The risk of a slowdown of the overturning circulation in the North Atlantic Ocean is lower than previously thought. New research suggests increased deep water formation in ice-free regions of the Arctic Ocean will keep the wheel spinning.

Climate change is reducing the formation of deep water in the Norwegian Sea and the Labrador Sea. This has led to a fear that the Gulf Stream and other currents in the North Atlantic Ocean will weaken.

But as sea ice disappears in the Arctic, more deep water is formed in newly ice-free regions. A new study shows that this may compensate for reduced deep water formation farther south.

«Arctic regions have been somewhat forgotten,» says Marius Årthun. «But the Arctic Ocean is no longer a calm, desolate desert.»

The researcher from the Geophysical Institute at the University of Bergen, and the Bjerknes Centre for Climate Research, has led the new study, published in the journal Science Advances. Colleagues from Bergen and the University of Oxford have contributed to the work.

Their findings help explain why the system of ocean currents called the Atlantic Meridional Overturning Circulation (AMOC), of which the Gulf Stream is a branch, so far has remained stable.

The northward flow of warm water depends on deep water flowing southward

The Atlantic Meridional Overturning Circulation is like a ribbon transporting warm and salty water northward at the surface, and back south at depth. The Gulf Stream is the most important surface current in the system.

The transport relies on water cooling and sinking in the north. In a warmer climate, less water is expected to cool sufficiently to sink in the Norwegian Sea and the North Atlantic. As a result, the circulation could weaken, and in the worst case, collapse.

Deep water formation in the Arctic compensates for southern reduction

The new study indicates that the Arctic Ocean may take over the role of forming deep water. The reason behind this is the disappearance of sea ice. In newly open areas surface water is exposed to cold, polar air. As the water cools, it gets denser and is mixed downwards into the depths.

In this way, formation of deep water in the Arctic Ocean may compensate for reduced deep water formation farther south. Because the overturning in the Arctic has been stronger than before, the supply of deep water to the Atlantic Ocean has remained stable.

«Theories of such a shift have existed,» says Marius Årthun. «But it has not been expected to occur until in a much warmer future. We had no idea that this had already been going on for several decades.»

His conclusion is based on data from 1993 to 2020.

Ice melt has its limit

The system is more resilient than feared. But what happens as more and more of the sea ice in the Arctic Ocean disappears? Will the formation of deep water move steadily northward?

«Atlantic water is pushing into the Arctic, but cannot do so forever,» says Marius Årthun. «How long can the Arctic keep the wheel spinning? Will we reach a point where that cog wheel is also worn out?»

He has no answer yet, but with his colleagues he will continue to investigate the stability of ocean currents in the North Atlantic.

«Many focus on weakening and collapse,» he says. «To study a process that helps hold back, is exciting.»