig M. ; Petrie, Brian ; Agnew, Tom A. ; Wong, Amy ; Ribergaard, Mads H.
    The process of open-ocean convection in the subpolar North Atlantic Ocean forms a dense water mass that impacts the meridional overturning circulation and heat flux, and sequesters atmospheric carbon. In recent years the convection has been shallow or nonexistent, which could be construed as a consequence of a warmer climate. However, in the winter of 2007-08 deep convection returned to the subpolar gyre in both the Labrador and Irminger Seas. Here we document this return and elucidate the reasons why it happened. Profiling float data from the Argo programme are used to document the deep mixing, and a variety of in-situ, satellite, and reanalysis products are analyzed to describe the conditions leading to the overturning. The transition to a convective state took place abruptly, without going through a preconditioning phase, which is contrary to general expectations. Changes in the hemispheric air temperature, tracks of storms, flux of freshwater to the Labrador Sea, and distribution of pack ice all conspired to enhance the air-sea heat flux, resulting in the deep overturning. This study illuminates the complexity of the North Atlantic convective system.
  • Preprint
    Rapid response of Helheim Glacier in Greenland to climate variability over the past century
    ( 2011-11-08) Andresen, Camilla S. ; Straneo, Fiamma ; Ribergaard, Mads H. ; Bjork, Anders A. ; Andersen, Thorbjorn J. ; Kuijpers, Antoon ; Norgaard-Pedersen, Niels ; Kjaer, Kurt H. ; Schjoth, Frands ; Weckstrom, Kaarina ; Ahlstrom, Andreas P.
    During the early 2000s the Greenland Ice Sheet experienced the largest ice mass loss observed on the instrumental record1, largely as a result of the acceleration, thinning and retreat of major outlet glaciers in West and Southeast Greenland2-5. The quasi-simultaneous change in the glaciers suggests a common climate forcing and increasing air6 and ocean7-8 temperatures have been indicated as potential triggers. Here, we present a new record of calving activity of Helheim Glacier, East Greenland, extending back to c. 1890 AD. This record was obtained by analysing sedimentary deposits from Sermilik Fjord, where Helheim Glacier terminates, and uses the annual deposition of sand grains as a proxy for iceberg discharge. The 120 year long record reveals large fluctuations in calving rates, but that the present high rate was reproduced only in the 1930s. A comparison with climate indices indicates that high calving activity coincides with increased Atlantic Water and decreased Polar Water influence on the shelf, warm summers and a negative phase of the North Atlantic Oscillation. Our analysis provides evidence that Helheim Glacier responds to short-term (3-10 years) large-scale oceanic and atmospheric fluctuations.