An age-old current in the deep Atlantic off Brazil poses an intriguing new puzzle.
This article first appeared in the Woods Hole Oceanographic Institution's Currents, Volume 4, Number 2, 1995.
Physical oceanographer Mindy Hall is staring at a vexing chart in an open folder in her office. The chart shows a jagged line climbing from left to right, a saw-toothed stairstep that traces deep water temperatures taken off equatorial Brazil. Its message is clear: between late 1992 and early 1994, the temperature of the Atlantic's deepest current showed an unexplained rise.
"The warming trend was totally unanticipated," Hall says, looking at the chart's irregular upward curve. "That's curious," she says soberly. "That's very curious."
Curious, indeed. The world's ocean currents and the amount of heat they hold play an integral role in the Earth's climate. This deep water, which was last at the surface almost five to ten years ago, was believed for decades to hold a steady temperature. This new, surprising temperature rise could either be a normal oscillation, just discovered, or a disturbing signal of global climate change.
Since first coming to WHOI in 1979, Hall, 37, has co-authored a landmark method for measuring how the ocean transports heat and has helped map the structure of the Gulf Stream. She has sketched the path of eddies in the Eastern North Pacific and plotted the intricate flows of the tropical Atlantic. In her outside interests, too, Hall is intrigued by things she can sense but cannot be see. She has a wide knowledge of modern and classical music, is an accomplished jazz saxophonist, and has a connoisseur's appreciation of fine wine.
But deciphering the enigmas of enormous, hidden water masses is a challenge Hall finds compelling. "How do you describe that kind of thing?" she says of her fascination with the deep sea. "It's just something that grabs me. It's interesting because it's not something that's sitting on the surface that I can observe."
Oceanographers sometimes liken the sea's circulation to a conveyor belt. In this simplified description, warm surface water in the Atlantic flows north, cools, sinks, flows south at depth, rounds Africa into the Indian and Pacific Oceans, then rises to the surface and journeys back to the Atlantic. As the "conveyor belt" circles the globe, it plays an important role in the global climate by converting warm water to cold and liberating heat to the atmosphere.
The "conveyor belt" model is accurate in its broadest outlines, but in detail the water's flow is far more complicated. In 1989 Hall was part of a team aboard WHOI's R/V Oceanus on a 2,400-mile trip across the Atlantic from Senegal to French Guiana. The ship stopped eighty-five times to take 1,500 water samples and over 200,000 conductivity, temperature, and depth measurements to within forty feet of the ocean bottom. For the next three years, Hall and WHOI/MIT graduate student Marjorie Friedrichs wove the data into a detailed profile of the water column along the ship's track. By comparing the results to previous studies, Hall and Friedrichs were able to describe eight ocean current layers and found that there are large, deep, cyclonic eddies on each side of the Mid-Atlantic Ridge. Some of the currents flow north, some flow south, some flow east and west through gaps in the Ridge. Others split, spin into gyres, branch and loop back upon themselves.
For the last eight months Hall has been examining data from the Deep Basin Experiment, a multi-year, international project that is mapping the water and heat flowing in and out of the Southern Hemisphere's Brazil Basin. The Deep Basin researchers are setting current meters and thermometers in passages that lead into the basin, releasing over two hundred floats to track the currents' paths, and taking extensive hydrographic samples in a broad grid over the basin.
"We hope to evaluate the mixing and diffusion rates in the deep water masses and where heat diffusion actually occurs," Hall says. "Does it occur mostly in the passages or over the entire basin? It might occur in the passages because the currents tend to be somewhat stronger there, and they might tumble around a bit and mix the water more efficiently."
Hall's data comes from current meters and thermometers deployed for eighteen months across a steep-sided 150-mile wide passage on the equator. The instruments, set between 9,750 and 14,500 feet deep, revealed that the currents speed up in the constricted passage but remain highly stratified, with fast, steady flows moving in opposite directions to each other.
The deepest current in the trough is the Antarctic Bottom Water (AABW). It forms at the surface of Antarctica's Weddell Sea when freezing sea water releases salt into the surrounding water. This near-freezing water - dense with its new load of salt - sinks, slides down Antarctica's continental shelf, and is pushed to the sea floor as more dense water forms above it. It then moves north like a sluggish, determined snake, wending its way steadily along the bottom.
Although AABW is the smallest layer at the equator, it makes up fifteen percent of the total and flows through the passage at 2.1 million cubic meters per second, a volume of water one hundred times larger than the outflow of the Mississippi River. But the new measurements show AABW's flow may be changing. During the experiment, its temperature rose at a rate of .07ºC over a thousand days, matching a similar rise observed 2,000 miles to the south by WHOI scientist Nelson Hogg. These are the first changes recorded in the AABW in over twenty years of monitoring.
"We don't really know why that should occur," Hall says. Unfortunately, the instruments have been removed from the water and there are no plans to replace them. "There's not a lot of money out there for this kind of work."
For the foreseeable future the puzzle will remain.
"Like all good oceanographers I was always starry-eyed about the ocean," Hall recalls. A native of McLean, Virginia, she spent summers on the Delaware shore and studied with oceanographer Orin Pilkney at Duke University. His enthusiasm inspired her; when she learned he was looking for volunteers for a Mediterranean research cruise in 1978, she hurried to sign up.
There was no pay, the food was bad, and the work - hauling up a fifty-foot piston corer full of ocean bottom mud - was hard. "I had no idea what a bad deal this was," she says with a grin. "But as far as I was concerned, this was the greatest thing I could do."
Two years later Hall was a WHOI summer student fellow, working with scientist Harry Bryden. Their first project established a new method for calculating ocean heat transport. "At the time I didn't appreciate how clever it was," Hall remembers. "Harry always claimed 'Oh, this paper is going to make you famous.' "Bryden was right; the calculation became a milestone in physical oceanography and has proven so indispensable that it has earned its own name: the Hall-Bryden method.
Hall went on to earn her doctorate in the MIT/WHOI Joint Program in Oceanography. She is married to WHOI Associate Scientist Larry Pratt, a physical oceanographer who came to WHOI as a student the same summer she did. Their offices are now across the hall from each other. But homelife with scientists is nothing new for Hall. Her grandfather and father were physical chemists, her mother has an accounting degree, and her two sisters earned Ph.D'.s from MIT in operations research. And being a woman in a male-dominated field doesn't faze her, thanks to years of training: she routinely outshone all the men in her high school math and physics classes, and "I played saxophone in a twenty-two piece jazz band that was entirely men except for me. And they weren't the types of men who were in my calculus class."
Hall still plays a mean alto sax and would love to rehearse regularly with a small jazz group, but she doesn't have the time. What with writing grant proposals, going to sea, sending instruments to the bottom, making observations, recording data, and then spending a few years figuring out what it all means, Halls days are filled to the brim. And though there's no other way to unravel the ocean's currents, the process has its benefits.
Hall says, "There are certainly times at sea that can't be matched by anything on shore because of the beauty of the surroundings, meaning warm, fine weather, and sunrises and sunsets. They all look the same when you take their pictures, but somehow you remember every single one."
Copyright © 2002 Tom Gidwitz