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Water density shifts can drive rapid changes in AMOC strength

In the Atlantic Ocean, a system of currents carries vast amounts of warm, salty surface water northward. As this water reaches higher latitudes and becomes colder, it sinks and joins a deep, southward return flow. This cycle, known as the Atlantic Meridional Overturning Circulation (AMOC), plays an important role in Earth's climate as it redistributes heat, nutrients, and carbon through the ocean.

Although scientists know that the strength of the AMOC—meaning how much water it transports—can vary over time and across regions, it has been unclear how changes in AMOC strength at high northern latitudes may or may not be linked to changes farther south.

in Geophysical Research Letters, Petit and team applied high-resolution climate modeling to uncover connections between AMOC variability at the midlatitude of 45°N and the current's behavior at higher subpolar latitudes. High-latitude AMOC observations used in the modeling were captured by the Overturning in the Subpolar North Atlantic Program (OSNAP) instrument array, a network of moorings and submersibles deployed across the Labrador Sea between Greenland and Scotland.

The researchers discovered that subpolar AMOC strength, as captured by OSNAP data, does not affect midlatitude AMOC strength. However, they did find that the density of the subpolar AMOC water beginning its journey back southward affected subsequent midlatitude AMOC strength.

Changes in the water's density at high latitudes appear to be driven by changes in atmospheric pressure that affect wind stress and buoyancy at the sea surface. The team's analysis indicates that within a time span of 1 year, these subpolar density changes propagate southward along the far western side of the North Atlantic, creating a steeper density gradient at midlatitudes and, ultimately, affecting AMOC strength there.

The findings suggest that OSNAP density measurements could be used to monitor midlatitude AMOC strength. The study's results could also help inform the design of future ocean-observing systems to deepen understanding of the ocean's role in Earth's climate, according to the researchers.