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ABSTRACT
The middle Pliocene warm period represents a unique time slice in which to model and understand climatic processes operating under a warm climatic regime. Palaeoclimatic model simulations, focussed on the United States of America (USA), for the middle Pliocene (ca 3 Ma) were generated using the USGS PRISM2 2º x 2º data set of boundary conditions and the UK Meteorological Office’s HadAM3 General Circulation Model (GCM).
Model results suggest that conditions in the USA during the middle Pliocene can be characterised as annually warmer (by 2º to 4ºC), less seasonal, wetter (by a maximum of 4 to 8 mm/day) and with an absence of freezing winters over the central and southern Great Plains. A sensitivity experiment suggests that the main forcing mechanisms for surface temperature changes in near coastal areas are the imposed Pliocene sea surface temperatures (SST’s). In interior regions, reduced Northern Hemisphere terrestrial ice, combined with less snow cover and a reduction in the elevation of the western cordillera of North America, generate atmospheric circulation changes and positive albedo feedbacks that raise surface temperatures. A complex set of climatic feedback mechanisms cause an enhancement of the hydrological cycle magnifying the moisture bearing westerly wind belt during the winter season (Dec., Jan., Feb.). Predictions produced by the model are in broad agreement with available geological evidence. However, the GCM appears to underestimate precipitation levels in the interior and central regions of the southern USA.
Alan M. Haywood. Postgraduate Research Institute for Sedimentology, The University of Reading, PO Box 227, Whiteknights, Reading, RG6 6AB,UK and Department of Meteorology, The University of Reading, Earley Gate, PO Box 243,Whiteknights, Reading, RG6 6BB,UK
Paul J. Valdes. Department of Meteorology, The University of Reading, Earley Gate, PO Box 243,Whiteknights, Reading, RG6 6BB,UK
Bruce W. Sellwood. Postgraduate Research Institute for Sedimentology, The University of Reading, PO Box 227, Whiteknights, Reading, RG6 6AB,UK
Jed O. Kaplan. Max-Planck-Institute for Biogeochemistry, Postfach 10 01 64, D-07701 Jena, Germany
Harry J. Dowsett. Eastern Earth Surface Processes, U.S. Geological Survey, 926A National Center, Reston, VA 20192, USA
KEY WORDS: Middle Pliocene, GCM, USA, evaluation.
Copyright: Palaeontological Association,
22 June 2001
Submission: 28 February 2001 Acceptance: 15 May 2001