Ocean eddies are growing

“The ocean temperatures east of Tasmania are some of the fastest rising in the world,” said Iain Suthers, a professor at the University of New South Wales (UNSW). Average winter sea temperatures have warmed to 12 degrees Celsius (54 degrees Fahrenheit). One result is that ocean eddies there are growing dramatically in both size and strength.

These satellite images present the complex qualities of cyclonic and anticyclonic eddies within Eddy Avenue, proving they are not simply cold core and warm core, respectively. The researchers used the Moderate Resolution Imaging Spectroradiometer (MODIS) sensor on the NASA Aqua satellite to track concentrations of chlorophyll a (shading). (Credit P. Oke)

Eddies are rotating blobs of water with warm or cold cores. They are the ocean’s high or low pressure systems, instrumental in transporting heat within the ocean. “They’re basically ocean weather,” said Peter Oke, a research scientist with the Commonwealth Scientific and Industrial Research Organisation (CSIRO) for Marine and Atmospheric Research.

Eddies form out of instabilities. Most result when the wavelike path of a current circles back onto itself, pinching off into spinning cylinders of water.

Swirling Ocean Eddies Are Vital To Ocean Life

Those vital eddies are self contained spinning worlds of intense biological and physical productivity. “We suspect that commercial fishermen know eddies well and truly. They can look for certain features and efficiently target their catch,” Oke who is part of a team studying them said. “We’re just filling in a bit of a gap.”

Subtle shifts in ocean temperature can significantly alter established ocean food chains. Like an underwater train, the East Australian Current (EAC) transports warm, low-nutrient waters from the Coral Sea southward into the Tasman Sea, where it floats above and displaces downward cold, nutrient-rich waters. Due to ocean warming it now extends further south by 350 kilometers (220 miles). “Species are being transported well outside of their range,” said Suthers.

Everett and his team set out to locate and quantify these vital eddies within the Tasman Sea, and then link ocean circulation to different biological elements: phytoplankton, zooplankton, and fisheries. Eddy Avenue was not part of the initial plan. “But when we started, Eddy Avenue jumped out,” said Jason Everett, a postdoctoral researcher at UNSW.

Located close to the southeast coast of Australia, Eddy Avenue has 20 to 30 percent more eddies than the surrounding waters. Here, these powerful vital eddies deviate from the global average with higher sea levels, faster rotations, and more chlorophyll, which means more nutrients to support a food chain.

High Speed Eddies

To understand the high incidence of eddies in Eddy Avenue, one has to understand the EAC. “In the Tasman Sea, the eddies get spun up quickly after the EAC leaves the coast,” Oke said. Typical eddies rotate at ten centimeters per second, but here they rotate at fifty centimeters per second, a slow walking speed.

“When the current separates from the coast, it gets complicated. It starts to wobble; it meanders. Rather than going in a relatively straight path like the Gulf Stream in the North Atlantic, it walks like a drunk man.” Not only does the EAC wobble, it U-turns. “Every boundary current has its own peculiarities. They’re like people,” Suthers said. “They have their own idiosyncrasies. The EAC is a bit anomalous.” About two-thirds of the current retroflects back up into the eastern Pacific, breaking up the EAC further. “It really is a current of eddies.” Such instabilities saturate Eddy Avenue with eddies.

Down underwater Salps in Australian Eddies

One commonly found eddy inhabitant the research team hauls in its fine mesh nets to are  salps.  If caught during their asexual solitary stage the team can keep them for only a few hours to determine their growth and fecundity.

These gelatinous, barrel-shaped filter feeders are supremely abundant, some claim they may be one of the most abundant forms of life in the sea, in their asexual stage they can release 240 buds in a week. They feed on a virtually unlimited carbon resource and thrive in rich phytoplankton areas.

Using satellite data from 1993 to 2008, the researchers charted the frequency and quantity of eddies within the Tasman Sea. They identified 30,000 eddies with over half being anticyclonic. The unproductive warm cores were expected to have low chlorophyll, while chlorophyll should have clouded the centers of cyclonic eddies. Sometimes this was the case, but the large number of eddies here complicated matters. “Now we’re starting to understand eddies aren’t simply warm or cold core,” Suthers said.

Sometimes cyclonic eddies had no cold core. As the EAC leaves the coast of Australia and breaks down into eddies, often a trace lingers—a fast slither weaving in and out of eddies. When it comes into contact with a cyclonic eddy, it floods it, capping it with warm water.

“The importance here is if you were to look at SST from satellite data, you might not see the cyclonic eddies,” Oke said. “It’s only by pulling other data types, the satellite altimetry and ocean color, that we can go ‘Ah that’s warm, but it’s still a cyclonic eddy.’”  Though this has happened before, the researchers had not seen it on this scale.

Suthers said, “Cyclonic eddies are far more involved. If you look at a pair of twins, the big bald twin is the anticyclonic eddy and then you’ve got the cyclonic twin that has a range of colors, sizes, and personalities. They’re far more biologically interesting.”

Click to enlarge

Both types of vital eddies interact with the continental shelf, but cyclonic eddies are able to entrain nutrient-rich water from the shelf, resulting in higher chlorophyll concentrations. Anticyclonic, for reasons yet undetermined, do not. “We didn’t expect that from the cyclonic eddies,” Everett said. “Our research points to two processes. You get uplifting, but closer to the coast within Eddy Avenue, there’s a second process: the entrainment of shelf water.”

Researchers once considered entrainment as a death trap, believing that when spawned fish were dragged from the coast, they would die. But entrainment provides a nutrient-rich environment with fewer predators. “Larval fish are growing faster and bigger within these smaller, coastal cyclonic eddies,” Everett said. “We’re in a neat position to see this in Eddy Avenue because of the number of eddies.”

All eddies propagate to the west. This is partially due to Earth’s rotation. To the west is Australia. So the eddies just bobble there. Bumping up against the coast, they sweep in high-nutrient water, over and over. It is a productive environment—little plankton incubators, if you like. “We have a nutrient source that is self contained, can endure for a long time, and be exploited by different fish populations,” Oke said.

For now the researchers are left with a bit of optimism. “Up until now, global climate models (GCMs) assumed that with global warming these would be less productive because the warmer layer of water would isolate upwelling and cap deep nutrient-rich water,” said Suthers. Eddy Avenue has unlocked another possibility. As currents increase, more energy will propagate south. “Eddies will either have to get bigger or there will be more of them,” Everett said. “We aren’t sure which at this stage.”

More eddies and bigger eddies point to the possibility of more chlorophyll, perhaps pulling more carbon out of the atmosphere into phytoplankton and sustaining fisheries. “That was something that came from this research,” Suthers said. “You bet, we’ve got a serious eddy production off the east coast of Australia and now it’s being incorporated, being realized, into GCMs.”

References
Everett, J. D., M. E. Baird, P. R. Oke, and I. M. Suthers. 2012. An avenue of eddies: Quantifying the biophysical properties of mesoscale eddies in the Tasman Sea. Geophysical Research Letters 39, L16608, doi:10.1029/2012GL053091.

NASA Ocean Biology Processing Group. MODIS Level 3 Ocean Color Web. 2012. Greenbelt, Maryland USA.