Yale study charts change in major ocean current

Matthew Thomas

NEW HAVEN — The strong currents in the Atlantic Ocean appear to be changing, at least partly because of the melting Arctic ice, which may in turn be accelerating that melting, according to a postdoctoral research associate at Yale University who recently co-authored a journal article about the phenomenon.

Matthew Thomas, who studies the forces that control ocean circulation, said that the two phenomena — a movement northward of the current that brings warm water from the tropics and melting Arctic ice — are interrelated and are caused by, and contribute to, climate change.

“Any change is bad because in our climate we have such a delicate balance with our environment in how we get food and clean, fresh water,” Thomas said. “Any change in the climate affects people and we have to adapt to it.”

Thomas and lead author Camille Lique of Laboratoire d’Océanographie Physique et Spatiale in France studied the current known as the Atlantic Meridional Overturning Circulation, a major contributor to the Earth’s climate, which has been weakening because of the rising atmospheric temperature. Their research, which uses a three-dimensional model to show more clearly how the northern and southern reaches of the AMOC are shifting toward the poles, was published in the November issue of Nature Climate Change.

“The overturning has a very important role in our climate system due to the heat that it carries northwards,” Thomas said. “In order to make predictions of the future of the climate, we need to understand how all of the different components of the climate system will respond, and the overturning is one of them.”

Thomas said the AMOC is part of what is broadly known as thermohaline circulation, which is often described as a large conveyor belt in which warm, salty water flows from the southern regions of the Atlantic into the Labrador, Irminger and Nordic seas, which are sub-Arctic bodies of water near Greenland and Iceland.

“Because there’s a transport from south to north and it’s warmer … it means that it’s carrying that warm water to Europe and [that is] why winter is warmer in Europe than Canada,” Thomas said.

As the current flows northward, it becomes colder and saltier and thus less dense. The Gulf Stream is part of this current, Thomas said. Once it reaches its northernmost point, the lighter, saltier water sinks into the deep ocean, where it then reverses course and flows southward, Thomas said.

But the polar ice cap has been melting as the result of increased carbon dioxide in the atmosphere. The increase in fresh water from the melted ice keeps the seawater less dense, hindering the ability of the current to sink to the lower regions of the ocean.

“The idea that the overturning would weaken under climate change has been well explored,” Thomas said. “The paper’s not so much [saying] that the overturning would weaken. What we’re trying to say is that the mechanism by which the future overturnings would work … will be different from what they are today.

“That connection between the surface water and the deep ocean is going to weaken because we’re making the surface water lighter,” Thomas said. Also, the reduction in Arctic ice allows the current to flow farther north, he said.

“The overturning circulation is sensitive to change and it’s one of the most sensitive to change” of the many aspects of climate, Thomas said. “It’s very important for us to get it right about climate change.”

Thomas and Lique’s paper states that the AMOC has weakened over the past 20 years and, according to computer models they developed, it is expected to weaken a further 25 to 30 percent by 2100.

“Our main result, I would say, from the paper is that the overturning circulation is predicted to move north to the Arctic and this has the potential to change the interaction between Arctic ice and the warm water coming from the south,” Thomas said.

“The reason that the overturning can move into the Arctic is because ice is melting there and that opens up the possibility for the heat transport by the overturning to then further interact with the ice, potentially further melting the ice,” he said.

But it’s unclear how much the current’s ability to flow farther north would contribute to the melting ice, which is also affected by the warmer atmosphere. “That really requires future research,” Thomas said.

However, there’s no doubt the ice and the overturning current interact, Thomas said. The ice “kind of protects the ocean [underneath it] from that loss of heat and moisture,” he said. Less ice “reveals the surface of the ocean to the atmosphere. When the ice melts, the atmosphere is now in contact with the ocean where the ocean was protected by the ice.”

Thomas said one of the ramifications of their work concerns how predictable the weakening ocean current will be. “Any kind of climate change can potentially have huge influences on people … agriculture, food security, water security, energy security,” he said. It can force migration as people seek better living conditions with enough safe food and water.

Thomas’ and Lique’s work on the north Atlantic Ocean currents is important because “the Arctic is changing faster than pretty much any place on Earth,” Thomas said. “Ice is very reflective to the sun and when there’s a lot of ice … it reflects a lot of sunlight back to space.”

Less ice means less heat is reflected, more sunlight is absorbed by the Earth, melting more ice and potentially causing a “runaway domino effect,” Thomas said. What is occurring is that the reflective nature of the Arctic is changing.

As the ice melts, the overturning circulation “now has a different relationship to the ice itself,” he said. “Now the overturning goes further north and will potentially have more interaction with the sea ice than it does today.”