Multistability and intermediate tipping of the Atlantic Ocean circulation

Area of Science:

• Climate Dynamics
• Oceanography
• Geophysics

Background:

• Climate tipping points (TP) are traditionally viewed as abrupt shifts triggered by a single dominant feedback at a critical threshold.
• Coupling between subsystems, additional feedbacks, and spatial heterogeneity can lead to earlier, smaller reorganizations.
• The Atlantic Meridional Overturning Circulation (AMOC) is a key component of Earth's climate system sensitive to tipping.

Purpose of the Study:

• To investigate the occurrence and nature of intermediate tipping points (ITP) in the AMOC before a full collapse.
• To explore the complexity of the AMOC stability landscape under increasing glacial melt forcing.
• To assess the predictability of tipping points and safe operating limits for climate subsystems.

Main Methods:

• Simulations using a primitive-equation ocean model.
• Analysis of 2.75 million years of model output to capture long-term dynamics.
• Investigating the AMOC's response to a gradual increase in glacial meltwater input.

Main Results:

• Abrupt, qualitative changes in AMOC variability (ITPs) were observed well before a complete collapse.
• A highly complex stability landscape was revealed, with regions supporting up to nine coexisting stable states.
• The sequence of ITPs leading to collapse is dependent on the rate of meltwater input, highlighting path dependency.

Conclusions:

• The AMOC exhibits intermediate tipping points, challenging the notion of a single critical threshold for collapse.
• The complex, rugged stability landscape implies that predicting safe limits for climate tipping points is highly challenging.
• Understanding these intermediate transitions is crucial for accurately assessing climate risks and developing mitigation strategies.