Multistability and Intermediate Tipping of the Atlantic Ocean Circulation

TL;DR

This paper reveals that the Atlantic Meridional Overturning Circulation (AMOC) can undergo multiple intermediate stable states before collapsing, influenced by complex feedbacks and meltwater input rates, complicating climate tipping point predictions.

Contribution

It uncovers a rugged stability landscape with up to nine coexisting states and demonstrates the occurrence of intermediate tipping points prior to AMOC collapse.

Findings

Multiple stable states coexist in the AMOC system.

Intermediate tipping points occur before the final collapse.

The path to collapse depends on meltwater input rate.

Abstract

Tipping points (TP) in climate sub-systems 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 sub-systems, 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 prior to 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.