Source: AGU Advances
The Atlantic Meridional Overturning Circulation (AMOC) is thought to be under potentially existential threat from climate change. But attempts to model what happens to AMOC as the oceans warm show divergent outcomes, leaving its fate unclear. Sun and Thompson use a hierarchy of models to show that the cross-hemispheric overturning could persist for centuries even as North Atlantic Deep Water (NADW) formation shuts down.
As an illustrative example, the researchers examined a simulation by the National Center for Atmospheric Research’s Community Earth System Model 2 (CESM2), a fully coupled Earth system model. In a scenario in which carbon dioxide levels had quadrupled, CESM2 showed a full shutdown of NADW formation but only a gradual slowdown of the cross-hemispheric overturning transport. The researchers found the same results when they ran an idealized, reduced-gravity ocean model that simulates the key dynamics governing the overturning circulation.
The reduced-gravity model indicated that this continued overturning is powered by a gradual deepening of density layers, or a rearrangement of the ocean’s stratification, rather than a steady balance of sinking and rising water, the authors say. That result means that though this circulation appears to remain active for centuries in their models, it may not endure forever. Rather, this deepening mechanism may simply sustain the circulation for several centuries until the ocean achieves a new density distribution. However, the authors note that attempting to predict the future of AMOC is still difficult and beyond their scope here.
One implication of this continued interbasin activity, according to the models, is the emergence of a clockwise overturning circulation in the Indo-Pacific Ocean. This circulation would redistribute heat, biogeochemical tracers, and ventilation throughout the ocean. For instance, it could result in an increase in heat transport to deeper layers, which would keep the surface cooler at the expense of intermediate water layers. In addition, less nutrient-rich water would be found in the upper ocean, and ventilation age—or the amount of time since a mass of water has been in contact with the atmosphere—is projected to increase substantially in the deep Atlantic. Together, these changes equate to a shift in global patterns of nutrients and ventilation that could have wide-ranging effects on ecosystems and global climate, the authors say. (AGU Advances, https://doi.org/10.1029/2026AV002462, 2026)
—Nathaniel Scharping (@nathanielscharp), Science Writer
