# Basin bifurcations, oscillatory instability and rate-induced thresholds for AMOC in a global oceanic box model

## TL;DR

This paper investigates the stability and bifurcation behavior of the Atlantic Meridional Overturning Circulation (AMOC) using a global oceanic box model, revealing subcritical Hopf bifurcations and rate-induced thresholds relevant for climate change scenarios.

## Contribution

It identifies the type of bifurcation causing AMOC collapse in a physically derived model and explores how rate-induced tipping points can occur without crossing bifurcation thresholds.

## Key findings

- AMOC loss of stability occurs via subcritical Hopf bifurcation.
- Rate-induced thresholds can occur even without crossing bifurcation points.
- The model is calibrated to both pre-industrial and doubled CO2 conditions.

## Abstract

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 {\em 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.