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Anomalies in temperature and salinity that originate in the midlatitude North Atlantic can affect the Atlantic Meridional Overturning Circulation (AMOC) in the Nordic Seas up to a decade later. A new study in Communications Earth & Environment shows that the anomalies that travel northward with the Atlantic Water are an important part of the system, and actively modulate both the inflow of warm water into the Nordic Seas and the overflow of dense water back into the deep Atlantic.

"Rather than being passive signals, we find that these anomalies are an inherent part of the system—they help control the strength of the Nordic Seas branch of the AMOC. This research will likely open the door to many new studies on how upstream changes shape the overturning circulation at high latitudes—and how changes in the Nordic Seas may, in turn, influence conditions further south," says Léon Chafik, first and corresponding author of the study and researcher at the Department of Meteorology, Stockholm University and the Bolin Center for Climate Research.

The overturning circulation in the Nordic Seas is a key high-latitude branch of the larger Atlantic Meridional Overturning Circulation (AMOC), which moves heat northward and influences the climate of Europe and the Arctic.

"In the study, we focus on what controls the strength of the AMOC in the Nordic Seas—something that has been poorly understood. Interestingly, all the time series we use in this study suggest that the Nordic Seas overturning is still stable, with no sign of long-term weakening. Understanding what drives variability in this region is essential, especially since it helps shape the future of the broader AMOC," says Chafik.

The research team reconstructed the northward transport of Atlantic Water into the Nordic Seas using 50 years of hydrographic observations—temperature and salinity profiles collected north and south of the Greenland–Scotland Ridge.

"By combining these data with satellite altimetry and current meter records, we were able to piece together how thermohaline anomalies travel northward along the Atlantic Water pathway and influence the Nordic Seas overturning as they go.

"What's unique about our approach is that we used variability in the inflow itself as a physical tracer—a way to follow these anomalies from their origin in the midlatitudes all the way to the Arctic gateways," says Chafik.

The results point to a slow but predictable chain of events, where changes in the North Atlantic can affect the high-latitude overturning circulation five to 10 years later. This makes thermohaline anomalies a potential source of climate predictability.

"One important outcome of our study is that satellite data—particularly altimetry—can be used to monitor these changes as they unfold," says Chafik.

This offers a cost-effective way to track the state of this system in near real time.

"We hope these satellite programs will continue to be supported and not be affected by government funding cuts, as is currently happening in some countries. In the long run, our findings could help improve forecasts of regional climate shifts, especially in northern Europe and the Arctic.

"They also highlight the need for sustained satellite and in-situ observations, and for better representation of high-latitude ocean processes in climate models," says Chafik.