Effects of LGM sea surface temperature and sea ice extent on the isotope-temperature slope at polar ice core sites

Stable water isotopes in polar ice cores are widely used to reconstruct past temperature variations over several orbital climatic cycles. One way to calibrate the isotope-temperature relationship is to apply the present-day spatial relationship as a surrogate for the temporal one. However, this method leads to large uncertainties because several factors like the sea surface conditions or the origin and the transport of water vapor influence the isotope-temperature temporal slope. In this study, we investigate how the sea surface temperature (SST), the sea ice extent and the strength of the Atlantic Meridional Overturning Circulation (AMOC) affect these temporal slopes in Greenland and Antarctica for Last Glacial Maximum (LGM, ~21 000 years ago) to preindustrial climate change. For that, we use the isotope-enabled atmosphere climate model ECHAM6-wiso [1, 2], forced with a set of sea surface boundary condition datasets based on reconstructions (GLOMAP [3] and Tierney et al. (2020) [4]) or MIROC 4m simulation outputs [5]. We found that the isotope-temperature temporal slopes in East Antarctic coastal areas are mainly controlled by the sea ice extent, while the sea surface temperature cooling affects more the temporal slope values inland. Mixed effects on isotope-temperature temporal slopes are simulated in West Antarctica with sea surface boundary conditions changes, because the transport of water vapor from the Southern Ocean to this area can dampen the influence of temperature on the changes of the isotopic composition of precipitation and snow. In the Greenland area, the isotope-temperature temporal slopes are influenced by the sea surface temperatures very near the coasts of the continent. The greater the LGM cooling off the coast of southeast Greenland, the larger the temporal slopes. The presence or absence of sea ice very near the coast has a large influence in Baffin Bay and the Greenland Sea and influences the slopes at some inland ice cores stations. We emphasize that the extent far south of the sea ice is not so important. On the other hand, the seasonal variations of sea ice distribution, especially its retreat in summer, influence the water vapor transport in this region and the modeled isotope-temperature temporal slopes in the eastern part of Greenland. A stronger LGM AMOC decreases LGM to preindustrial isotopic anomalies in precipitation in Greenland, degrading the isotopic model-data agreement. The AMOC strength does not modify the temporal slopes over inner Greenland, and only a little on the coasts along the Greenland Sea where the changes in surface temperature and sea ice distribution due to the AMOC strength mainly occur.

[1] Cauquoin and Werner, J. Adv. Model. Earth Syst., 13, https://doi.org/10.1029/2021MS002532, 2021.

[2] Cauquoin et al., Clim. Past, 15, 1913–1937, https://doi.org/10.5194/cp-15-1913-2019, 2019.

[3] Paul et al., Clim. Past, 17, 805–824, https://doi.org/10.5194/cp-17-805-2021, 2021.

[4] Tierney et al., Nature, 584, 569–573, https://doi.org/10.1038/s41586-020-2617-x, 2020.

[5] Obase and Abe-Ouchi, Geophys. Res. Lett., 46, 11 397–11 405, https://doi.org/10.1029/2019GL084675, 2019.