Basin bifurcations, oscillatory instability and rate-induced thresholds for Atlantic meridional overturning circulation in a global oceanic box model

• Published in: Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences
• Main Authors: Alkhayuon, H, Ashwin, P, Jackson, LC, Quinn, C, Wood, RA
• Format: Article in Journal/Newspaper
• Language: English
• Published: Royal Soc London 2019
• Subjects: Mathematical Sciences; Applied mathematics; Dynamical systems in applications; North Atlantic
• Online Access: https://doi.org/10.1098/rspa.2019.0051; http://ecite.utas.edu.au/149466

The Atlantic meridional overturning circulation (AMOC) transports substantial amounts of heat into the North Atlantic sector, and hence is of very high importance in regional climate projections. The AMOC has been observed to show multi-stability across a range of models of different complexity. The simplest models find a bifurcation associated with the AMOC on state losing stability that is a saddle node. Here, we study a physically derived global oceanic model of Wood et al. with five boxes, that is calibrated to runs of the FAMOUS coupled atmosphere-ocean general circulation model. We find the loss of stability of the on state is due to a subcritical Hopf for parameters from both pre-industrial and doubled CO 2 atmospheres. This loss of stability via subcritical Hopf bifurcation has important consequences for the behaviour of the basin of attraction close to bifurcation. We consider various time-dependent profiles of freshwater forcing to the system, and find that rate-induced thresholds for tipping can appear, even for perturbations that do not cross the bifurcation. Understanding how such state transitions occur is important in determining allowable safe climate change mitigation pathways to avoid collapse of the AMOC.