Model sensitivity to North Atlantic freshwater forcing at 8.2 ka

Abstract

We compared four simulations of the 8.2 ka event to assess climate model sensitivity and skill in responding to North Atlantic freshwater perturbations. All of the simulations used the same freshwater forcing, 2.5 Sv for one year, applied to either the Hudson Bay (northeastern Canada) or Labrador Sea (between Canada's Labrador coast and Greenland). This freshwater pulse induced a decadal-mean slowdown of 10–25% in the Atlantic Meridional Overturning Circulation (AMOC) of the models and caused a large-scale pattern of climate anomalies that matched proxy evidence for cooling in the Northern Hemisphere and a southward shift of the Intertropical Convergence Zone. The multi-model ensemble generated temperature anomalies that were just half as large as those from quantitative proxy reconstructions, however. Also, the duration of AMOC and climate anomalies in three of the simulations was only several decades, significantly shorter than the duration of ~150 yr in the paleoclimate record. Possible reasons for these discrepancies include incorrect representation of the early Holocene climate and ocean state in the North Atlantic and uncertainties in the freshwater forcing estimates

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

“…The associated sudden input of a large amount of freshwater into the North Atlantic caused a salinity/ density reduction of the ocean surface waters and consequently a transient slowdown of the Atlantic meridional overturning circulation (AMOC; Ellison et al, 2006;Kleiven et al, 2008). This resulted in reduced northward heat transport, leading to a pronounced cooling in the North Atlantic realm (Alley and Agústsd ottir, 2005;Morrill and Jacobsen, 2005;Rohling and P€ alike, 2005), which is also evident in climate model simulations (Bauer et al, 2004;Morrill et al, 2013b;. Several proxy-based studies have therefore focused on investigating the causal mechanisms, absolute dating, duration, amplitude, spatio-temporal characteristics and environmental consequences of the 8.2 ka event (e.g.…”

“…200e400 years after the end of a freshwater perturbation (Renold et al, 2010;Stouffer et al, 2006). Although the AMOC response to freshwater forcing in general and particularly the occurrence of an enhanced AMOC resumption after a freshwater perturbation strongly depends on model setup and sensitivity as well as the input parameters (Morrill et al, 2013b), some modelling studies have proposed reasonable trigger mechanisms for such an enhanced AMOC resumption. On the one hand, it could be related to northward salt transport by the subtropical gyre, causing the recovery of salinity in the high latitudes (Vellinga et al, 2002).…”

“…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.…”

Early-Holocene to present palaeoenvironmental shifts and short climate events from the tropical wetland and lake sediments, Kukkal Lake, Southern India: Geochemistry and palynology

“…Several other proxy records of similar length are available for comparison in the region of the SWM and the East Asian Monsoon (EAM). Oxygen isotope records from radiometrically-dated speleothems (Cheng et al, 2009;Liu et al, 2013;Fleitmann et al, 2003Fleitmann et al, , 2004Fleitmann et al, , 2007Morrill et al, 2013aMorrill et al, , 2013b show a general unidirectional decrease in δ 18 O since about 8000 yr BP ( The speoleothem records also show a trough in the δ 18 O between 8500 and 8000 yr BP., interpreted in part as a decrease in monsoon intensity beginning at 8200 yr BP and lasting 100 to 150 yr. An event close to this time is present in the Kukkal sediments at an interpolated age of 8700 yr BP, where it is marked by the replacement of savanna vegetation by grassland assemblage, and inflections in the curves of major oxide abundances. Given the uncertainties of the age interpolation, the proxy records may all be recording the same event.…”

Sea level in time and space: revolutions and inconvenient truths

“…Quantifying the sea‐level rise that occurred during such events (Kendall et al ., ; Törnqvist and Hijma, ) provides a critical sensitivity test for models of the AMOC (e.g. Morrill et al ., ). Late Holocene sea‐level data have been used to test and calibrate semi‐empirical predictive sea‐level models (Kemp et al ., ; Bittermann et al ., ).…”