NASA Satellite Spots Unique Ocean Eddy and a Bounty of Food for Fish

University of Hawaiʻi
Posted: Apr 17, 2001

Scientists using NASA and other satellite data discovered an unusuallong-lasting, whirlpool-like ocean eddy that generated a dramatic increasein the marine food supply off the Hawaiian coast in 1999.

The eddy, named Loretta, began spinning up in the Alenuihaha Channelbetween the islands of Hawaii and Maui during mid-May 1999 and maintaineda presence in the lee of the Hawaiian Islands until January 2000. Over the8-month period, the eddy's churning motion brought up a great amount ofnutrients from the ocean depths, enhancing the plankton population on theocean's surface, and providing a banquet for marine life.

Several organizations collaborated to track Loretta, and other Hawaiianeddies and their ecological benefits. The University of Hawai'i at Manoa,NASA and the National Oceanic and Atmospheric Administration's (NOAA) NationalMarine Fisheries Service (NMFS) integrated information from two independentsatellite sensors that measure sea surface temperature (SST) and ocean color.NASA's Sea-viewing Wide Field-of-View Sensor (SeaWiFS) satellite trackedocean chlorophyll, and NOAA's Geostationary Operational Environmental Satellite-10(GOES) satellite tracked sea surface temperatures. Data from shipboard measurementstaken aboard the NOAA ship Townsend Cromwell were also used to track Loretta.

"Eddies naturally occur in this locale for periods of several weeksto a few months, but Loretta persisted for 8 months according to satellitedata," said Bob Bidigare of the University of Hawaii. After January2000, Loretta started to move slowly westward, and eventually weakened beyonddetection, but not before bringing a tremendous amount of deep-sea nutrientsto the surface of the ocean

Eddies are usually 30-125 miles (50-200 km)in diameter, and resemblehurricanes in the water. Like hurricanes, each eddy is given a name to keepbetter track of it. In 1999, researchers named the eddy "Loretta."Around the Hawaiian Islands, eddies are caused when northeasterly tradewindsinteract with the topography of the islands. Eddies occur most frequentlyin the Alenuihaha Channel off the Kona coast of the Big Island of Hawaii,where they can be observed throughout the year.

Eddies bring an increase in organisms that comprise the marine food web,attracting fish and fishermen. The swirling motion of eddies cause nutrientsthat are normally found in colder, deeper waters to come to the surface.Normally surface waters are nutrient-limited, and when an eddy occurs thecold water upwelling substantially increases chlorophyll and plankton production,as it did with Loretta.

NASA's SeaWiFS satellite tracked Loretta's movement by monitoring chlorophyll.Phytoplankton are single-celled ocean plants, smaller than the size of apinhead that contain chlorophyll. Increases in phytoplankton cause higherlevels of the green chlorophyll pigment, which in turn change the colorof the ocean surface. Although microscopic, phytoplankton can bloom in suchlarge numbers that they can change the color of the ocean so much that theycan be measured from satellites.

By looking at the color of an area of the ocean, the concentration ofphytoplankton can be estimated. Because phytoplankton changes an ocean'scolor, they are ideal candidates for tracking eddies and currents, detectingpollution, and observing meteorological events. SeaWiFS generated 8-daycomposite pictures of the ocean color that showed scientists where the nutrientsand the eddy was located.

NOAA's Coast Watch program also monitored Loretta using imagery fromthe GOES-10 satellite to generate 3-day composites of sea surface temperatures.By watching where the colder water moved, they were able to track Loretta'smovements. According to Seki, "Some of the strongest temperature gradientsassociated with Loretta occurred during late August-early September 1999.The sea surface temperature in the center of Loretta was 23.5 degrees Celsius(74.3 degrees Fahrenheit), a lot cooler than the waters outside of the eddy."

Fishermen constantly monitor eddies using satellite data because theyare such large circulation features that they are difficult to see withthe naked eye. When an eddy is spotted, fishermen set their hooks and linesaround and through these features. Because of the increase in food, eddiesare known to increase the concentration of fish and thus, fishermen harvestgreater catches.

A paper authored by Michael Seki of the U.S. NMFS appears in the April15, 2001 issue of Geophysical Research Letters, and details the increasedplankton observed in response to eddies in the open ocean near Hawaii. Co-authorsof the paper include Michael Seki, Jeffrey Polovina, and Russell Brainardof NMFS, Honolulu Laboratory; Robert Bidigare and Carrie Leonard of theUniversity of Hawaii, Department of Oceanography; and David Foley of theJoint Institute of Marine and Atmospheric Research, University of Hawaiiand NOAA.

This project is collaborative research effort between the NMFS and NASAfunded projects. The study was also partially supported by the Pelagic FisheriesResearch Program administered through the University of Hawaii, School ofOcean and Earth Science and Technology.

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