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Atlantic Subsurface Temperatures: Response to a Shutdown of the Overturning Circulation and Consequences for Its Recovery

J. MignotPotsdam Institute for Climate Impact Research, Potsdam, Germany

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A. GanopolskiPotsdam Institute for Climate Impact Research, Potsdam, Germany

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A. LevermannPotsdam Institute for Climate Impact Research, Potsdam, Germany

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Abstract

Using the coupled climate model of intermediate complexity, CLIMBER-3α, changes in the vertical thermal structure associated with a shutdown of the Atlantic meridional overturning circulation (AMOC) are investigated. When North Atlantic Deep Water formation is inhibited by anomalous freshwater forcing, intermediate depth ventilation can remain active and cool the subsurface water masses (i.e., the “cold case”). However, if intermediate ventilation is completely suppressed, relatively warm water coming from the south penetrates to a high northern latitude beneath the halocline and induces a strong vertical temperature inversion between the surface and intermediate depth (i.e., the “warm case”). Both types of temperature anomalies emerge within the first decade after the beginning of the freshwater perturbation. The sign of subsurface temperature anomaly has a strong implication for the recovery of the AMOC once the anomalous freshwater forcing is removed. While the AMOC recovers from the cold case on centennial time scales, the recovery is much more rapid (decadal time scales) when ventilation is completely suppressed and intermediate depths are anomalously warm. This is explained by a more rapid destabilization of the water column after cessation of the anomalous flux due to a strong vertical temperature inversion. A suite of sensitivity experiments with varying strength and duration of the freshwater perturbation and a larger value of background vertical diffusivity demonstrate robustness of the phenomenon. Implications of the simulated subsurface temperature response to the shutdown of the AMOC for future climate and abrupt climate changes of the past are discussed.

* Current affiliation: LOCEAN, Unité Mixte de Recherché, CNRS-IRD-UPMC-MNHN, Université Pierre et Marie Curie, Paris, France

+ Additional affiliation: Institute of Physics, Potsdam University, Potsdam, Germany

Corresponding author address: J. Mignot, LOCEAN, Université Pierre et Marie Curie, Case courrier 100, 4 place Jussieu, 75252 Paris CEDEX 05, France. Email: juliette.mignot@locean-ipsl.upmc.fr

Abstract

Using the coupled climate model of intermediate complexity, CLIMBER-3α, changes in the vertical thermal structure associated with a shutdown of the Atlantic meridional overturning circulation (AMOC) are investigated. When North Atlantic Deep Water formation is inhibited by anomalous freshwater forcing, intermediate depth ventilation can remain active and cool the subsurface water masses (i.e., the “cold case”). However, if intermediate ventilation is completely suppressed, relatively warm water coming from the south penetrates to a high northern latitude beneath the halocline and induces a strong vertical temperature inversion between the surface and intermediate depth (i.e., the “warm case”). Both types of temperature anomalies emerge within the first decade after the beginning of the freshwater perturbation. The sign of subsurface temperature anomaly has a strong implication for the recovery of the AMOC once the anomalous freshwater forcing is removed. While the AMOC recovers from the cold case on centennial time scales, the recovery is much more rapid (decadal time scales) when ventilation is completely suppressed and intermediate depths are anomalously warm. This is explained by a more rapid destabilization of the water column after cessation of the anomalous flux due to a strong vertical temperature inversion. A suite of sensitivity experiments with varying strength and duration of the freshwater perturbation and a larger value of background vertical diffusivity demonstrate robustness of the phenomenon. Implications of the simulated subsurface temperature response to the shutdown of the AMOC for future climate and abrupt climate changes of the past are discussed.

* Current affiliation: LOCEAN, Unité Mixte de Recherché, CNRS-IRD-UPMC-MNHN, Université Pierre et Marie Curie, Paris, France

+ Additional affiliation: Institute of Physics, Potsdam University, Potsdam, Germany

Corresponding author address: J. Mignot, LOCEAN, Université Pierre et Marie Curie, Case courrier 100, 4 place Jussieu, 75252 Paris CEDEX 05, France. Email: juliette.mignot@locean-ipsl.upmc.fr

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