 | |  |
|
| Enlarge ImageData from satellite altimeters, which measure sea surface heights, show depressions (blue) and bumps (red) that mark cold- and warm-water eddies in the ocean on June 17, 2005. Researchers tracked the southwestward motion of eddy A4 by ship from June 24 to Sept. 12. (Courtesy of Dennis McGillicuddy, WHOI, and the Colorado Center for Astrodynamics Research) |
|
|  |
|
| Enlarge ImageThe research vessels Oceanus and Weatherbird II worked in tandem for two summers, tracking different aspects of eddy dynamics. The ships met at sea several times during the two-year operation. (Photo by Josh Eaton, WHOI) |
|
|  |
|
| Enlarge ImageDennis McGillicuddy helps prepare to lower the Video Plankton Recorder (VPR) from R/V Oceanus. The VPR found billions of microscopic plants blooming in eddy A4. (Photo by Stephan Duller, Rutgers University) |
|
|  |
|
| Enlarge ImageResearchers deploy a sampling sled to detect chemical tracers, which helped them track how eddies mix water layers of the ocean. (Photo by David Ciochetto, Dalhousie University) |
|
|  |
|
| Enlarge ImageFrom left, Leo Byckovas, Brian Guest, Blair Greenan, Penelope Howe, and Nathan Buck prepare to lower an "integrating sampler." Arrays of these samplers were towed through the ocean to track how the eddy had spread harmless chemical tracers. (Photo by David Ciochetto, Dalhousie University) |
|
|  |
|
| Enlarge ImageSarah Bender, a Rutgers University researcher, processes water samples in the laboratory on R/V Oceanus. She measured how nutrients affect the abundance of microscopic plants in the eddy. (Photo by Valery Kosnyrev, Woods Hole Oceanographic Institution) |
|
|  |
|
| Enlarge ImageR/V Oceanus Bosun Jim McGill (top, blue shirt) and Able Seaman Bill Hoff (yellow shirt) operate the winch to deploy a conductivity-temperature-depth (CTD) water sampler into the Sargasso Sea, as WHOI oceanographer Dennis McGillicuddy (bottom left) and Rutgers Postdoctoral Scholar Diana Nemergut look on. (Photo by Donglai Gong, Rutgers University) |
|
|
Related Links |
|
|
In the 87 days that Dennis McGillicuddy and colleagues spent in the
Sargasso Sea in the summer of 2005, they were tossed around or chased
by four hurricanes and two tropical storms: Franklin, Harvey, Irene,
Maria, Nate, and Ophelia.
Not one of those massive storms was as powerful as the one swirling in the water beneath them.
From June to September, McGillicuddy and a team of more than 20
scientists from Woods Hole Oceanographic Institution and five other
marine science labs tracked an eddy named A4. It was the oceanic
equivalent of a hurricane—a huge mass of water spinning like a
whirlpool, moving through the ocean for months, stretching across more
than 62 miles (100 kilometers), stirring up a vortex of water and
material from the depths to the surface.
“Eddies are the internal weather of the sea,” says McGillicuddy, an
associate scientist in the WHOI Applied Ocean Physics and Engineering
Department. But unlike destructive hurricanes, eddies can be
productive. As certain types of eddies stir the ocean, they draw
nutrients up from the deep, fertilizing the waters to create blooms of
microscopic marine plants in the open ocean, where little life was once
thought to exist.
“The open ocean is twice as productive as we can explain based on what
we know about nutrients in the water,” said McGillicuddy. “Where do all
the nutrients come from to make these oases in the oceanic desert?”
Pump up the volume (of nutrients)
The Sargasso Sea—south and east of the Gulf Stream—forms the geographic
center of the North Atlantic Ocean. It is warmer, saltier, bluer, and
clearer than most other parts of the North Atlantic, except for the
floating mats of sargassum
seaweed that gave the sea its name. For centuries, prevailing wisdom
was that such open ocean waters were mostly desert-like, unproductive
regions.
A lecture on the Sargasso Sea in the early 1990s sparked McGillicuddy’s
curiosity. In the talk, Bill Jenkins, a senior scientist in the WHOI
Marine Chemistry and Geochemistry Department, pointed out that
scientists were finding more oxygen being produced and consumed in the
open ocean than anyone expected. The suspects were phytoplankton,
microscopic marine plants that produce oxygen in photosynthesis, and
zooplankton (microscopic animals) and bacteria, which use oxygen as
they consume plants and organic detritus that sink to the seafloor.
Scientists found 10 times more microscopic life in the Sargasso Sea
than anyone could explain, given the dearth of nitrate, phosphate,
trace metals, and other nutrients that plants need to grow in sunlit
surface waters. Researchers slowly developed the hypothesis that
vortices of cold or warm water—eddies—might somehow act as a biological
pump.
“I had proposed a problem, and Dennis suggested a solution,” Jenkins
said. “He had the clever idea that eddies were perturbing the layers of
the water column, mixing different waters, and bringing nutrients up
from below.” The upwelling of nutrients into the euphotic zone (the top
330 feet or 100 meters of the ocean, where light penetrates) would
stimulate prodigious blooms of phytoplankton, which attract zooplankton
and other animals up the food chain.
The Eddies Dynamics, Mixing, Export, and Species composition (EDDIES) project was born.
Into the eye of the oceanic storm
“Dennis has wanted to do this experiment since he was a graduate
student,” said Dave Siegel, a longtime collaborator with McGillicuddy
and an oceanographer from the University of California, Santa Barbara
(UCSB).
McGillicuddy mustered chemists, biologists, and physical oceanographers
from WHOI, UCSB, Rutgers University, Bermuda Biological Station for
Research (BBSR), Virginia Institute of Marine Sciences, Dalhousie University, and the
University of Miami. Together, they secured $3.5 million from the
National Science Foundation, as well as five months of ship time over
two years on the WHOI-operated research vessel Oceanus and the BBSR-operated Weatherbird II.
The goal: to make detailed chemical, biological, and oceanographic
measurements of a specific eddy by getting right into the middle of it.
“We didn’t want to just sit on the fence and watch from one point,”
said Ken Buesseler, chairman of the WHOI Department of Marine Chemistry
and Geochemistry. “Eddies move and develop, so we decided to follow a
parcel of ocean as it moved. This was the first time anyone has really
studied an eddy in this way.”
Toward a holistic view of scientific problems
Eddies are distinct parcels of water that move and jostle within the
ocean, much like warm and cold air masses or high- and low-pressure
systems in the atmosphere. Eddies are formed by differences in ocean
temperature and salinity that give water different densities. Like oil
and water, water masses of different densities tend to keep separate,
rather than mix.
The largest eddies can contain up to 1,200 cubic miles (5,000 cubic
kilometers) of water and can last for months to a year. Earth’s
rotation—the Coriolis force—gives eddies their spin.
To hunt for their target, McGillicuddy and colleagues used data from
satellites, whose measurements of sea surface heights show telltale
signs of eddies. Warm-water eddies form bumps in the ocean; cold-water
eddies form depressions. The team examined several eddies and settled
on anticyclone No. 4, or A4, a “mode water” eddy (see "The Hunt for 18° Water")
that stretched some 93 miles (150 kilometers) in diameter at the
surface.
The EDDIES program took a truly integrated approach, combining many
tools—satellites, ships, moorings, drifters, robotic vehicles, computer
models—and many types of scientists.
From June 20 to Sept. 14, 2005, the researchers zigzagged across the
eddy as it drifted southwest about 3.7 miles (6 kilometers) per day.
The team on Oceanus buzzed
around collecting water and nutrient samples, measuring current speeds
and directions, and towing WHOI biologist Cabell Davis’ Video Plankton
Recorder through the turbulent swirl. Bill Jenkins and his lab mates
measured natural chemical markers such as tritium, an indicator of the
amount of plant-fueling nitrate being raised from the depths. WHOI
Senior Scientist Jim Ledwell, an expert on using tracers in the ocean,
injected sulfur hexafluoride, a harmless chemical, into the middle of
the eddy and tracked how it spread up, down, and across the sea.
At the same time, a research team on Weatherbird II
made targeted measurements in the core of the eddy, measuring plant and
animal productivity, the movement of particles, and thorium, a
radioisotope that marks how much organic material is sinking from
surface waters. Siegel used a radiometer to measure whether the eddy
was disturbing the light penetrating the blue water.
“Ocean scientists are moving toward a more holistic view of their
research problems,” said Siegel. “Ocean science grows by filling in the
cracks between disciplines. If you put a smart and diverse group of
people together in a boat, a lot of good things can happen. People
start to think outside of their own little research worlds, and
together we can tell scientific stories that we couldn’t put together
individually.”
Stirring up a rich soup
Fueled by nutrients from the deep, diatoms bloomed to concentrations
10,000 to 100,000 times the norm—among the highest ever observed in the
Sargasso Sea.
At the same time, the team was surprised to find historically low
concentrations of oxygen in the depths, a sign of zooplankton and
bacterial population explosions. It also meant that an awful lot of
heat-trapping carbon dioxide may have been drawn out of the atmosphere
and ocean surface, transformed by phytoplankton, and sunk to the bottom
of the ocean.
Six months after the last EDDIES researcher stepped off Oceanus,
the scientists are still assessing and analyzing the wealth of data
they collected on A4. The team met in February 2006 at the
international Ocean Sciences Meeting in Hawaii to share observations
and collectively make sense of what they saw. Ultimately, the goal is
to develop high-resolution computer models—McGillicuddy’s
specialty—that can simulate and predict the full range of eddy dynamics.
The EDDIES project is a critical step toward comprehending these great
ocean storms, whose sheer size and scale are daunting. During the
expedition, tropical storm Harvey made a direct hit in early August,
cutting a path right across eddy A4. The eddy hardly felt Harvey; the
monstrous atmospheric storm never came come close to breaking up the
potent, voluminous swirl of water in the ocean.
—Mike Carlowicz
The EDDIES project received funding
from the Chemical Oceanography, Biological Oceanography, and Physical
Oceanography branches of the National Science Foundation.
Posted: April 13, 2006 [top] |
