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Southern Ocean origin for the resumption of Atlantic thermohaline circulation during deglaciation

Abstract

During the two most recent deglaciations, the Southern Hemisphere warmed before Greenland1,2. At the same time, the northern Atlantic Ocean was exposed to meltwater discharge3, which is generally assumed to reduce the formation of North Atlantic Deep Water4,5. Yet during deglaciation, the Atlantic thermohaline circulation became more vigorous, in the transition from a weak glacial to a strong interglacial mode6. Here we use a three-dimensional ocean circulation model7 to investigate the impact of Southern Ocean warming and the associated sea-ice retreat8 on the Atlantic thermohaline circulation. We find that a gradual warming in the Southern Ocean during deglaciation induces an abrupt resumption of the interglacial mode of the thermohaline circulation, triggered by increased mass transport into the Atlantic Ocean via the warm (Indian Ocean) and cold (Pacific Ocean) water route9,10. This effect prevails over the influence of meltwater discharge, which would oppose a strengthening of the thermohaline circulation. A Southern Ocean trigger for the transition into an interglacial mode of circulation provides a consistent picture of Southern and Northern hemispheric climate change at times of deglaciation, in agreement with the available proxy records.

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Figure 1: Modelled overturning streamfunction, as well as NADW convection sites and horizontal surface velocities in the Atlantic for the glacial control climate (LGM_CTRL).
Figure 2: Temporal changes of NADW export at 30° S, and North Atlantic temperature, salinity and convection energy loss.
Figure 3: Differences between LGM_100 and LGM_CTRL, and meridional overturning streamfunction in LGM_100 after 3,000 model years.
Figure 4: Atlantic NADW export at 30° S for the different freshwater pulse experiments and stability diagrams for the Atlantic THC.

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Acknowledgements

We thank H. Jansen, S. Mulitza and M. Prange for suggestions. L. Könnecke, S. Schubert, M. Butzin and S. Blessing are acknowledged for their technical support. This work was supported by BMBF through the DEKLIM project ‘climate transitions’.

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Correspondence to Gregor Knorr.

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Supplementary information

41586_2003_BFnature01855_MOESM1_ESM.mov

Supplementary Movie 1: Temporal anomaly evolution of salinity, temperature and horizontal velocities between experiment LGM_100 and LGM_CTRL. Temperature and velocity represent surface anomalies. The salinity anomaly is averaged over the upper 800 m. The blue point on the NADW export curve serves for orientation in time. (MOV 1905 kb)

41586_2003_BFnature01855_MOESM2_ESM.mov

Supplementary Movie 2: Temporal evolution of North Atlantic deep water (NADW) convection sites in experiment LGM_100 and horizontal velocity anomaly between experiment LGM_100 and LGM_CTRL at the ocean surface. The blue point on the NADW export curve serves for orientation in time. (MOV 719 kb)

41586_2003_BFnature01855_MOESM3_ESM.pdf

Supplementary Figure: Time Series of NADW export at 30°S for our deglaciation scenarios with alternated wind and temperature forcing. In this set of experiments, the global sea ice cover has been prescribed and the change to interglacial conditions has been applied instantaneously. LGM_SH – changed temperature, sea ice and wind stress; LGM_SH_TICE – changed temperature and sea ice; LGM_SH_WIND – changed wind stress. The experiments with Southern Ocean warming south of 30°S are represented by the red curves. In experiment LGM_NH (green curve), interglacial values in temperature, sea ice and wind stress are applied north of 30°N. The glacial (LGM_CTRL_ICE) and interglacial (PD) reference states are indicated by the black curves and crosses. (PDF 94 kb)

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Knorr, G., Lohmann, G. Southern Ocean origin for the resumption of Atlantic thermohaline circulation during deglaciation. Nature 424, 532–536 (2003). https://doi.org/10.1038/nature01855

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