The Unseen Forces Driving Deep-Sea Climate Change

Exploring how swirling ocean currents are intensifying heatwaves and cold snaps deep below the surface

After many years of moving across continents and states, we decided to settle in the Washington, DC, area two years ago. We had my husband’s family there, old-time friends, and lots of career opportunities. After having children, we needed all of those.

And as we were planning our move, we took over one of our long-time dreams: buying a house.

We were quickly surprised by how expensive the area had gotten, so we had to settle for a house that needed some fixes in the medium to short term. Most buyers didn’t want to take over the project; they wanted something ready to show off. But we knew we could make it work.

Among everything that needed to be done, though, was the foundation. Built-in 1950, the house had a few additions, including a second floor. However, the foundation was never updated, reinforced, or insulated. The house was sinking, and the lack of insulation meant huge temperature variations between different areas.

As much as we wanted to work on our kitchen and bathrooms, that had to be our priority. Even if nobody would see how pretty our crawl space was now, what lay underneath was crucial to what could be done on top.

This is a lesson we learned as homeowners. And it’s also one that can be easily applied to the science of climate change. Is climate change affecting the deep ocean — the foundations of this planet — as much as it affects the surface?

When you think of a heatwave, you probably picture scorching summer days on land. But did you know the ocean experiences its own version of heatwaves and even cold snaps deep below the surface?

These extreme temperature events don’t just affect the surface waters that we can observe easily — they’re also happening in the depths of the ocean, where many marine creatures live, far from the reach of satellites.

New research is helping us understand these subsurface temperature extremes, showing how swirling ocean currents, called eddies, play a major role in creating and intensifying these events. These eddies, large loops of moving water that can stretch hundreds of kilometers wide, transport heat and cold to deeper layers of the ocean.

A study published in Nature, offers fresh insights into how these temperature extremes affect the ocean’s twilight zone, a vital habitat for marine life between 200 and 1,000 meters deep.

But first, let’s take a step back. How did they study this?

The team behind this study relied on a combination of data sources to understand what’s happening beneath the waves. For example, they collected over two million temperature profiles from across the globe over three decades.

This rich dataset came from long-term moorings — buoys tethered to the ocean floor that continuously measure temperature at various depths — and Argo floats, which are robotic divers that travel deep into the ocean, collecting temperature and salinity data before resurfacing. Yes, this was a pretty fancy study.

However, these data sources clearly showed temperature changes happening far below the surface. The researchers then matched these temperature readings with the location and timing of ocean eddies, using satellite images to track the movement of these massive currents. This allowed them to see how eddies influence temperature extremes in the deeper layers of the ocean.

In other words, it is a study of correlation and causation with statistical tests to ensure they were measuring what they thought they were.

So what did all these fancy instruments find?

Well, it turns out that eddies are responsible for about half of the marine heatwaves and cold spells happening in the ocean’s twilight zone. These powerful currents either push warm water down or lift cold water up, depending on their rotation.

As a consequence, in areas where eddies are more common, like the East Australian Current, the Kuroshio Current, and the Gulf Stream, temperature extremes are happening more frequently in the deeper layers of the ocean.

One of the most surprising findings was how common these temperature swings are below the surface, often happening independently from surface temperature events. “We found that over 80% of subsurface marine heatwaves and cold spells do not coincide with surface signals,” the researchers noted. This shows that the deep ocean can have its own separate climate events, which can go undetected if we rely only on surface temperature measurements.

But there’s more to this. The twilight zone is home to an incredible amount of marine life, including many species of fish that make daily migrations to the surface to feed. It’s a crucial habitat, not just for fish but also for other organisms like jellyfish, squid, and plankton.

As you can imagine, many of these species rely on stable conditions to survive. However, research shows that temperature extremes driven by eddies are disrupting these conditions, potentially stressing marine ecosystems.

“Eddy currents are the main driver for nearly half of all deep ocean heatwaves and cold spells,” the study found. This is important because warmer oceans produce stronger eddies, creating more intense heatwaves and cold spells.

As these extremes become more frequent, marine organisms living in these depths might struggle to adapt. But there’s more. Heat waves in the ocean can lower oxygen levels and reduce nutrient availability, which could lead to major shifts in marine ecosystems over time. A cascade of unfortunate events.

While surface marine heatwaves are getting a lot of attention due to their visible impacts — like coral bleaching — this research highlights that we need to be just as concerned about what’s happening below the surface.

The deep ocean is often overlooked, but it’s a critical part of the marine ecosystem, supporting an immense variety of life and critical for the whole marine food chain.

As the research team noted, the twilight zone “has more fish biomass than the rest of the ocean combined.” Of course, the findings suggest that the twilight zone, which plays a major role in the ocean’s food chain, could be under significant threat as these subsurface heatwaves and cold snaps become more intense.

Moving on, this research could help scientists predict where and when deep ocean heatwaves and cold snaps are likely to occur. Since eddy currents can be tracked by satellites, there’s potential to develop early warning systems for these events, helping us protect the most at-risk ecosystems. Just like we predict and prepare for weather events on land, we should also be ready for underwater events.

Understanding these deep ocean dynamics is a key step in managing the impacts of climate change on marine life.

By digging deeper — literally — we’re starting to get a clearer picture of our foundations. How much the ocean is changing beneath the surface as our planet warms, and what that might mean for the future of our oceans, has implications for all of us.