Ocean circulation is one of the big mysteries in our quest to predict future climate changes. We know that oceans drive carbon cycles and weather, but how quickly? And in what ways? A massive new study has provided new answers.
What's incredible about this study of the powerful Southern Ocean's circulation, called DIMES, is that scientists were able to map the exact pathways of its eddies over several years. They poured a harmless, inert chemical over a mile of the sea bottom, and tracked its spread. You can see in the video above how that chemical spread over just one year.
This study will improve our ability to predict climate changes dramatically because it maps what might happen to other substances — like, say, carbon — as it drifts through the oceans.
The Southern Ocean plays an outsize role in containing global warming, swallowing an estimated 10 percent of the heat-trapping carbon dioxide that humans pour into the atmosphere. But the ribbon of water surrounding Antarctica may be absorbing less carbon than it used to, a study in the journal Science suggested in February, possibly because strengthened winds are dredging up more sunken carbon from the seafloor and causing it to saturate the surface waters. Because subtle changes can trigger a feedback loop in fluid dynamics, some researchers think the Southern Ocean could eventually switch from absorbing carbon dioxide to emitting it (as may have occurred in the ancient past), which would further escalate global temperatures.
The Southern Ocean has a powerful effect on Earth’s climate because it “provides a connection between the atmosphere and the deep ocean,” said Andrea Burke, a marine chemist doing postdoctoral work at California Institute of Technology who is not involved with DIMES. It circles Antarctica, enabling surface winds to drive it eastward in a continuous loop. The Antarctic Circumpolar Current, as it’s called, has an average or “mean flow,” while buildups of surplus energy erupt into eddies — circular currents tens of miles across that stir the water and, in a feedback process, reinforce the mean flow.
Because cold, dense water is farther below the ocean’s surface toward the equator than near Antarctica, ocean layers of constant density slope upward as one moves north to south across the Southern Ocean. Eddies and the mean flow draw water from the depths to the surface along these southward inclines, then drive it down again as it moves northward — a conveyor belt called an “overturning circulation” that scientists say is the biggest on Earth.
These circulations conspire to make the Southern Ocean a remarkably efficient absorber of greenhouse gases, which are swallowed at the surface and channeled to the seafloor. And as a driver of global ocean currents, the Southern Ocean bolsters the impact of the other oceans on the climate, too.
But because of the complexity of ocean dynamics, climate change effects — strengthening surface winds (also caused by the hole in the ozone layer) and the 0.8 degrees C (1.4 degrees F) rise in average global temperatures since the start of the Industrial Revolution, for example — could drastically alter these circulations decades from now.
“Understanding the feedbacks between the mean flow and the eddies is critical to understanding future climate change,” said Emily Shuckburgh, an applied mathematician at the British Antarctic Survey and a DIMES principal investigator whose research over the past decade has highlighted the complex role played by eddies in ocean dynamics.