A new approach for simulating the paleo-evolution of the Northern Hemisphere ice sheets

Abstract

Abstract. Offline forcing methods for ice-sheet models often make use of an index approach in which temperature anomalies relative to the present are calculated by combining a simulated glacial-interglacial climatic anomaly field, interpolated through an index derived from the Greenland ice-core temperature reconstruction, with present-day climatologies. An important drawback of this approach is that it clearly misrepresents climate variability at millennial timescales. The reason for this is that the spatial glacial-interglacial anomaly field used is associated with orbital climatic variations, while it is scaled following the characteristic time evolution of the index, which includes orbital and millennial-scale climate variability. The spatial patterns of orbital and millennial variability are clearly not the same, as indicated by a wealth of models and data. As a result, this method can be expected to lead to a misrepresentation of climate variability and thus of the past evolution of Northern Hemisphere (NH) ice sheets. Here we illustrate the problems derived from this approach and propose a new offline climate forcing method that attempts to better represent the characteristic pattern of millennial-scale climate variability by including an additional spatial anomaly field associated with this timescale. To this end, three different synthetic transient forcing climatologies are developed for the past 120 kyr following a perturbative approach and are applied to an ice-sheet model. The impact of the climatologies on the paleo-evolution of the NH ice sheets is evaluated. The first method follows the usual index approach in which temperature anomalies relative to the present are calculated by combining a simulated glacial-interglacial climatic anomaly field, interpolated through an index derived from ice-core data, with presentday climatologies. In the second approach the representation of millennial-scale climate variability is improved by incorporating a simulated stadial-interstadial anomaly field. The third is a refinement of the second one in which the amplitudes of both orbital and millennial-scale variations are tuned to provide perfect agreement with a recently published absolute temperature reconstruction over Greenland. The comparison of the three climate forcing methods highlights the tendency of the usual index approach to overestimate the temperature variability over North America and Eurasia at millennial timescales. This leads to a relatively high NH icevolume variability on these timescales. Through enhanced ablation, this results in too low an ice volume throughout the last glacial period (LGP), below or at the lower end of the uncertainty range of estimations. Improving the representation of millennial-scale variability alone yields an important increase in ice volume in all NH ice sheets but especially in the Fennoscandian Ice Sheet (FIS). Optimizing the amplitude of the temperature anomalies to match the Greenland reconstruction results in a further increase in the simulated icesheet volume ...

The Antarctic Ice Sheet response to glacial millennial-scale variability

“…Ice-core records from the Greenland Ice Sheet (GrIS) during the LGP show the characteristic signal of DO events: a rapid warming of more than 10 K on decadal timescales followed by a slow cooling that can last from several centuries to thousands of years (e.g., Dansgaard et al, 1993). Modeling studies (e.g., Ganopolski and Rahmstorf, 2001;Rahmstorf, 2002;Shaffer et al, 2004) and reconstructions (Barker et al, 2015;Böhm et al, 2015;Henry et al, 2016;McManus et al, 2004) support the hypothesis that DO events were caused by reorganizations of the Atlantic Meridional Overturning Circulation (AMOC), with enhanced (reduced) North Atlantic Deep Water (NADW) formation during interstadials (stadials) transporting more (less) heat into high northern latitudes. In addition, marine sediment records across large areas of the North Atlantic show quasi-periodic deposition of ice-rafted detritus (IRD) (Hemming, 2004) known as Heinrich (H) events.…”

Submarine melt as a potential trigger of the North East Greenland Ice Stream margin retreat during Marine Isotope Stage 3

“…This approach has been adopted by many ice-sheet models to represent transient past precipitation when they are not coupled to a climate model (e.g. Banderas et al, 2018;Charbit et al, 2002Charbit et al, , 2007Colleoni et al, 2014;Marshall et al, 2000;Marshall and Peltier, 2002;Marshall and Koutnik, 2006;Philippon et al, 2006;Zweck and Huybrechts, 2005). Surface ablation is calculated by the simple positive degree day (PDD) scheme (Reeh, 1989).…”

“…The former is depth-dependent; the latter also depends on the distribution of salinity in the water column. Introducing B ref in the equation is a simplification made to avoid the choice of values to be assigned to these two variables, that might be challenging and unconstrained (Beckmann and Goosse, 2003). For the sake of simplicity, T clim,ocn − T f can be considered to be spatially (horizontally and vertically) constant, in the way that B ref is defined to scale directly with κ.…”

Submarine melt as a potential trigger of the NEGIS margin retreat during MIS-3

“…The α(t) index is derived from the Greenland temperature reconstruction for the Holocene (Vinther et al, 2009), the North Greenland Ice Core Project (NGRIP) reconstruction for the LGP (Kindler et al, 2014) (Montoya and Levermann, 2008). The precipitation field is obtained following a similar approach based on the ratio of LGM and present-day precipitation, scaled by α (Banderas et al, 2018):…”