Researchers have observed a bizarre feature in the ocean’s water circulation that had thus far been confined to textbooks and research papers.
Whirlpools, also called eddies, each a few hundred kilometres wide are relatively common formations in the ocean. But for the first time, researchers have observed two whirlpools very close to each other as if paired, one spinning clockwise and the other counterclockwise.
“Eddies can be viewed as ships, transporting heat, salt and nutrients from one part of the ocean to the other,” Anirban Guha, an assistant professor at the department of mechanical engineering in IIT Kanpur, told The Wire. “They predominantly travel [from east to west] with a very slow speed – in the order of centimetres per second.
When two eddies link up, however, they can move ten-times as fast and in the opposite direction, too. So, as Chris Hughes, an oceanographer from the University of Liverpool and one of the researchers involved in the study, told Popular Science, “They carry water in unusual directions across the ocean.”
When he and his colleagues were poring over satellite data, it was this unusual combination of travel speed and direction that gave the eddies away. These eddies – called modons – remain stable and don’t break up because of the speed at which they’re able to move across the water, sort of like a bicycle that doesn’t tip over when it’s moving. “Almost all … eddies drift slowly westwards but this little feature was going quickly eastwards,” Hughes told Popular Science.
He and his colleagues spotted a pair of modons travel across the Tasman Sea, a distance of about 2,000 km, in six months.
Oceanographers predicted decades ago that modons could exist in the sea. The underlying math was elucidated by in the early 20th century by Sir Horace Lamb. That’s why modons are also called a Lamb dipole. However, it wouldn’t be until the 1970s that oceanographers would take them seriously.
In satellite data and imagery spanning the last 25 years, Hughes and co. found evidence of nine modons. Eight were found around Australia and one was found to the southwest of South Africa, in the Atlantic Ocean.
“The current work by the British researchers stands out because, even with not-so-great-resolution data, they were able to identify modons in the southern ocean,” said Guha.
“It turns out that when a clockwise and an anticlockwise eddy come [close] to form a couple, technically a vortex dipole, they together travel at a speed nearly 10-times faster than their individual speeds.”
The circulation of water and the mixing of temperatures and gases ensure that these elements are evenly distributed across the water. When they’re not, their imbalance can drive changes in the weather. This is one reason oceanographers look out for more stable structures, which can help keep things the way they are. Modons are one kind of structure. They derive their stability from the fact that its whirlpools’ tails are connected underwater, forming a giant ‘U’-shaped tube of swirling liquid.
The researchers speculate that modons might even act like a trap. “My thinking is that these linked, fast moving eddies could ‘suck up’ small marine creatures and carry them at high speed and for long distances across the ocean,” said Hughes. His team also suspects that some organisms might be adapting to the nutrient source readily available within the eddies. “It’s quite possible there are shoals of particular types of fish following these eddies for their special conditions,” according to him. Some of the satellite images he inspected showed coloured eddies, suggesting a bloom of plankton could be tumbling about inside the modon.
While it would be interesting to study them further, especially for their impact on the weather, the data is wanting. “The spatial and temporal resolution of satellite data [available for finding modons] is still quite coarse, and not very conducive for identifying modons,” according to Guha.
At the same time, what the British group was able to find – nine modons in the southern ocean over 24 years – suggests they might not be very disruptive. As Guha said, “If modons are exotic and geographically localised, then I don’t think they will greatly contribute to the global ocean circulation.”
It’s not like the little things about modon are fully understood, either. How do modons merge and dissipate? What happens when they stop moving?
The nine modons that have been identified were also probably the clearest. Since there is no sharp cut off to what may be considered a modon, there are many transient pairs of vortices that merit further scrutiny, particularly to the south and southwest of Australia and in the Agulhas Return Current region. We also need to find out if modons occur only in the southern hemisphere.
The paper was published in the journal Geophysical Research Letters on December 28, 2017.
Vishwam Sankaran is a freelance science writer.