Research using Earth system and eddy-permitting coupled ocean–sea-ice models shows that freshening in the subarctic Atlantic Ocean and a weakening of the AMOC contribute to increased temperature and salinity in the South Atlantic.
The Weakening of the Atlantic Meridional Overturning Circulation research shows that observational studies indicate a persistent weakening since the mid-20th century, while climate models generally depict a stable circulation. The Atlantic Meridional Overturning Circulation (AMOC) plays a key role in transporting heat northward within the Atlantic Ocean, influencing global climate patterns.
These changes occur on decadal timescales and are driven by the propagation of Kelvin and Rossby waves. The extent to which global warming has impacted the strength of the AMOC over the past century remains debated.
The AMOC circulates water from north to south and back in a long cycle within the Atlantic Ocean. This circulation brings warmth to various parts of the globe and also carries nutrients necessary to sustain ocean life.
The circulation process begins as warm water near the surface moves toward the poles (such as the Gulf Stream in the North Atlantic), where it cools and forms sea ice. As this ice forms, salt is left behind in the ocean water. Due to the large amount of salt in the water, it becomes denser, sinks down, and is carried southwards in the depths below. Eventually, the water gets pulled back up towards the surface and warms up in a process called upwelling, completing the cycle.
According to NOAA, if the AMOC does continue to slow down, however, it could have far-reaching climate impacts. For example, if the planet continues to warm, freshwater from melting ice at the poles would shift the rain belt in South Africa, causing droughts for millions of people. It would also cause sea level rise across the U.S. East Coast.
Incorporating higher estimates of meltwater input in historical simulations improves the alignment between observational data and models, indicating a slowdown of 0.46 sverdrups per decade since 1950, the research finds.
Projections that include subarctic meltwater input suggest the AMOC could weaken by 33% compared to pre-industrial levels under 2 °C of global warming, a threshold that may be reached within the next decade.
Such changes would have significant implications for climate systems and ecosystems, the research highlights.
