Satellite sensors
giving scientists
better look at eddy
off Big Island

By Helen Altonn
Star-Bulletin

A strong eddy named Loretta for "dogged determination and tenacity" has been spinning more than 90 days off the Big Island.

"Essentially, it's like a hurricane in the ocean," says Jeffrey Polovina, ecosystem environmental investigations director at the National Marine Fisheries Service's Honolulu Laboratory.

Hawaii Coastwatch coordinator Dave Foley, Fisheries Service scientist with the Joint Institute of Marine and Atmospheric Research, says the effects of the eddy on the ocean are similar to those of a hurricane over land.

"It's one of the most dramatic things," he said.

Eddies frequently form off the Kona Coast, spit up by currents and winds around the northern tip of the Big Island, Polovina said.

The scientists said they're getting a good look at them for the first time because of two new satellite sensors.

One collects high resolution sea surface temperature data. Another measures phytoplankton, single cell plants in the ocean's food chain.

Foley is collaborating with University of Hawaii oceanographer Bob Bidigare, who has NASA funding to study the eddies.

"We sort of specialize in combining physics and biology," Foley said.

The eddies are spinning cylinders of water, usually 50 to 80 miles in diameter, that provide a feeding and nursery habitat for billfish, tunas and larvae of coral reef fish, the scientists said.

Loretta is about 100 miles across, Polovina said. Its edge fluctuates but is probably about 20 miles off Kona, he said.

It is spinning counterclockwise and pumping nutrient-rich water to the surface that's about 2 degrees colder in the center than at the edge, he said.

About six or so eddies occur annually, typically lasting about 20 days, Foley said. "On that basis, we figure the ocean is in that state maybe 50 percent of the time. Now it's going on strong for 90 days."

"What's curious about this one," Polovina said, "is it's sort of wobbled around, gone to Kahoolawe and come back. It has never broken away completely. ... We have a persistent tradewind field that keeps intensifying this thing.

"Maybe it's not that unusual, but we were never able to watch it as long as we have before."

The scientists are anxious to see how fish are affected by the eddy, Polovina said, adding that it must provide productive fishing grounds around the edges.

"I think if the billfish tournament was on this year, which unfortunately it's not, there would be some interesting data come out," he said.

Foley changed Eddy 9903's name to Loretta and decided to give other eddies names as part of an EddyWatch service he provides for the joint institute operated by the University of Hawaii and National Oceanic and Atmospheric Administration.

He receives and distributes satellite data to decision-makers and resource managers.

Instead of getting only two pictures daily from the polar-orbiters, Foley said images can be obtained every half hour with a new GOES weather satellite sensor that reduces cloud contamination.

"It's amazing. We can string together two day's worth and come up with a composite image," he said.

Phytoplankton concentrations and pigments are measured with SeaWIFS (sea viewing wide field of view sensor), on a SeaStar satellite supported by NASA.

Chlorophyll was measured in the phytoplankton at 0.5 milligrams per cubic meter, Foley said. Typically, in Hawaiian waters, it would be 0.075, so it's almost a factor of 10 higher, he pointed out.

"The water is more like coastal California where it is all green ... Water here (normally) is very blue because not much is living in it."

With the chlorophyll level at 0.5, he said, "it is much more fertile. A lot more life is going on in general."

He said bigger fish tend to like clear water and feed and hang out on the eddy's edges. There are anecdotal reports that "fish were jumping out of the water" when the eddy moved offshore at Kona, he said.

Scientists are interested in the eddy spinning up water, like Loretta, because of nutrients it's delivering to phytoplankton in the lighted zone, Bidigare said.

Sea eddy What: EddyWatch images and information. Web site: http://coastwatch.nmfs.hawaii.edu/