By Robert Monroe
Imagine the ocean as a giant swimming pool — devoid of topographical features like seamounts and trenches and with smooth walls instead of jutting continental shelves or jagged coastlines.
If you’re in the community of oceanographers who model the large-scale circulation of the oceans, that’s pretty much how you have to imagine them. Their size and complexity have presented a stiff challenge to those who would dare to try to mimic on computers how water moves and understand ocean dynamics. The challenge is to write computer code sophisticated enough to capture the myriad variables that move a unit of water from one place to another. What ocean modelers have traditionally ended up with is something that looks like a rudimentary computer game like Pong when what they desire is the resolution of an Xbox.
But in a new age of supercomputing, ocean circulation modelers are making first steps in seeing their subject as it really is. Christopher Wolfe and Paola Cessi, physical oceanographers at Scripps Institution of Oceanography, UC San Diego, have come up with an explanation for the way water moves in layers between the poles. The researchers are taking advantage of a new ability to simulate ocean dynamics at a scale of a few kilometers.
Though that may still sound like a pixelated picture, its improved realism in portraying intermediate-sized phenomena such as large swirls known as eddies is allowing the researchers to revise long-standing theories of large-scale circulation, which in turn could help the world understand what keeps warm places warm and cold places cold.
Some would say the epiphany is happening not a moment too soon. There is increasing evidence of rapid melt-off of ice sheets in the world’s two biggest repositories, Antarctica in the south and Greenland to the north, spurring climate modelers to devise a number of what-if scenarios. The evidence has triggered a variety of doomsday theories that a freshwater dump would disrupt the climate patterns we’ve grown accustomed to, plunging temperate areas of the world, especially Europe, into frigidity.
Now Wolfe and Cessi have made enough progress to be able to advance theories of what two big puddles of fresh water at either end of the ocean would do to ocean circulation. In most of the scenarios they come up with, the effects on global climate would be significant.
“At this point, based on global climate predictions, circulation could either speed up or slow down or do nothing,” said Wolfe, a postdoctoral researcher. “That’s something we’d really like to know and that’s the question we’re trying to answer.”
OCEAN CIRCULATION 2.0
Before ocean circulation modelers can make predictions about patterns of water circulation in a changed world, they need to understand the fundamentals of how it works now. A new interpretation has emerged in the past 10 years.
The accepted view is that there is a wind-driven flow of water around the world’s oceans called meridional overturning circulation. It moves heat from the warm tropical oceans to the frigid poles.
Even though the Pacific Ocean basin is bigger, the Atlantic is where most of this action takes place. There the major current of warm water known as the North Atlantic Current flows in the topmost 1,000 meters (3,300 feet) of ocean. It crosses the north Atlantic and acts as a temperature regulator that keeps England a few degrees warmer than it would be otherwise.
The more complex part of the story is what happens at the deepest depths where actual field data are harder to come by. Oceanographers know that surface water gets heavier as it flows toward the North Pole; the water becomes denser as it cools and evaporation makes it saltier so the water becomes even more dense. It sinks as it approaches Greenland at the northernmost part of the Atlantic basin. It plunges toward the seafloor then U-turns southward. The water is stratified into bands of varying density along lines known as isopycnals.
Until recently the classic interpretation has been that water in the world’s ocean basins is more or less self-contained, the deepest water flows upward around the equator and begins the cycle anew. A growing number of oceanographers, however, have come to believe that Atlantic Ocean water that sank near Greenland actually travels the nearly 15,000-kilometer (9,300-mile) span of the Atlantic to the Antarctic Circumpolar Current region on the other end of the world. Cessi likens it to a freeway of north- and southbound lanes, one that doesn’t exist in the Pacific Ocean because of its different rainfall. In the Pacific, there is no northbound surface water, nor southbound deep water, so water at the bottom of that ocean doesn’t travel as freely.
In the Atlantic, southbound lanes of stratified water reach the Antarctic Circumpolar Current region, the only place in the world where the oceans meet unfettered by continents. In this violent roundabout, fierce winds over the ocean that give southern latitudes names like the Furious 50s blow with enough intensity to force deep water to the surface. Dense water that had been hugging the seafloor for most of the trip gets thrust upward when it approaches Antarctica. Bands of heavier and lighter water are suddenly alongside each other, defying gravity, until surface turbulence reshuffles the different kinds of water like cards in a deck and the reorganized water heads north again.
This perpetual commute between north and south can be interrupted by pulses of buoyant water at either pole due to fresh water melting from ice caps.
FROM MOLASSES TO MAPLE SYRUP
To visualize phenomena such as large-scale pulses of buoyant water, scientists need to replicate oceans in computer models. Lack of understanding and limitations in computing power means that most of the details required to truly understand the implications of such a melt-off have to be left out of the model. Most simulations have required the oceans to be represented as pools of molasses, where water moves on a millennial pace with no accounting of the swift rapidly changing currents that accompany all water movements.
Among the most common of its rapid movements is the eddy, a whirlpool-like swirl of water that punctuates otherwise orderly flows of water. Its size is the mesoscale, ranging from tens to hundreds of kilometers across. The atmospheric analog of eddies appears in nightly weather reports when the weathercaster shows a satellite image of a swirling low- or high-pressure system moving in. In the oceans, the movement of eddies reaches the seafloor, though their strongest velocity is usually within the top 1,000 meters (3,300 feet) of the surface.
If winds and differences in the buoyancy of water are what set oceans in motion to begin with, eddies are like the flywheels that keep the motion going. Without a realistic understanding of eddies, oceanographers can’t really simulate the oceans at the speeds at which water really moves. So Wolfe and Cessi elected to try to produce a computer simulation, using supercomputers at Lawrence Berkeley National Laboratory in Berkeley, Calif, Argonne National Laboratory and Oak Ridge National Laboratory. They obtained 20,000,000 CPU-hours and used a model that is highly faithful to the movement of eddies in real life. They also decided, however, to leave their computerized ocean in more or less the shape of a rectangular swimming pool and shrink its scale to about half its real size, creating what Cessi dubs a “hobbit ocean.” The computational power needed to simulate eddy activity and include a geographically-correct basin would require a devotion of resources still not available among the world’s supercomputers.
But Cessi and Wolfe say the high-resolution view of eddies produces a significantly more realistic view of how oceans move than anyone has been able to replicate so far. Already the two believe that there is sufficient evidence to suggest that large-scale circulation patterns adjust over decades or centuries rather than over thousands of years, which implies that changes in circulation are something that we could conceivably witness within a few generations rather than at some point in the distant future. Cessi notes with pride that the pair’s modeling approach has sped up the oceans from a molasses pace to something a little runnier, not real water yet but maybe more like maple syrup.
“Our contribution was to resolve scales as small as five kilometers,” said Cessi. “I don’t think anyone has done a calculation with such high resolution and for an extended period of time.”
The Scripps scientists chose this course after noticing that many oceanographers have in recent decades explored what would happen if Northern Hemisphere ice sheets were to suddenly melt and dump loads of freshwater into surrounding oceans. Doing so, they have concluded that an infusion of fresh water slows circulation in the Atlantic.
But for unknown reasons, few have considered the equally plausible scenario that a warming world would create a similar melt-off in Antarctica as well. The two discovered that if Antarctic melt produced a larger amount of freshwater, the circulation would speed up.
Recent observations suggest that these are not hypothetical scenarios. The opposing ice masses are melting at an accelerating rate. A 2009 analysis showed that in Greenland, the rate of annual mass loss increased from 137 gigatons per year in 2002-03 to 286 gigatons per year between 2007 and 2009. In Antarctica, the mass loss increased from 104 gigatons per year between 2002 and 2006 to 246 gigatons per year between 2006 and 2009.
A FULL TURN OF THE WHEEL
In the manner of most scientists, what Cessi and Wolfe understand best so far is what more they’d still like to nail down. They believe they can demonstrate that eddies alter the paths of water when strata return to the surface in the Antarctic Circumpolar Current. There is still not enough evidence on which is more likely: the meridional overturning circulation slowing down, which would stifle the delivery of warm water to northern Europe, or it speeding up, which would make parts of Europe warmer.
“We need a combination of increasing our understanding of what-if scenarios and observations that provide some idea of the rate of melting in both areas,” said Cessi.
The researchers haven’t been at it long enough to reach a bankable conclusion. Simulations with basins the size of the world’s oceans can simulate about a decade, which in Atlantic Ocean time probably describes scarcely one-tenth of a full cycle. As computers become more powerful, it might be possible to have a reliable forecast tool in less than 10 years or so. Along the way, the researchers hope to confirm their notion that there are two competing pools of buoyant water on the northern and southern ends of the Atlantic Ocean and then understand which of those pools is a bigger influence on circulation.
“We’ve increased the number of unknowns in the problem,” said Wolfe. “What we’ve done is take false certainty and replace it with accurate uncertainty. The whole project going forward in the next few years is to create actual certainty.”