Multi-century simulations of LGM and present day climate using an accelerated coupled GCM carrying water isotope tracers, with comparisons to ocean sediment/ice cores and observations.

摘要

440 and 540-year climate simulations of the Last Glacial Maximum (21 kyBP) and Present Day were done with a 5 x 4 degree Coupled General Circulation Model (13-level ocean, 9-level atmosphere), without flux corrections and fully carrying Oxygen 18 as water isotope tracer. To run the CGCM to near-equilibrium in reasonable time, CGCM acceleration schemes were implemented and tested: Reduced Gravity, Distorted Physics, program parallelization, and starting from another equilibrium. Up-to-date era-appropriate boundary conditions were prepared for the CGCM: insolation, greenhouse gases, and, as a function of sea level, land/ocean mask, straits, land/glacier elevations, ocean depths, and mean ocean O18 and salt concentrations.; Testing "forward modelling" of climate proxies, CGCM ocean results were input into a model of foraminifera O18 ratio and the results compared to a database of ocean sediment core foraminifera O18, compiled as part of this research. CGCM precipitation O18 ratio results were compared directly to an also-compiled ice core O18 database. For PD, CGCM results were compared to gathered observations.; Parallelization was found to be the least-problematic acceleration scheme; use of the others possibly affecting the equilibrium reached. Forward modelling of forarninifera O18 was found to be a useful technique. It was determined that the LGM to PD change (LGM-PD) in mean ocean O18 was -1.0 permil, although LGM-PD deep ocean O18 from core pore water was found not to match well the CGCM's. LGM-PD mean tropical SST was determined to be 3.6 C, significantly greater than CLIMAP's. LGM-PD SST from alkenones and foraminifera Mg/Ca were found not to match well the CGCM's or CLIMAP's, regardless of month. Terrestrial temperature proxies, which contradict CLIMAP, were found to match well the CGCM. Contrary to its proxies, North Atlantic meridional overturning circulation was found to be stronger at the LGM than PD. Consistent with ice core borehole paleothermometry, the spatial relationship of surface air temperature versus snow O18 was found to be significantly different from the temporal one at a location.