Thousands of metres below the ocean's surface, there are tiny waves moving through the water.
Much like breaking waves at the beach, these small waves within the ocean must eventually break. When they do, they create turbulence and mixing, similar to what you feel from a big wave breaking on the beach.
This might seem far removed from your everyday life. In fact, for a long time scientists have assumed this deep ocean turbulence only mattered over long time scales - that is, centuries to millennia.
But our new research, published in Nature Communications, shows this isn't always the case. In fact, what happens deep below the ocean's surface can change what happens above it - even over the course of a single year.
Yet the tools scientists use to understand how the climate is changing do not currently consider the effects of these tiny, crucial movements.
Studying different scales
For our study, we used a combination of previously collected physical and chemical measurements to examine the many different scales on which deep ocean turbulence shapes the global climate system, with a particular focus on short timescale impacts.
Chlorofluorocarbons (CFCs), chemicals once used in refrigerants and aerosols before being banned in the 1980s, entered the ocean from the atmosphere at a known time and rate. Because CFCs don't occur naturally in seawater, their presence in the ocean is only possible through contact with the atmosphere and subsequent transport by ocean currents and mixing.
By measuring CFC concentrations at depth today, we can calculate how much time has elapsed since these deep waters last mixed with the surface , and how quickly they moved around the globe. In turn, this gives us a better understanding of how heat, carbon and nutrients are transferred between the atmosphere and ocean, and how they are transported.
In just 40 years, some deep waters have transported CFCs from Antarctica to the mid-Pacific and north Indian Ocean.
More targeted experiments used a dye, physically injected into the ocean at a known location and depth, to track the transport and movement of ocean waters directly.
In one such experiment, dye was injected into a deep canyon in the Rockall Trough, near the United Kingdom. Rather than simply dispersing as expected, the dye rose towards the ocean surface, climbing as much as 100 metres a day .
A missing piece of the puzzle
Understanding this small-scale turbulence is crucial for several reasons.
For one, nutrients such as nitrate and phosphate underpin the marine food web. And if they're not getting pulled from the deep ocean to the surface, that web could collapse.
That would devastate entire ecosystems, as well as global fisheries. In turn, it would have a significant impact on global food security.
The way that heat is transferred from the deep ocean to shallower waters and back also affects how Arctic and Antarctic ice melts. And that in turn affects sea level rise, storm intensity and flooding levels around the world.
However, right now global climate models do not do a very good job at capturing these small-scale processes. In fact, when we compared real world measurements from the CFCs and the injected dyes against our model predictions, a clear difference emerged: climate models significantly underestimated how much mixing, and how much vertical movement of water, is actually happening.
That's because they must estimate the effects of small-scale processes such as deep ocean turbulence using relatively simple approximations - a method scientists call parameterisations. Many of these parameterisations date back to the 1990s.
The way clouds form is another process where tiny physical events have outsized impacts on the climate - and this too is a problem for climate models, for much the same reasons.
Improving our models
Climate models should be updated with new parameterisations that take into account our improved theoretical understanding of deep ocean mixing. That would make them even more useful for understanding our climate - and informing our decisions about the future.
Observing small-scale mixing - the microphysics of the ocean - is still challenging. But we have made significant progress over the past decade.
Thanks to regional and global observation programs, and advances in high-performance computing, our understanding of mixing and its larger-scale impacts has evolved rapidly.
But significant obstacles remain in the way of fully unravelling how it impacts the climate.
As mixing observations are still very rare, we need to find ways to overcome this observational bottleneck, and target resources to where they can accelerate progress fastest.
Elizabeth Ellison receives funding from the National Environmental Science Programme.
Laura Cimoli receives funding from Schmidt Sciences LCC and the UK Advanced Research and Invention Agency.
