Weakening Atlantic Ocean Currents Disrupt Tropical Monsoons

The Atlantic Ocean currents that carry heat to the Northern Hemisphere may be shutting down due to climate change, a new study from the Technical University of Munich in Germany suggests, throwing the tropical monsoon system into disarray within at least a century.

The Atlantic Overturning Current (AMOC) is a giant ocean current, including the Gulf Stream, that pumps heat and salt from the South Atlantic to the North Atlantic. Maya Ben-Yami, a climate researcher specializing in climate tipping points at the Technical University of Munich in Germany and lead author of the study, likens the AMOC to a ventilator. So the collapse of the AMOC could have a huge impact on the transport of heat in the Earth system. The collapse of the AMOC has the potential to cause climate change around the globe, but the Northern Hemisphere and tropical monsoon regions are on the front lines, Ben-Yami said. Researchers have long suspected that a weakening of the AMOC would disrupt the tropical monsoon system, but the new study offers a much more detailed picture of what could happen.

Global warming threatens the AMOC by melting ice and ice sheets, which cause freshwater to flow into the North Atlantic. This dilutes the salinity of the top layers of water and prevents it from sinking to the ocean floor, where it would normally push the current south.

Tropical monsoons occur in a narrow band of low-pressure atmospheric conditions that surrounds the Earth near the equator. Trade winds from the Northern and Southern Hemispheres blow into this band, called the Intertropical Convergence Zone (ITCZ), resulting in heavy rains and thunderstorms for several months of the year.

The ITCZ ​​is linked to ocean temperatures and therefore to the AMOC, says Ben-Yami. The ITCZ ​​forms from warm air rising from the ocean, so it forms over the hottest places on Earth, undulating up and down along the equator with the seasons.

“Because the Earth is tilted, the warmest spot on Earth moves up and down. So the huge rain band around the planet also moves up and down,” Ben-Yami said.

If the AMOC slows or stops, it won’t deliver the same amount of heat to the Northern Hemisphere, meaning ocean temperatures there will be colder. And if the Northern Hemisphere gets colder, the hottest places on Earth will move further south. The ITCZ ​​will follow, still undulating but closer to Antarctica, bringing vital rainfall. “Right now, we have areas that are used to getting a lot of rain during the rainy season,” she says, adding that this may not last as the entire system shifts south.

To simulate the impact of an AMOC collapse on the tropical monsoon, Ben-Yami and her colleagues used eight state-of-the-art climate models to conduct experiments called “watering.” Watering, she said, is the equivalent of dumping freshwater into the North Atlantic to model the effects of ice melt, and the researchers did this over a simulation period of 50 years until the AMOC collapsed. The team published their findings on September 3 in the journal Earth’s Future.

The models indicate that the AMOC collapse will disrupt tropical monsoon systems around the planet. In West Africa, India, and East Asia, rainy seasons will become shorter and less intense as the ITCZ ​​shifts south. These results are consistent with previous predictions, but the climate changes in South America surprised the researchers, Ben-Yami said.

“The more interesting results are for the Amazon,” Ben-Yami said. There, the model predicts a significant delay in the annual monsoon, as well as reduced rainfall. “A three-month delay in the rainy season could be very bad for the ecosystem,” she said.

When the AMOC collapsed in the models, the researchers turned off the watering simulations and observed the system for another 100 years. Despite the lack of freshwater inflow, the tropical monsoon did not return to its original state, suggesting that the effects of the AMOC collapse are irreversible for at least a century.

“The effects we have in this paper are irreversible in 100 years,” said Ben-Yami, adding that this is a long time.

Previously, a study by author Johannes Lohmann from the Niels Bohr Institute at the University of Copenhagen warned that AMOC could reach a “tipping point” much sooner than expected and the consequences could be catastrophic.

The alarming increase in freshwater from melting ice from Greenland poses a major threat to the AMOC. If the current were to stop completely, tropical monsoon patterns would change, rainfall in the Northern Hemisphere would decrease, and the North Atlantic would see more storms. It would also raise concerns about other climate tipping points, such as the collapse of the polar ice caps or the drying out of the Amazon rainforest.