Researchers and crew members struggle to deploy a spar buoy in rough seas during a January 2006 cruise of R/V Atlantis in the North Atlantic. The buoy measured the exchange of heat and moisture between the atmosphere and the ocean. (Photo by Terry Joyce, Woods Hole Oceanographic Institution)
Atmospheric scientist Jim Edson launches a radiosonde—an instrumented weather balloon—from R/V Atlantis. (Photo by Dave Stuebe, Woods Hole Oceanographic Institution)
Terry Joyce is looking for Val Worthington’s water.
In 1959, Woods Hole oceanographer Valentine Worthington gave a name
and identity to a long-observed but poorly understood phenomenon of the
North Atlantic Ocean. Analyzing data from as far back as the H.M.S.
Challenger expedition of the 1870s, Valentine described how the
interior of the Sargasso Sea contained distinct parcels of water with
remarkably constant salinity, density, and temperature—roughly 18°C
(64° F). To Worthington, the appropriate name for this quirky mass was
simple and straightforward: 18° water.
In the early 1970s, Worthington persuaded colleagues and funding
agencies to mount an expedition to study 18° water. He saw connections
between these peculiar water masses and the circulation of the entire
North Atlantic, as well as the weather above it. But when he finally
went to sea in 1974 and 1975, he couldn’t find what he was looking for.
Researchers now know that 18° water is produced by a critical energy
transfer between warm Gulf Stream water and the cold winter atmosphere.
Unfortunately for Worthington, he went to sea in the midst of a warm
winter, an ebb time in the assembly line of production of 18° water.
Decades later, his successors in the Physical Oceanography
Department at Woods Hole Oceanographic Institution and from eight other
oceanographic institutions have launched a far-reaching program to
examine the formation and evolution of Worthington’s famous water and
how it might influence North Atlantic climate. The CLIVAR Mode Water
Dynamics Experiment (CLIMODE) began its own series of expeditions in
November 2005, and this time researchers seem to be finding what their
predecessor was looking for. An oceanic layer cake
During winter, chilly winds blow from the Arctic and North American
interiors out to sea, where the warm Gulf Stream rides east over the
Sargasso Sea. The cool, dry winds pull heat and moisture from the
Stream and carry them off to Europe and North Africa. The cooler, salty
water left behind forms a layer on top of the ocean that can extend
1,300 feet (400 meters) deep.
Spring and summer heat eventually warms the surface again, but the
chilled waters formed in winter do not dissipate. They are denser than
warm waters, so they sink—not to the seafloor, but to the middle of the
sea. This layer of water hangs suspended between warm surface waters
and even colder deep waters, wedged like butter cream in the midst of a
layer cake.
Oceanographers call 18° water and other water masses like it “mode
water,” a term for a mass of water that has homogeneous
temperature and other characteristics. Mode waters are like pools
within the deeper and wider pool of world oceans, and they form in the
middle latitudes all over the world. (The Kuroshio Current in the
northwest Pacific and the Agulhas Current in the southwest Indian Ocean
are also famous for mode-water creation.) Once they form and sink,
layers of mode water remain relatively intact for several years and are
swept around by the ocean’s circulation.
As Worthington wrote in 1959, “the 18° water is of more than usual
interest” because it can often be found hundreds of miles to the south
and west of where it is formed, “in places where the winter surface
temperature is far higher than 18° and there is no possibility of it
being formed locally.” In fact, Worthington’s 18° water can be found as
far south as the Caribbean.
Terry Joyce, a WHOI senior scientist who worked down the hall from
Worthington for many years, said 18° water is a bit like a memory bank
for North Atlantic climate: It freezes a memory of conditions from the
winter when it was formed and carries around the ocean.
“At the end of each winter season, the Sargasso Sea puts this water
away as if it were going to a safe deposit box, storing it away for
sometime later,” Joyce said. Researchers believe this stored water can
later influence and regulate how quickly or slowly the ocean and
atmosphere can change in future seasons. After migrating to warmer
climes or drifting back to the Gulf Stream, it eventually mixes with
and tempers warmer surface waters.
Hoping for bad weather
The CLIMODE team of investigators is doing everything it can to
chronicle and understand the water that so excited their predecessor.
They have so many questions.
• How much heat is gained, lost, and stored in the
ocean-atmosphere exchange that creates 18° water? How high into air and
deep below the ocean surface does this interaction reach?
• Where does 18° water go after it forms? How does it move horizontally through the ocean?
• How big are these wedges of water, and how do they affect other elements of ocean circulation?
• What role does sinking mode water play in
sequestering carbon dioxide, nutrients, and heat from the surface in
the ocean depths, and how does that affect climate and marine life from
year to year?
From 2005 through 2007, the CLIMODE team will spend 107 days at sea,
crisscrossing the Gulf Stream by ship and spreading instruments all
over it. The project, funded by the National Science Foundation, is a
mix of satellite-based observations, ship-based observations led by
Joyce, and computer modeling led by John Marshall of the Massachusetts
Institute of Technology.
The first two cruises—19 days in November 2005 and 14 days in
January 2006 on the WHOI-operated research vessels Oceanus and
Atlantis—were typical of what is to come. Joyce and colleagues from the
Scripps Institution of Oceanography lowered
conductivity-temperature-depth (CTD) samplers and water-sampling
bottles into the sea dozens of times to characterize the ingredients,
dimensions, flows, and properties of the 18° layer.
WHOI researchers Dave Fratantoni and John Lund and NOAA scientist
Rick Lumpkin cast surface drifters and special bobbing floats that can
drift horizontally and vertically with the water as it sinks, meanders,
and sometimes gets stirred up into circularly spinning currents called
eddies (see "The Oceans Have Their Own Weather Systems").
To measure the interaction between sea and sky, WHOI’s Bob Weller
and the University of Connecticut’s Jim Edson deployed a moored weather
buoy and 30 instrumented weather balloons. A specialized buoy was
released by Edson and WHOI oceanographer John Toole to drift with the
Gulf Stream; it was then recaptured after simultaneously measuring
conditions above and below the ocean’s surface.
All the while, the researchers were hoping for 35- to 40-knot winds
and 5°C (40°F) air temperatures that could cool and churn the sea
enough to start the mode-water assembly line. Nature did not disappoint
this winter, and the CLIMODE team is hoping for cold, windy conditions
when it goes back to sea for another six weeks next winter.
The National Science Foundation provided funding for
CLIMODE. The five-year program includes researchers from Woods Hole Oceanographic Institution, the University
of Connecticut, Florida State University, Massachusetts Institute of
Technology, the University of Washington, Duke University, the University of
Miami, Oregon State University, and the Scripps Institution of Oceanography.
