Dynamics of Spontaneous (Multi) Centennial‐Scale Variations of the Atlantic Meridional Overturning Circulation Strength During the Last Interglacial

A Kessler,N Bouttes,J Tjiputra,U S Ninnemann,D M Roche

doi:10.1029/2020pa003913

Abstract

AbstractRecent reconstructions of bottom water δ13C during the last interglacial (LIG) suggest short‐lived variability in the Atlantic meridional overturning circulation (AMOC). Spontaneous (multi) centennial‐scale variability of the AMOC simulated in the Earth system model of intermediate complexity iLOVECLIM are investigated for that period. The model simulates abrupt AMOC transitions occurring at 300 years frequency and correspond to a switch of the AMOC vigor between a strong (∼17 Sv) and a weak (∼11 Sv) state. The onset of these abrupt transitions is associated with changes in orbital forcings resulting in the decline of summer insolation in the high latitudes of the North Atlantic and affecting the sea ice cover in two key deep convection regions: (1) the northern Nordic Seas (NNS) and (2) the northwest North Atlantic (NWNA). Northward inflow of Atlantic surface water increases the convection depth in (1) and strengthens the Greenland Iceland Norway (GIN) Seas overturning circulation. Subsequent ocean‐atmosphere interactions involving sea ice, ocean heat release, anomalies of evaporation‐precipitation, and wind stress over the Nordic Seas lead also to an increase in deep convection in (2), followed by increase in the AMOC strength.

Highlights

Large and abrupt changes in the Atlantic meridional overturning circulation (AMOC) associated with dramatic climate shifts may have repeatedly occurred during the past Glacial cycle (Rahmstorf, 2002) and have been associated with freshwater input from ice‐rafted detritus into the North Atlantic (Bond & Lotti, 1995; Broecker, 1994; Heinrich, 1988) and disturbances in the North Atlantic Deep Water (NADW) formation (Henry et al, 2016)
We define the northern branch of the AMOC as the Greenland Iceland Norway (GIN) Sea overturning circulation (GSOC) index, which we characterize by the maximum of the overturning stream function north of 70°N
Our findings reveal that abrupt transitions of the AMOC from strong to weak states may have been plausible during the last interglacial (LIG) under natural greenhouse gas and orbital forcings

Summary

Introduction

Large and abrupt changes in the Atlantic meridional overturning circulation (AMOC) associated with dramatic climate shifts may have repeatedly occurred during the past Glacial cycle (Rahmstorf, 2002) and have been associated with freshwater input from ice‐rafted detritus into the North Atlantic (Bond & Lotti, 1995; Broecker, 1994; Heinrich, 1988) and disturbances in the North Atlantic Deep Water (NADW) formation (Henry et al, 2016). It is found that a majority of climate models feature an AMOC that imports fresh water into the North Atlantic, that is, monostable (Drijfhout et al, 2011; Weaver et al, 2012), in contrast to observations‐based analysis, which suggest that the current AMOC is in bistable regime (Bryden et al, 2011; Garzoli et al, 2013; McDonagh & King, 2005; Weijer et al, 1999) This means that the KESSLER ET AL

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