How Atlantic Ocean Current’s Slowing Down May Affect the Tropics

A slowdown of the Atlantic Meridional Overturning Circulation (AMOC), one of the Earth’s most crucial ocean circulation systems, could turn some of the world’s rainiest regions, including the Amazon, Central America, and West Africa, into arid zones suffering from severe droughts in the future, according to a study published in Nature in July 2025.

What is the AMOC, and why does it matter

The AMOC acts as a vast conveyor belt, moving warm, salty water from the tropics northward into the Atlantic Ocean and returning cooler water southward at depth. This process plays a vital role in regulating global temperatures, redistributing heat between the hemispheres, and maintaining the position of the tropical rain belt, a narrow band of intense precipitation near the equator.

The study combined climate models with palaeoclimate records dating back 17,000 years, the last time the AMOC significantly slowed due to natural causes. Evidence from cave formations, as well as lake and ocean sediments, revealed how rainfall patterns shifted during that ancient slowdown, providing crucial clues to how a modern decline could reshape tropical climates.

What happens when the AMOC slows down

As the planet warms, melting polar ice and increased rainfall add fresh water to the North Atlantic, reducing ocean water density and potentially slowing down the AMOC. However, scientists caution that the system has only been monitored for about two decades, making it difficult to draw definitive conclusions about long-term changes.

According to Pedro DiNezio, the study’s lead author and an associate professor in the Department of Atmospheric and Oceanic Sciences at the University of Colorado Boulder, “A few years ago, this monitoring system recorded signs of a decline in the AMOC, but it later rebounded. So we weren’t sure if it was just a fluke. The problem is, we haven’t been measuring the ocean long enough to detect meaningful long-term change.”

While the scientists remain uncertain whether the AMOC has already slowed, climate models suggest that global warming will eventually weaken it. Based on palaeoclimate evidence, the study projects that a weakened AMOC could lead to:

• Up to a 40% reduction in annual rainfall in parts of the Amazon rainforest,
• Significant decreases in precipitation over Central America, and
• Drier conditions across West Africa.

“This study is an excellent example of how we can use palaeoclimate data to better understand how the Earth system works,” said Chijun Sun, co-author and assistant professor of Earth and Planetary Sciences at the University of California, Davis. “Improving our knowledge of these potential changes is crucial to preparing for the future.”

The broader implications for the planet

The potential drying of the Amazon is particularly concerning. As one of the planet’s largest carbon sinks, the Amazon plays a key role in absorbing carbon dioxide from the atmosphere.

Prolonged droughts could weaken this function and even transform the rainforest into a carbon source, releasing massive amounts of emissions and creating a feedback loop that accelerates climate change.

The study underscores how climate change may either hasten or influence the AMOC’s weakening.

As DiNezio warns, “We still have time, but we need to rapidly decarbonise the economy and make green technologies widely available to everyone in the world. The best way to get out of a hole is to stop digging.”

Read the complete study: “Tropical response to ocean circulation slowdown raises future drought risk“.