Nordic Seas Overturning Circulation strengthens as Atlantic Meridional Overturning Circulation weakens, new study

While the AMOC, a major Atlantic current system, has weakened, its northern branch, the NOC, has remained stable over the past century, with models projecting a slight strengthening in the future. The NOC carries water past Iceland into the Nordic Seas before returning to the Atlantic, flowing as dense deep water over underwater ridges between Greenland, Iceland and Scotland.

“The stability of the NOC and its projected increase have been viewed by some as a contradiction to the weakening AMOC. But our findings tell us the opposite. The strengthening of the NOC is a physical consequence of AMOC weakening,” said co-author Stefan Rahmstorf of PIK.

“Our model results indicate that a density-driven mechanism links these opposing trends. A weakened AMOC leads to reduced salt transport into the subpolar North Atlantic, lowering the density of water there, and strengthening the NOC by increasing the density contrast with the waters further north,” explained lead author Sasha Roewer, PIK researcher when the study was conducted and now with the Max Planck Institute for Meteorology.

Using detailed climate model data and a simplified model of the Atlantic and Nordic Seas, the researchers investigated how changes in water density link the AMOC and NOC.

According to the model simulations, the NOC may keep strengthening as a result of AMOC weakening. But only until deep convection in the Nordic Seas shuts down – a change that could then trigger the collapse of both currents.

“A strengthening of the NOC is not a sign of a stable AMOC, but rather a symptom of its weakening and perhaps even a precursor of its shutdown, with profound impacts for the global climate,” Stefan Rahmstorf concluded.