Characterising meridional overturning bistability using a minimal set of state variables

1 A close approximation of key state variables and salt fluxes for both the 2 North Atlantic Deep Water (NADW) “on” and “off” states in a General Circu3 lation Model (GCM) is constructed, yielding a natural stability condition. Here, 4 stability is linked to the effect of feedbacks on infinitesimal salinity anomalies 5 on the average Atlantic salinity. The stability condition simply states that the 6 total advective salt feedback must be negative in each steady state, ensuring sta7 bility by damping the growth of infinitesimal salinity perturbations. However, 8 a decomposition of the salt feedback into three components shows that only 9 the interaction between the mean salinity and infinitesimal perturbations of the 10 meridional flow have the potential to render a state unstable, holding the key to 11 state transitions. In contrast, the interaction between the mean meridional flow 12 and infinitesimal salinity perturbations yields a negative (stabilising) component 13 feedback. Similarly, the gyre salt flux also stabilises the overturning states. Fur14 thermore, the nodes limiting the “on” and “off” state regimes in the GCM can 15 be accurately computed based on linear fits of basic state variables and the gyre 16 salt flux. It is shown that the NADW “on” state closest to collapse must be con17 tained within a neighbourhood of fresh water exporting states. Finally, the role 18 of temperature in the bistability structure is elucidated. 19