Simulated vs. reconstructed Last Glacial Maximum surface conditions: impact on oxygen isotopes in ice cores, speleothems and precipitation

Abstract:

For simulations of the Last Glacial Maximum (LGM, about 19,000 to 23,000 years ago) with coupled climate models, the structure of simulated anomalies in sea-surface temperature and sea-ice concentration often differs from that of reconstructed anomalies: It is more zonal and shows a smaller polar amplification of the glacial cooling, in the Arctic as well as in the Southern Ocean. To study the impact of these systematic and consistent differences on oxygen isotopes in ice cores, speleothems and precipitation, we prepared pre-industrial and LGM sea-surface boundary conditions based on observations and reconstructions. For the LGM, we used a new global climatology of the ocean surface during the Last Glacial Maximum mapped on a regular grid (GLOMAP), which is an extension of the Glacial Atlantic Ocean Mapping (GLAMAP) reconstruction of the Atlantic SST based on the results of the Multiproxy Approach for the Reconstruction of the Glacial Ocean Surface (MARGO) project and several recent estimates of the LGM sea-ice extent. We employed two different atmosphere-only general circulation models (NCAR iCAM3 and MPI ECHAM6-wiso) with oxygen isotopes as prognostic variables. One of them (iCAM3) was also forced with simulated sea-surface conditions from a coupled GCM (CCSM3). The resulting atmospheric fields reflect the richer structure and larger zonal and meridional gradients in the reconstructions. There is also a clear difference in the δ¹⁸O of precipitation between the two models for the same sea-surface conditions. From our the model-data comparison, we conclude: (a) iCAM3 with reconstructed sea-surface conditions performed better for ice cores in polar regions. (b) Regarding ECHAM6-wiso and iCAM3 with reconstructed sea-surface conditions, iCAM3 performed slightly better for some (sub-) tropical ice cores and speleothems. Overall, the model-data fit for both models forced by reconstructed sea-surface conditions was comparably good and much better than for forcing iCAM3 with simulated LGM sea-surface conditions.