Research using MITgcm reveals surprising shifts in Atlantic Ocean circulation under sudden climate forcing.

Reporting by Helen Hill for MITgcm

In a new study published in the Journal of Climate, researchers Chiung-Yin Chang (Princeton) and Malte Jansen (U. Chicago) have leveraged the MIT General Circulation Model (MITgcm) to explore how the Atlantic Meridional Overturning Circulation (AMOC)—a key driver of global climate—responds to sudden surface warming and an intensified hydrological cycle. Their findings reveal complex, and at times counterintuitive, dynamics that could reshape how scientists assess the risk of abrupt climate transitions.

The AMOC, often likened to a planetary conveyor belt, transports warm surface waters northward and returns cold, dense waters southward at depth. This circulation regulates climate patterns across the Atlantic basin and beyond. Disruptions to the AMOC have been implicated in past abrupt climate events and are a major concern in future climate projections.

To investigate the AMOC’s sensitivity to rapid climate forcing, Chang and Jansen turned to the MITgcm as an open-source, high-resolution numerical model developed at MIT and widely used in oceanographic and climate research. The model’s flexibility allowed the team to simulate an idealized ice–ocean system under tightly controlled conditions, isolating the effects of two key perturbations: uniform surface warming and an amplification of the evaporation-minus-precipitation (E − P) pattern, which reflects changes in the hydrological cycle.

“The MITgcm was essential for this study,” said Chang. “It allowed us to explore the fundamental physics of ocean circulation in a clean, idealized setting, while still capturing the nonlinear feedbacks that make the AMOC so sensitive to climate change.”

The simulations revealed that both warming and intensified E − P initially weaken the AMOC by reducing surface water density in the North Atlantic, thereby suppressing deep-water formation. However, the response to E − P changes was significantly smaller—about ten times less—when scaled according to the Clausius–Clapeyron relation, which describes how atmospheric moisture capacity increases with temperature.

At equilibrium, the results diverged from previous studies. Sustained warming led to a weakened AMOC, driven by dominant warming in the North Atlantic that outweighed the effects of reduced brine rejection in the Southern Ocean. In contrast, an amplified hydrological cycle actually strengthened the AMOC, consistent with a “passive response” theory in which enhanced salinity gradients reinforce circulation.

Yet the study also uncovered a critical caveat: a negative salt advection feedback that limits the strengthening effect by dampening salinity contrasts. More dramatically, when both warming and E − P changes were applied simultaneously at large amplitudes, the AMOC collapsed entirely. This collapse was driven by a positive salt advection feedback—a mechanism that, under certain conditions, can flip from stabilizing to destabilizing the circulation.

To better understand this tipping behavior, the researchers used simplified box model theory to identify the thresholds at which feedbacks change sign. These theoretical insights, combined with the idealized simulations enabled by MITgcm, provide a powerful framework for interpreting the behavior of more complex Earth system models.

“Our work shows that the AMOC’s response to climate forcing is highly nonlinear and depends on a delicate balance of feedbacks,” said Chang. “The MITgcm gave us the tools to dissect these dynamics in a way that would be difficult with more comprehensive but less flexible models.”

Questions/ comments email: cychang@princeton.edu

To find out more about this work​ contact Jenny.

Story image credit: Aaron Ulsch

About the Researchers

Chiung-Yin Chang is a postdoc in the Program in Atmospheric and Oceanic Sciences, Princeton University, Princeton, New Jersey. She began using MITgcm in 2019. Malte Jansen is an Associate Professor in the Department of Geophysical Sciences at the University of Chicago. He has been using MITgcm since 2008.

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