When someone mentions waves, we are most likely to think of the beach and surfers riding breaks to shore, not the waves deep beneath the ocean's surface. Now, new research has shed light on the important role underwater waves play in climate change.
Much of the heat and carbon produced by humans is absorbed by the ocean. How much is absorbed depends on underwater turbulence, which can either push heat and carbon down deep or pull it upward toward the surface. It helps to think of the ocean like the layers of a cake, with the colder, denser water at the bottom and the warmer, lighter water on top. Water circulates from warm to cool and back again as if on a conveyer belt.
In the North Atlantic, the dense (cold) waters travel southward to the equator, while the lighter (warmer) waters travel in the opposite direction. The Atlantic section of this conveyer belt is called the Atlantic Meridional Overturning Circulation (AMOC) and it plays a key role in regulating global heat and carbon loads. Usually, heat and carbon travel horizontally along one ocean layer that is the same density, but they can also move vertically between layers of different densities. This is how deeper water is brought to the ocean’s surface.
A new study has examined how underwater waves in the Atlantic Ocean affect the movement of heat and carbon.
“The Atlantic Ocean is special in how it affects the global climate,” said Dr Ali Mashayek, co-author of the study. “It has a strong pole-to-pole circulation from its upper reaches to the deep ocean. The water also moves faster at the surface than it does in the deep ocean.”
The movement of heat and carbon up through layers of different densities is caused mainly by underwater waves. While the phenomenon of underwater waves is well-known, how they move heat and carbon around is poorly understood.
Over the last several decades, studies have investigated whether AMOC is responsible for the Arctic losing ice cover while simultaneously causing ice sheets to grow in Antarctica.
In the current study, researchers used data from various observational platforms, including remote sensors, ships, and autonomous buoys. They found that heat from the North Atlantic reaches the Antarctic much faster than was previously thought. Further, turbulence within the ocean, particularly from large underwater waves – some of which can be as high as 1,640 ft (500 m) – affects climate.
The researchers found that the movement of heat and carbon between ocean layers was brought about by small-scale turbulence, something that is not properly represented in climate models, which use quantitative methods to simulate the interactions of atmosphere, oceans, ice, and land to predict future climate. The turbulence mostly affected the deep waters moving from the Atlantic Ocean southward to the Southern Ocean. The researchers concluded that there is a high chance that heat and carbon carried by this water will move across different density levels.
“Climate models do account for turbulence, but mostly in how it affects ocean circulation,” said lead author Dr Laura Cimoli. “But we’ve found that turbulence is vital in its own right, and plays a key role in how much carbon and heat gets absorbed by the ocean, and where it gets stored.”
The study’s findings mean rethinking the importance of small-scale turbulence in climate models.
“Many climate models have an overly simplistic representation of the role of micro-scale turbulence, but we’ve shown it’s significant and should be treated with more care,” Mashayek said.
The researchers are calling for turbulence sensors to be included on observational arrays worldwide so scientists can more accurately predict climate change.
The study was published in the journal AGU Advances, and the video below features Jonathan Sharples, a Professor of Oceanography at the University of Liverpool in the UK, which wasn't involved in the study, providing an easy-to-understand explanation of AMOC.
Source: University of Cambridge
