Unexpected New Striations in the Earth's Ocean Currents.

Sailors and scientists have been mapping ocean currents for centuries, but it turns out they’ve missed something big. How big? The entire ocean is striped with 100-mile-wide bands of slow-moving water that extend right down to the seafloor, according to a recent study.

The currents appear as jets, a few hundred kilometers wide, extending over thousands of kilometers and flowing in an alternating more or less east-west direction.

Scientists have discovered a zebra-stripe pattern of deep, wide and slow currents that cut east-west across the planet's oceans, each like a plodding conveyer belt at the airport passenger terminal.

The previously unsuspected currents stand in sharp contrast to the heat- and wind-driven express trains such as the Gulf Stream, which typically flow in a circular pattern.

“Our finding was so unbelievable that our first proposal submitted to the National Science Foundation failed miserably because most reviewers said ‘You cannot study what does not exist,’” Maximenko said. “The striations are like ghosts. To see them one needs to believe in them. No doubt, armed with our hint, scientists will start finding all kinds of striations all around the world.”

Some scientists initially dismissed the stripes as statistical artifacts, but Maximenko’s team dug deeper, looking for a similar pattern in water temperature measurements from two test areas in the Pacific.

It may seem hard to believe, but the oceans have stripes. These stripes are not visible without looking very closely, but they are visible through their effect on currents, temperatures, and sea surface heights. They were first spotted in a careful analysis of the Mean Dynamic Ocean Topography (MDOT) dataset, but had to be confirmed by looking directly at ocean buoy and vertical temperature profile data.

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Finding these stripes was difficult because they are very subtle features that are superimposed on much larger ocean current, topography, and temperature fields. The stripes have velocities around 1-1.5cm/s, while major ocean currents often travel at 40-50cm/s. The change in sea surface height from one stripe to the next is roughly four centimeters—globally, the average sea surface height varies by one to two meters. Likewise, the temperature at a depth of 100 meters varied by 12 degrees Celsius in one study area, while the variation across stripes was approximately one degree.

So how did they find these stripes? Essentially, they just filtered out any larger spatial scale features. Most changes in sea surface height, velocity, and temperature occur over thousands of kilometers. By using two consecutive high-pass filters, the team was able to remove the larger scale features.

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[W]hile the exact cause of these features is unknown at present, they are present in the ocean general circulation model run on the Earth Simulator supercomputer in Japan. By running the same high pass filter over the model output they were able to isolate the same features they saw in the ocean data.

Nikolai A. Maximenko of the University of Hawaii at Manoa and colleagues developed a precise new method for measuring the topography of the ocean surface by combining data from satellites and from the movements of more than 10,000 drifting oceanographic buoys. In doing so, the team generated detailed maps, in which they first noticed the peculiar striations.

Using data from satellites and drifting buoys, the scientists found that weak but persistent currents run horizontally across oceans worldwide, moving particles east or west.

Niiler credits the long-term and comprehensive ocean current measurements made over more than 20 years by the Global Drifter Program, now a network of more than 1,300 drifting buoys designed by him and administered by the National Oceanic and Atmospheric Administration (NOAA) for detecting these new current patterns on a global basis.

Between 1992 and 2003, Peter Niiler of the Scripps Institution of Oceanography in San Diego, California, and colleagues collected data from more than 10,000 drifting ocean buoys, which they tracked with satellites. As expected, the buoys’ movements were influenced mainly by known global currents, which are driven by wind and by differences in the temperature and salinity of seawater. But when the team analysed the data, it emerged that something else had been subtly influencing the buoys’ paths. It turned out that there were alternating strips of water running eastward or westward, a bit like parallel moving sidewalks. Niiler recalls his reaction: “My God, we’ve never seen these before.”

Indeed, though barely detectable, the striated currents are real. They flow past each other in opposing directions at 130 feet per hour—just one-tenth to one-hundredth the speed of major ocean currents—and subtle changes in temperature demarcate their boundaries.

Dozens of previous research missions in those areas missed the currents, Maximenko said.

One reason might be their weakness.

With a speed of about .02 mph or 1 centimeter a second, they are dwarfed by normal ocean currents and eddies, some of which have speeds of about 30 centimeters a second near Hawaii.

Scientists also found the currents travel nearly half a mile down, possibly even reaching the sea floor. While feeble, the currents are about 124 miles wide and travel thousands of miles.

These stripes are interesting for a number of other reasons. For one thing, they persist to at least 700 meters deep based on the temperature profile data ... while we do not have measured data deep enough to verify this, the model suggests that these features are coherent all the way to the sea floor.

Maximenko says a new computer model has corroborated some features of the observed striations, but his team is still mystified by their orientation, location, and strength.

What causes the striped flows remains a puzzle. “They are a fascinating new aspect to the ocean’s circulation, but the jury is still out on the mechanisms leading to their formation,” says Geoff Vallis of the Geophysical Fluid Dynamics Laboratory at Princeton University.

The precise cause of these stripes remains a mystery, although the authors have a few thoughts on the matter. They suggest that the stripes may be caused by a form of inertial waves known as Rossby waves that are driven by the coriolis force. They may be a more general phenomenon, too, as others have compared these stripes to the cloud bands that are observed in the atmosphere of Jupiter.

Their cause remains a mystery.

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Scientists are still trying to explain their existence. One theory compares the striations to cloud bands on Jupiter that form from turbulence in the atmosphere. Maximenko considers this theory unlikely, since land masses interfere in the ocean.

The scientists plan another study, funded by NASA and with more scientists added to the team, that will try to determine the cause of the stripes and their effects.

The new maps of ocean circulation produced by a combination of drifter and satellite measurements will eventually be the yardstick for judging the accuracy of the circulation patterns portrayed by climate and ocean ecosystem models -a major deficiency in current simulations-and to generate substantially more reliable forecast products in climate and ecosystem management. Niiler noted, for example, that there are a large number of computer models that can simulate equatorial currents, but fail in the attempt to accurately simulate the meandering flow of the California Current and the striations that exude from it.

“This research presents the next challenge in ocean modeling,” says Niiler. “I’m looking forward to the day when we can correctly portray most ocean circulation systems with all climate and ecosystem models.”

The discovery is important, he says, because even weak currents can have large effects on global climate and on the flow of food and creatures in the oceans.

Besides uncovering a surprising and little-understood ocean feature, the findings could significantly improve high-resolution models that help researchers understand trends in climate and in marine ecosystems, the scientists say.

The spatial high-resolution view of the linkage between the striations and the larger scale patterns of currents could improve predictions of ocean temperatures and hurricane paths.

The linkage between the striations and the larger scale patterns of currents could improve predictions of sea temperatures and hurricane paths, the scientists say. The striations also delineate ocean regions where uptake of carbon dioxide is greatest, they add.