Underwater waves as tall as 1,640 ft impact the ocean’s carbon budget

The ocean's deep underwater waves, some of which are as tall as 1640 ft (500 meters), significantly influence how the ocean stores heat and carbon, according to research published in AGU Advances.

The discovery suggests the urgent need for turbulence sensors in global observational arrays to improve climate models, which have misrepresented how the waves impact anthropogenic heat- particularly heat reaching the Atlantic Ice Sheet. 

What are the ocean's giant underwater waves?

The ocean absorbs the majority of the heat and carbon that humans emit. However, the amount of carbon and heat that can be absorbed depends on how turbulent the ocean's interior is since both can be either pushed deep below or dragged toward the surface.

"Beneath the surface of the water, there are jets, currents, and waves – in the deep ocean, these waves can be up to 500 meters high, but they break just like a wave on a beach," said first author Dr. Laura Cimoli from Cambridge's Department of Applied Mathematics and Theoretical Physics in a press release. 

Although these underwater waves are well known, their significance in the movement of heat and carbon is not entirely understood.

In particular, researchers have looked into whether the Atlantic Meridional Overturning Circulation (AMOC) may play a role in why the Arctic has lost so much ice cover while some Antarctic ice sheets have been increasing over the past several decades.

This phenomenon might be explained by the fact that it takes several hundred years for heat absorbed by the North Atlantic to reach the Antarctic.

Now, a team led by Cambridge University has shown that heat from the North Atlantic can reach the Antarctic far more quickly than previously believed. They combined remote sensing, ship-based observations, and data from autonomous floats.

They also found that ocean turbulence - powerful underwater waves - significantly impacts climate.

The press release highlighted that the ocean is divided into layers like a cake, with colder, denser water at the bottom and warmer, lighter water at the top. Most heat and carbon transfer in the ocean occurs within a specific layer. Still, heat and carbon can also flow between layers of varying densities, bringing deep waters to the surface.

The researchers discovered that small-scale turbulence, a feature not fully accounted for in climate models, facilitates the flow of heat and carbon across layers.

As an example, Dr. Ali Mashayek, a co-author from Cambridge's Department of Earth Science, explained that this turbulence influences the rate at which anthropogenic heat is absorbed by the Antarctic Ice Sheet.

"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," stated Mashayek. 

The full study was published in AGU Advances on March 7 and can be found here.

Study abstract:

Diapycnal mixing shapes the distribution of climatically important tracers, such as heat and carbon, as these are carried by dense water masses in the ocean interior. Here, we analyze a suite of observation-based estimates of diapycnal mixing to assess its role within the Atlantic Meridional Overturning Circulation (AMOC). The rate of water mass transformation in the Atlantic Ocean's interior shows that there is a robust buoyancy increase in the North Atlantic Deep Water (NADW, neutral density γn ≃ 27.6–28.15), with a diapycnal circulation of 0.5–8 Sv between 48°N and 32°S in the Atlantic Ocean. Moreover, tracers within the southward-flowing NADW may undergo a substantial diapycnal transfer, equivalent to a vertical displacement of hundreds of meters in the vertical. This result, confirmed with a zonally averaged numerical model of the AMOC, indicates that mixing can alter where tracers upwell in the Southern Ocean, ultimately affecting their global pathways and ventilation timescales. These results point to the need for a realistic mixing representation in climate models in order to understand and credibly project the ongoing climate change.