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Climate Dynamics

, Volume 39, Issue 9–10, pp 2127–2142 | Cite as

Characterising meridional overturning bistability using a minimal set of state variables

  • Willem P. SijpEmail author
Article

Abstract

A close approximation of key state variables and salt fluxes for both the North Atlantic Deep Water (NADW) “on” and “off” states in a General Circulation Model (GCM) is constructed, yielding a natural stability condition. Here, stability is linked to the effect of feedbacks on infinitesimal salinity anomalies on the average Atlantic salinity. The stability condition simply states that the total advective salt feedback must be negative in each steady state, ensuring stability by damping the growth of infinitesimal salinity perturbations. However, a decomposition of the salt feedback into three components shows that only the interaction between the mean salinity and infinitesimal perturbations of the meridional flow have the potential to render a state unstable, holding the key to state transitions. In contrast, the interaction between the mean meridional flow and infinitesimal salinity perturbations yields a negative (stabilising) component feedback. Similarly, the gyre salt flux also stabilises the overturning states. Furthermore, the nodes limiting the “on” and “off” state regimes in the GCM can be accurately computed based on linear fits of basic state variables and the gyre salt flux. It is shown that the NADW “on” state closest to collapse must be contained within a neighbourhood of fresh water exporting states. Finally, the role of temperature in the bistability structure is elucidated.

Keywords

AMOC Atlantic Atlantic meridional overturning circulation Bistability Non-linear system Climate change NADW formation NADW shutdown Gulf stream shutdown North Atlantic deep water Deep sinking Convection shutdown Poleward heat transport Halocline catastrophe Box model Two stable states Critical points Non-linear theory Saddle nodes Limit point 

Notes

Acknowledgments

We thank the University of Victoria staff for support in usage of the their coupled climate model. This research was supported by the Australian Research Council and the Australian Antarctic Science Program. This research was undertaken on the NCI National Facility in Canberra, Australia, which is supported by the Australian Commonwealth Government. We thank Jonathan M. Gregory for hosting several visits to the University of Reading, UK, and many stimulating discussions.

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Copyright information

© Springer-Verlag 2011

Authors and Affiliations

  1. 1.Climate Change Research Centre (CCRC)University of New South WalesSydneyAustralia

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