Climate Simulation for 125 kBP with a Coupled Ocean-Atmosphere General Circulation Model

M Montoya (Institute of Meteorology, University of Hamburg, Bundesstrasse 55, D-20146; ph. 49-40-4123-3531; fax 49-40-4123-3531; Internet:; H von Storch (Institute of Meteorology, University of Hamburg, Bundesstrasse 55, D-20146; ph. 49-40-4123-3531; fax 49-40-4123-3531; Internet: T J Crowley (Department of Oceanography, Texas A&M University, College Station, TX 77843; ph. 409-847-8879; fax 409-847-8879; Internet:$ (AGU Sponsor: H von Storch)

Coupled general circulation models (CGCMs) are the state-of-the-art tools to quasi-realistically simulate climate under a given set of boundary conditions, and to study climate response to altered boundary conditions. Due to their high computational cost, millennia CGCM integrations have seldom been performed, and those which exist have a maximum length of the order of one thousand years. Therefore, CGCMs are for the moment not suited for the study of millennial-scale climate change. However, CGCMs can provide valuable information about the physical mechanisms and the links between the different components of the climate system which are ultimately responsible for millennial-scale global climate change. Paleoclimate simulations and the comparison of their results against the evidence from the geological record can help to understand reconstructed paleoclimatic features in terms of concrete physical mechanisms, and provide a tool to validate general circulation models (GCMs) under boundary conditions independent from those under which models are built.

In this line, we have integrated a coupled ocean-atmosphere GCM over several hundred years to simulate climate conditions at the last interglacial maximum (Eemian, 125,000 years ago). The model has been fitted to present climatic conditions by invoking a flux correction scheme. Simulated sea surface temperatures (SSTs) are found to be consistent to within 0.3 C with reconstructed SSTs from CLIMAP. It is concluded that the model operates satisfactorily under moderately changed boundary conditions in spite of the use of a flux correction scheme. The results reflect the expected surface temperature changes (with respect to the control run) due to the amplification (reduction) of the seasonal cycle of insolation in the northern (southern) hemisphere, with enhanced summer warming over northern land masses. However, significant cooling also occurs in northern winter, with the net effect that temperature changes almost cancel out in the annual mean. Main atmospheric circulation changes include intensified summer monsoon precipitation and circulation, related to the enhanced land-sea contrast in the northern hemisphere, and reduced midlatitude westerlies and synoptic variability, related to the decreased meridional temperature gradient throughout the troposphere. A slight intensification of the meridional overturning circulation in both the Atlantic and Pacific basins is found at the Eemian. As to the question of climatic instability at the Eemian raised by evidence from the GRIP ice core and which is still a matter of debate, the model reproduces a large summer and all-year-round warming (up to 3-4 C) at high northern latitudes and particularly east Greenland which could potentially trigger partial melting of the Greenland ice sheet. This idea could be tested in collaboration with ice sheet modelers.