Transient simulations,empirical reconstructions and forcing mechanisms for the Mid-holocene hydrological climate in southern Patagonia |
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Authors: | Sebastian Wagner Martin Widmann Julie Jones Torsten Haberzettl Andreas Lücke Christoph Mayr Christian Ohlendorf Frank Schäbitz Bernd Zolitschka |
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Institution: | (1) GKSS Research Centre, Institute for Coastal Research, Max-Planck-Str.1, 21502 Geesthacht, Germany;(2) Institute of Geography, University of Bremen, Bremen, Germany;(3) Institute of Chemistry and Dynamics of the Geosphere V, Research Centre Jülich, Jülich, Germany;(4) Seminar for Geography and its Didactics, University of Cologne, Cologne, Germany |
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Abstract: | This study investigates the atmospheric circulation in transient climate simulations with a coupled atmosphere–ocean general
circulation model (GCM) for the mid-Holocene (MH) period 7–4.5 ka BP driven with combinations of orbital, solar and greenhouse
gas forcings. The focus is on southern South America. Statistical downscaling models are derived from observational data and
applied to the simulations to estimate precipitation in south-eastern Patagonia during the MH. These estimates are compared
with lake level estimates for Laguna Potrok Aike (LPA) from sediments. Relative to pre-industrial conditions (i.e. 1550–1850),
which show extraordinarily high lake levels, the proxy-based reconstructed lake levels during the MH are lower. The downscaled
simulated circulation differences indicate higher LPA precipitation during the MH from March to August, higher annual means,
and reduced precipitation from September to February. Thus the reconstructed lower LPA lake levels can not be explained solely
by the simulated precipitation changes. Possible reasons for this discrepancy are discussed. Based on proxy data from southern
South America hypotheses have also been proposed on the latitudinal position of the southern hemispheric westerlies (SHWs).
In agreement with some of these hypotheses our simulations show an increased seasonal cycle of the latitudinal position of
the SHWs during the MH, which can be explained by the orbital forcing. The simulations also show stronger SHWs over southern
Patagonia during austral summer and weaker SHWs during winter. The downscaling model associates weaker SHWs with increased
precipitation in the LPA region. However, this relationship is only moderate, and therefore the downscaling model does not
support the assumption of a strong link between mean SHWs and precipitation over south-eastern Patagonia, which is the basis
of many proxy-based hypotheses about the SHWs. |
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