Hunting A Tempest in the Deep: The Story of the Mid-Ocean Dynamics Experiment


Floating west of Bermuda on a clear day in 1959, a group of oceanographers noticed something odd. Henry Stommel, James Crease, and John Swallow, aboard the navy-hulled research ship Aries, had just deployed a swarm of oceanographic floats at different depths. Swallow had designed the floats and thought he knew what they would do: drift slowly north on leisurely currents believed to dominate the deep sea. Yet almost immediately, the floats began moving in every direction, clipping along at speeds ten times more than anticipated.

Their test had failed—and yet, it was “the most fruitful oceanographic experiment of the decade,” said oceanographer Alan Robinson. The Aries scientists didn’t yet know it, but they had discovered mesoscale eddies: massive, fast-moving ocean “storms,” as large as 300 miles wide.

Oceanographers were intrigued. Over the next decade, they pondered how they might capture an eddy—measure it, map it, and trace its source. To do this, they had to conduct an experiment unlike any they’d ever known: MODE, the Mid-Ocean Dynamics Experiment.

E.W. Seabrook Hull, in the October 1970 Ocean Science News, called MODE “a task of scientific consequence for which the nation presently has neither the knowledge nor the quantity and excellence of instrumentation necessary.” John Swallow’s floats, for example, required a ship nearby to track them, making them too limiting for a long-term experiment. The MODE scientists needed a set of tools that did not exist, tools they had to invent from scratch.

Perhaps the most daunting prospect was the collaboration the experiment demanded. Creating a complete oceanographic picture of an eddy required scientists from different places and different branches of the field, each with their own approach.

“We’re independent people, anxious to protect our individuality and freedoms,” said Stommel, then at MIT. “We’re not sure we like this kind of big science. We’re not used to it.” Despite their trepidation, the scientists knew that a mid-ocean eddy was quarry too big to capture alone.

Planning for MODE was launched in 1971 and proposals began flooding in. Ideas ranged widely, from upgrades of common oceanographic tools to wild new methods of measurement. One proposal suggested measuring phytoplankton productivity by dropping Secchi disks, which measure water transparency, from a nuclear-powered blimp.

Among the bustle of preparation, there was room for doubt—time for everyone to wonder if their new instruments would work, if their data would make sense, if the eddies would even be there. One scientist, speaking to a documentary crew, said he had nightmares about not getting his instruments back. Another compared the experiment to “trying to catch a whale with a kitchen sieve.”

“We may just have got it wrong,” researcher Francis Bretherton told the camera. “And if we’re wrong, and the real eddies are only half the size we think they are, we’re going to come back at the end and…” he looked away for a moment before grinning. “I think I’ll go to South America.”

By March 1973, after “four years of talking, planning, testing, arguing and worrying,” as Stommel described it, four ships steamed from Massachusetts, Rhode Island, Florida, and England, headed for their eddy hunting grounds: a piece of ocean southwest of Bermuda that looked much like any other. To that patch of blue, scientists brought fifty of the Swallow floats that had first detected eddies, but also their next generation: SOFAR floats, tracked over long ranges by acoustic communication. With them was a set of instruments capable of detecting currents, bottom pressures, temperatures, and water composition, many of these devices going to sea for the first time.

As the scientists had feared, not everything went smoothly. Some instruments failed; several were lost at sea; some of the data were unintelligible.

However, what did come back brought word of something incredible. Rushing right below the ships, the data said, was indeed what MODE had been chasing: a tempest in the deep, stronger than any thunderstorm and with a thousand times its lifespan. The experimenters had netted a mesoscale eddy.

In August 1971, the MODE scientific council wrote that “it is highly probably that mesoscale eddy processes are of vital importance.” Presciently, they also made a prediction: that understanding eddies would “advance our knowledge of, and ability to cope with, world climate.” Thanks to MODE, we know today that mesoscale eddies mix salt, nutrients, and heat through the ocean. They carry warm water from the surface of the ocean to the deep sea, and whisk it from the tropics to the poles, helping regulate our climate. Without mesoscale eddies, subtropical and temperate latitudes would likely be much chillier places.

In addition to the knowledge MODE produced, its scientists discovered something else: ”a way to work together…each contributing his individual skills to a common goal,” said Stommel. “The results seem to be much greater than each of us might have done in his own piece of ocean, in his own time.”

Together, the researchers found, they could capture an ocean storm in a bottle—and when poured out, reveal a piece of the watery workings of our planet.