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SATELLITE
SPOTS UNIQUE OCEAN EDDY AND A BOUNTY OF FOOD FOR FISH
Scientists
using satellite data discovered an unusual long-lasting, whirlpool-like
ocean eddy that generated a dramatic increase in the marine
food supply off the Hawaiian coast in 1999.
The
eddy, named Loretta, began spinning up in the Alenuihaha Channel
between the islands of Hawaii and Maui during mid-May 1999
and maintained a presence in the lee of the Hawaiian Islands
until January 2000. Over the 8-month period, the eddy’s churning
motion brought up a great amount of nutrients from the ocean
depths, enhancing the plankton population on the ocean’s surface,
and providing a banquet for marine life.
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1: Satellite remote sensing of the long-lived eddy Loretta.
Two-day composite of GOES sea-surface temperature during 3-4
September 1999 (LEFT) and eight-day composite of SeaWiFS chlorophyll
during 29 August - 5 September 1999 (RIGHT). Click on
picture to enlarge image.
Several
organizations collaborated to track Loretta, and other Hawaiian
eddies and their ecological benefits. The University of Hawaii,
NASA and the National Oceanic and Atmospheric Administration’s
(NOAA) National Marine Fisheries Service (NMFS) integrated
information from two independent satellite sensors that measure
sea surface temperature (SST) and ocean color. NASA’s Sea-viewing
Wide Field-of-View Sensor (SeaWiFS) satellite tracked ocean
chlorophyll, and NOAA’s Geostationary Operational Environmental
Satellite-10 (GOES) satellite tracked sea surface temperatures.
Data from shipboard measurements taken aboard the NOAA ship
Townsend Cromwell were also used to track Loretta.
"Eddies
naturally occur in this locale for periods of several weeks
to a few months, but Loretta persisted for 8 months according
to satellite data," said Bob Bidigare of the University
of Hawaii. After January 2000, Loretta started to move slowly
westward, and eventually weakened beyond detection, but not
before bringing a tremendous amount of deep-sea nutrients
to the surface of the ocean.
Eddies
are usually 30-125 miles (50-200 km) in diameter, and resemble
hurricanes in the water. Like hurricanes, each eddy is given
a name to keep better track of it. In 1999, researchers named
the eddy "Loretta." Around the Hawaiian Islands,
eddies are caused when northeasterly tradewinds interact with
the topography of the islands. Eddies occur most frequently
in the Alenuihaha Channel off the Kona coast of the Big Island
of Hawaii, where they can be observed throughout the year.
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Image
2: Conceptual diagram showing the modulation of sea
surface temperature by variations of wind speed in the lee
of the islands (graphics by Brooks Bays). The yellow arrows
represent intensified winds in the channels, yielding cooler
surface temperatures (light blue); in the calm lee, warmer
surface temperatures are observed (pink). These variations
of wind speed induce divergent and convergent surface currents
(horizontal blue arrows), which in turn lift or depress the
thermocline (vertical blue arrows), eventually leading to
the formation of clockwise (anticyclonic) and counter-clockwise
(cyclonic) eddies (gray curved arrows). Click on pic
to slightly enlarge image. Copyrighted image courtesy
of Dr. Pierre Flament of the Department of Oceanography at
the University of Hawaii.
Eddies bring an increase in organisms that comprise the marine
food web, attracting fish and fishermen. The swirling motion
of eddies cause nutrients that are normally found in colder,
deeper waters to come to the surface. Normally surface waters
are nutrient-limited, and when an eddy occurs the cold water
upwelling substantially increases chlorophyll and plankton
production, as it did with Loretta.
The
SeaWiFS satellite tracked Loretta’s movement by monitoring
chlorophyll. Phytoplankton are single-celled ocean plants,
smaller than the size of a pinhead that contain chlorophyll.
Increases in phytoplankton cause higher levels of the green
chlorophyll pigment, which in turn change the color of the
ocean surface. Although microscopic, phytoplankton can bloom
in such large numbers that they can change the color of the
ocean so much that they can be measured from satellites.
By
looking at the color of an area of the ocean, the concentration
of phytoplankton can be estimated. Because phytoplankton changes
an ocean's color, they are ideal candidates for tracking eddies
and currents, detecting pollution, and observing meteorological
events. SeaWiFS generated 8-day composite pictures of the
ocean color that showed scientists where the nutrients and
the eddy was located.
NOAA’s
Coast Watch program also monitored Loretta using imagery from
the 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’s movements. According to
Seki, "Some of the strongest temperature gradients associated
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 they
are such large circulation features that they are difficult
to see with the naked eye. When an eddy is spotted, fishermen
set their hooks and lines around and through these features.
Because of the increase in food, eddies are known to increase
the concentration of fish and thus, fishermen harvest greater
catches.
A
paper authored by Michael Seki of the U.S. NMFS appears in
the April 15, 2001 issue of Geophysical Research Letters,
and details the increased plankton observed in response to
eddies in the open ocean near Hawaii. Co-authors of the paper
include Michael Seki, Jeffrey Polovina, and Russell Brainard
of NMFS, Honolulu Laboratory; Robert Bidigare and Carrie Leonard
of the University of Hawaii, Department of Oceanography; and
David Foley of the Joint Institute of Marine and Atmospheric
Research, University of Hawaii and NOAA.
This
project is collaborative research effort between the NMFS
and NASA funded projects. The study was also partially supported
by the Pelagic Fisheries Research Program administered through
the University of Hawaii, School of Ocean and Earth Science
and Technology.
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