Multistability and intermediate tipping of the Atlantic Ocean circulation

Abstract

Tipping points (TP) in climate subsystems are usually thought to occur at a well-defined, critical forcing parameter threshold, via destabilization of the system state by a single, dominant positive feedback. However, coupling to other subsystems, additional feedbacks, and spatial heterogeneity may promote further small-amplitude, abrupt reorganizations of geophysical flows at forcing levels lower than the critical threshold. Using a primitive-equation ocean model, we simulate a collapse of the Atlantic Meridional Overturning Circulation (AMOC) due to increasing glacial melt. Considerably before the collapse, various abrupt, qualitative changes in AMOC variability occur. These intermediate tipping points (ITP) are transitions between multiple stable circulation states. Using 2.75 million years of model simulations, we uncover a very rugged stability landscape featuring parameter regions of up to nine coexisting stable states. The path to an AMOC collapse via a sequence of ITPs depends on the rate of change of the meltwater input. This challenges our ability to predict and define safe limits for TPs.

Uncertainties too large to predict tipping times of major Earth system components from historical data

“…There has long been a discussion about whether the AMOC, when investigated as a complex system under external forcing of, e.g., GMT (or better, regional freshwater forcing), exhibits multiple stable states ( 34 – 36 ). Transitions between such stable states could be bifurcation induced and thus abrupt and irreversible.…”

The Effect of Indian Ocean Surface Freshwater Flux Biases On the Multi-Stable Regime of the AMOC

“…The rate of forcing is important here and if this is much faster than the equilibration time scale of the steady state, the approximations of the regime boundaries become worse and also rate-induced tipping may occur (Lohmann et al, 2021). Such quasi-equilibrium experiments have been performed with many ocean-only models (Cini et al, 2024;Lohmann et al, 2024), Earth System Models of Intermediate Complexity (EMICs) (Rahmstorf et al, 2005) and the FAMOUS model, the latter being a Global Climate Model (GCM) with a relatively coarse horizontal ocean resolution of 2.5° × 3.75° (Hawkins et al, 2011). AMOC hysteresis behaviour has also been investigated in the Community Climate System Model (CCSM3) (Hu et al, 2012) and recently in the Community Earth System Model (van Westen and Dijkstra, 2023;.…”