Large sensitivity to freshwater forcing location in 8.2 ka simulations

Abstract

The 8.2 ka event is a key test case for simulating the coupled climate response to changes in the Atlantic Meridional Overturning Circulation (AMOC). Recent advances in quantifying freshwater fluxes at 8.2 ka from the proxy record have improved the realism of the forcing magnitude in model simulations, yet this forcing is still generally applied in an unrealistic geographic manner, across most of the Labrador Sea rather than just along the Labrador coast. Previous simulations with eddy-resolving ocean models have come to conflicting conclusions regarding the ability of such a coastally confined flow to impact the AMOC. These simulations have also not incorporated full atmosphere models nor have they used the new meltwater forcing values for 8.2 ka. We use the Community Climate System Model, version 3, with an ocean model resolution only slightly coarser than that used in previous eddy-resolving simulations, to test the sensitivity to freshwater forcing location. When revised freshwater forcing is applied across the Labrador Sea, the AMOC is reduced bỹ 40% and climate anomalies compare well with proxy records of the 8.2 ka event in terms of magnitude and duration. When the forcing is added just along the Labrador coast, however, most meltwater joins the subtropical gyre and travels to the subtropics with minor impact to the AMOC (~10% decrease). It is likely that model biases in the placement of the North Atlantic Current remain an important limitation for correctly simulating the 8.2 ka event.

Deglaciation of the Laurentide Ice Sheet from the Last Glacial Maximum

“…Climate modelling indicates that meltwater discharge events routed into the Labrador Sea (e.g. from meltwater runoff and glacial lakes) could cause a cooling of up to 1.5°C over the Labrador Dome (Morrill et al, 2014), but with minimal cooling (<0.5°C) along the western margin of the ice sheet, west of Hudson Bay. This negative feedback mechanism has thus been invoked to explain the relatively stability of the Labrador Dome, whilst the western margin of the ice sheet continued to retreat (Ullman et al, 2016).…”

The Greenland Ice Sheet during the last glacial cycle: Current ice loss and contribution to sea-level rise from a palaeoclimatic perspective

“…This freshwater outburst slowed North-Atlantic thermohaline circulation and decreased northward heat transport by the North-Atlantic current (e.g. Kleiven et al, 2008;LeGrande and Schmidt, 2008;Hoffman et al, 2012;Morrill et al, 2014;Jennings et al, 2015), which caused a significant regional cooling centred around 8.2 ka BP (e.g. Wiersma and Renssen, 2006;Thomas et al, 2007).…”

Evidence for higher-than-average air temperatures after the 8.2 ka event provided by a Central European δ18O record

“…Furthermore, the results of climate model simulations for the 8.2 ka event are still ambiguous with respect to the strength and duration of the AMOC slowdown and the following temperature decrease, mostly not matching the proxy evidence (Morrill et al, 2013b). The limitations of climate models in correctly reproducing the full spatio-temporal pattern of climatic changes around 8.2 ka BP are supposedly related to a suite of different factors, involving the complexity and resolution of the models, the probably non-linear response of the AMOC to freshwater forcing (LeGrande and Schmidt, 2008) and a number of not yet well-constrained in-/ external forcings (Morrill et al, 2013b), including the volume and rate of freshwater discharge and its exact routing in the North Atlantic (Li et al, 2009;Morrill et al, 2014;, the possible role of freshwater background forcing from the melting Laurentide Ice Sheet (Matero et al, 2017;Wagner et al, 2013), the ocean circulation mode around 8.2 ka BP (Born and Levermann, 2010;Morrill et al, 2013b) and the early Holocene climate background state (LeGrande et al, 2006). Hence, there still remain many uncertainties regarding the amplitude and pattern of the AMOC slowdown during the 8.2 ka event and its subsequent recovery as well as regarding the associated climatic changes.…”