The Atlantic Meridional Overturning Circulation (AMOC), an ocean current system that transports heat from the tropics to the North Atlantic, plays a vital role in regulating the global climate. Most climate models project a decline in AMOC strength under anthropogenic greenhouse gas warming. However, it remains unclear whether the AMOC has slowed over the past century, and if so, when this slowdown began.

To address this issue, a research team from the Institute of Oceanology of the Chinese Academy of Sciences (IOCAS), in collaboration with scientists from the Scripps Institution of Oceanography and the University of California, San Diego, analyzed observational data, climate models, and ocean simulations. They uncovered a key "fingerprint" of AMOC slowdown: mid-depth (1,000–2,000 meters) warming in the equatorial Atlantic Ocean.

The study was recently published in Communications Earth & Environment.

Using the Massachusetts Institute of Technology General Circulation Model (MITgcm), the researchers traced how AMOC-related signals propagate rapidly toward the equator. Their results showed that an AMOC slowdown triggers subsurface warming in the Subpolar North Atlantic. This warming then generates baroclinic Kelvin waves, which travel equatorward along the western boundary of the North Atlantic. Upon reaching the equator, these waves propagate along the equatorial region, ultimately causing the distinct mid-depth warming.

"Our findings reveal that the equatorial Atlantic serves as a critical crossroads for AMOC-related dynamical signals to spread across the global ocean," said Prof. LI Yuanlong, co-corresponding author of the study.

Further analysis using climate models confirmed that this mid-depth warming is highly correlated with AMOC changes over decadal to longer timescales, and the warming consistently accompanies AMOC weakening across a decade period. Importantly, mid-depth temperature changes provide a more stable and reliable indicator of AMOC strength than surface-based proxies, which are easily affected by atmospheric variability.

By examining observational data dating back to 1960, the researchers identified a clear mid-depth warming trend in the equatorial Atlantic. This trend has stood out from natural ocean variability since the early 2000s, suggesting that the AMOC likely began to weaken in the late 20th century.

"The equatorial mid-depth temperature signal is dynamically driven, detectable in observational data, and has already emerged from natural variability," noted Dr. REN Qiuping, first author of the study. "It offers a valuable new metric for monitoring AMOC changes in a warming world."

This study provides evidence that the Atlantic overturning circulation is currently slowing. It also highlights a novel, remote mid-depth fingerprint that can be used to track the AMOC's future evolution, the researchers noted.

Relationship between AMOC and its proxies in climate models. (Image by IOCAS)