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April 26, 1999
Oceanographer develops simple model of ocean flows
Finding could improve understanding of global warming
PRINCETON, N.J. -- A Princeton researcher has developed a simple model for predicting how changes in one part of the ocean could affect ocean currents and climate around the globe.
The work, published in the March 26 issue of Science, could give scientists a better understanding of the greenhouse effect and global warming.
"You don't have to change ocean currents very much to get a big change in climate," says Anand Gnanadesikan, the author of the study. Without ocean currents, for example, northern Europe would not be as livable. The latitude of Nice, France, is the same as Portland, Maine, but it's much warmer. About half that temperature difference is caused by ocean currents that pick up warmth at the equator and then release it in the North. Yet researchers are still trying to develop a clear picture of what drives these currents.
Gnanadesikan has sketched that picture, and its basic elements are so simple that a scientist could guess how a certain event would affect the oceans and climate using a single equation, or no equation at all. That gives people in other fields, such as climate research and marine biology, an opportunity to see how ocean currents affect their work. Previous models were too complex and often yielded conflicting answers, Gnanadesikan says.
The Atlantic Ocean is made up of four main layers. The warm surface waters and the very cold bottom waters do not have much affect on ocean currents, according to Gnanadesikan. In between, however, there are two layers that are like giant underwater rivers: the upper is warm, "light" water that's not very salty; the lower is what's called North Atlantic deep water, which is a little colder and saltier and therefore a little heavier. The interaction between these middle layers drives much of the world's climate.
The scenario Gnanadesikan describes goes like this: Rain in the Southern Ocean near Antarctica creates a pool of "freshened," light water -- layer number two -- that gets blown northward by strong winds. As this water moves toward the equator, it gets warmer. When it gets even farther north, however, it starts giving up its heat, which gives Europe that important warming.
As the water cools, it sinks and becomes layer number three. This water starts flowing south again in the form of North Atlantic deep water. Some churns back upward on its way past the equator, but most does not rise until it returns to the Antarctic. The result is a big conveyor belt of heat that leaves the Northern Hemisphere much warmer -- and more populated -- than the Southern Hemisphere.
This picture was not always so clear. Understanding the flow of the ocean is a major research theme at the Geophysical Fluid Dynamics Laboratory, a US government lab located on Princeton's Forrestal Campus and funded by the National Oceanographic and Atmospheric Administration. The lab is closely affiliated with the University's Program in Atmospheric and Oceanic Sciences, where Gnanadesikan is a staff member.
Gnanadesikan's model builds on a large body of work that other GFDL researchers have developed over the past 30 years, and it exemplifies an approach to oceanography that is unique to GFDL and Princeton.
Most scientists studying ocean currents have taken a much more detail-oriented approach in which they develop a complex model for how a relatively small section of the ocean behaves. These models rely on observational data and might take into account such factors as regional geography, wave motion or temperature conditions.
The approach some at GFDL have taken is to use general principles of physics and fluid dynamics, treating the oceans as one big system. Gnanadesikan compares his approach to macro versus microeconomics.
"It's very important for people to tie together various strands of research and put it into a bigger picture," says Kirk Bryan, a senior visiting research scientist at Princeton and one of the earliest researchers in the field of ocean currents. "One problem with numerical simulations is that these models are so complex and unwieldy, it's hard to see what's going on," he says.
Gnanadesikan has done his share of thinking about the details. He was a physics major as an undergraduate at Princeton, but then went off to do his graduate work at the MIT-Woods Hole Oceanographic Institution Joint Program, where he was part of a group that went on ocean voyages to gather data. Now in his office at Princeton, where he has been since 1995, the only decoration is a spherical orange float with some mangled rope attached -- the remains of his first failed attempt to deploy a data-gathering buoy. "This is a reminder of my career as an observational oceanographer," he says. "You'll notice I'm now a numerical oceanographer."
His confidence with the "big picture" becomes apparent as he talks about his work. Back at home, he reaches under the table and snags his daughter's beach ball, which is printed like a globe, to illustrate how the oceans move. "You can have very tiny density differences that result in very large flows," he says, explaining that the amount of northern water sinking down, changing from light to dense, is more than 20 times the flow of all the world's rivers combined. The big question that Gnanadesikan's work answers is where all that water goes.
Devising models of ocean currents is only about a one third of Gnanadesikan's research. He spends another third of his time working out the details for how to translate physical models, such as his own, into mathematical representations that can be plugged into a computer that simulates ocean currents. The final portion of his work has to do with ocean biology, which is very dependent on currents. Deep ocean waters are very rich in nutrients; as they flow to the top, they fertilize the ocean from below, giving rise to plant and fish populations.
Gnanadesikan hopes that his new work will fuel discussions in all three areas. Bryan believes that it will. "There are certain details of it that I don't quite agree with," Bryan says. "But I think it will be important. It will stir up conversation."