Effects of Last Glacial Maximum (LGM) sea surface temperature and sea ice extent on the isotope–temperature slope at polar ice core sites
Authors: Cauquoin Alexandre , Abe-Ouchi Ayako , Obase Takashi , Chan Wing-Le , Paul André , Werner Martin 
Published: June 2023

Abstract. 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 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, forced with a set of sea surface boundary condition datasets
based on reconstructions (e.g., GLOMAP) or MIROC 4m simulation outputs. 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 the temporal slope values inland more. On the
other hand, ECHAM6-wiso simulates the impact of sea ice extent on the EPICA Dome C (EDC) and
Vostok sites through the contribution of water vapor from lower latitudes.
Effects of sea surface boundary condition changes on modeled
isotope–temperature temporal slopes are variable in West Antarctica. This is
partly due to the transport of water vapor from the Southern Ocean to this
area that can dampen the influence of local temperature on the changes in
the isotopic composition of precipitation and snow. In the Greenland area,
the isotope–temperature temporal slopes are influenced by the sea surface
temperatures near the coasts of the continent. The greater the LGM cooling
off the coast of southeastern Greenland, the greater the transport of water
vapor from the North Atlantic, and 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 core stations. The extent of the sea ice far south slightly influences
the temporal slopes in Greenland through the transport of more depleted
water vapor from lower latitudes to this area. The seasonal variations of
sea ice distribution, especially its retreat in summer, influence the
isotopic composition of the water vapor 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 modifies the temporal slopes over inner Greenland slightly
and by 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.