Characterising meridional overturning bistability using a minimal set of state variables
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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.
KeywordsAMOC 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
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.
- Broecker WS (1991) The great ocean conveyor belt. Oceanography 4:79–87Google Scholar
- DeVries P, Weber SL (2005) The Atlantic fresh water budget as a diagnostic for the existence of a stable shut down of the meridional overturning circulation. Geophys Res Lett 32. doi: 10.1029/2004GL021450
- Dijkstra HA (2007) Characterization of the multiple equilibria regime in a global ocean model. Tellus 59a:695–705Google Scholar
- Dijkstra HA (2008) Scaling of the Atlantic meridional overturning in a global ocean model. Tellus 60a:749–760Google Scholar
- Fuerst JJ, Levermann A (2011) Minimal model of a wind- and mixing-driven overturning—threshold behaviour for both driving mechanisms. Clim Dyn 37. doi: 10.1007/s00382-011-1003-7
- Levermann A, Fuerst JJ (2010) Atlantic pycnocline theory scrutinized using a coupled climate model. Geophys Res Lett 37. doi: 10.1029/2010GL044180
- Levermann A, Griesel A (2004) Solution of a model for the oceanic pycnocline depth: scaling of overturning strength and meridional pressure difference. Geophys Res Lett 31. doi: 10.1029/2004GL020678
- Manabe S, Stouffer RJ (1999) Are two modes of thermohaline circulation stable?. Tellus 51A:400–411Google Scholar
- Pacanowski R (1995) MOM2 Documentation user’s guide and reference manual: GFDL Ocean Group Technical Report 3, 3rd edn. NOAA, GFDL, PrincetonGoogle Scholar
- Sijp WP, England MH, Gregory JM (2011a) Precise calculations of the existence of multiple AMOC equilibria in coupled climate models part I: equilibrium states. J Clim (Accepted)Google Scholar
- Sijp WP, Gregory JM, Tailleux R, Spence P (2011b) The key role of the western boundary in linking the amoc strength to the north-south pressure gradient. J Phys Oceanogr (In Revision)Google Scholar