Ice sheet-ocean interactions at 40 kyr BP : Insights from a coupled ice sheet-climate model of intermediate complexity.

This study examines the interactions between the Northern Hemisphere ice sheets and the ocean during the last glacial period. Using the iLOVECLIM climate model of intermediate complexity coupled with the GRISLI ice sheet model, we explore the consequences of an amplification of the melt rates beneath ice shelves on ice sheet dynamics and the associated feedbacks. First, the amplification of oceanic basal melt rates leads to significant freshwater release from both increased calving and basal melt fluxes. Grounding line retreat and dynamic thinning occur over the Eurasian and Iceland ice sheets, while the oceanic perturbation fails to trigger a grounding line migration over the coasts of Greenland and the eastern part of the Laurentide ice sheet. Second, similarly to hosing experiments with no coupling between the climate and the ice sheets, the influx of fresh water temporarily increases sea-ice extent, reduces convection in the Labrador Sea, weakens the Atlantic meridional overturning circulation, lowers surface temperatures in the Northern Hemisphere, and increases the subsurface temperatures in the Nordic Seas. Third, the freshwater release and latent heat effect on ocean temperatures lead to a decrease in ice sheet discharge (negative feedback) for the Greenland and Eurasian ice sheets. In the experiments, the Laurentide ice sheet does not feature significant volume variations. Nonetheless, we show that we are able to trigger a grounding line retreat and a North American ice sheet volume decrease, by imposing ad-hoc constant oceanic melt rates in a second set of perturbation experiments. However, the Hudson Strait ice stream also does not exhibit the past dynamical instability indicated by the presence of Laurentide origin ice rafted debris in the North Atlantic sediment records.  This suggests that the fully coupled model is too stable, specifically in the Hudson Bay region. To help address this issue, different modelling choices regarding the basal ice sheet dynamics are considered. This emphasizes the need for further research using fully coupled models to explore the triggering mechanisms of massive iceberg discharges and to clarify the role of the ocean in these events.