Quarterdeck 4.1
Conclusion
World Ocean Circulation Experiment
Studying the ocean's role in climate change
by Carri T. Hill and Piers Chapman
[...Continued from Part 1]
Physical oceanographer's toolbox
WOCE scientists conduct research using many tools that operate at different
scales. For example, satellites like ERS-1 and TOPEX/POSEIDON provide global
coverage of ocean surface topography, surface winds, and sea-surface temperature.
High-quality data from the TOPEX/POSEIDON satellite (see back cover) allow
us to track changes in seasonally varying currents, including the development
of the "Great Whirl," a large clockwise eddy that appears in the
Somali Current during the southwest monsoon. [Note
1]
Sea-level gauges and temperature probes deployed
by voluntary observing ships also provide global coverage and a means of
verifying the satellite data. Repeated measurements of temperature within
the top 1000 meters of the ocean allow us to estimate changes in the total
heat stored, for example, in the North Atlantic.[Note
2]
Most observations, however, are made at the
scale of a single ocean basin or smaller. Fleets of surface drifters, free-floating
instrument packages, record and transmit data about surface currents. In
some cases these drifters also report high-quality temperature and atmospheric
pressure data, which are important for operational weather forecasters.
Below the surface, neutrally buoyant floats track ocean flow at a depth
of 1000 meters to provide both a statistical representation of where currents
at that depth flow and a reference point against which current velocities
at other depths can be calibrated. The floats deployed in the Pacific Ocean,
for example, suggest that most flow away from the basin margin at 1000 meters
is zonal (east-west rather than north-south). [Note
3 ]
Most oceanographers are familiar with moored
current meters and hydrographic data obtained by research vessels. WOCE
also relies on these sampling systems. Scientists participating in the hydrographic
program (the largest single component of WOCE) collect data along a series
of lines extending coast-to-coast across all the major ocean basins.
The goal is to establish a database that describes the distribution of density
and chemical tracers in the oceans during the 1990s. These distributions
can be used to highlight the sources of the ocean's water masses, patterns
of movement, and time scales for water renewal. For example, measurements
of 14C in seawater show how the upper layers of the ocean have been affected
by the atmosphere, or ventilated, during the past 30 years. [Note
4]
Moored current meters provide data on short-term
variability in flow at particular choke points in the ocean's circulation.
The data also provide statistics that describe the size and number of eddies
close to the choke points. Data from a mooring near Abaco in the Bahamas,
for example, provide insight into the flow of the deep western boundary
current in the North Atlantic. [Note 5]
WOCE also supports research to improve our ocean
modeling capability. Only by incorporating field data into models can we
increase our ability to predict ocean behavior. Presently this is limited
by a lack of understanding of certain critical ocean processes and by the
lack of computing power needed to cope with a fine-scale model of the global
ocean. Progress is being made on both fronts, however, as well as in the
assimilation of data into models. We now have several models that can resolve
ocean eddies and provide reasonably realistic views of known ocean features.
[Note 6]
Cruising for data
To date, U.S. WOCE has concentrated on field work in the Pacific Ocean and,
most recently, the Indian Ocean. The Indian Ocean expedition began in early
December 1994 with a cruise across the Antarctic Circumpolar Current (ACC)
and concluded in late January 1996-some 50,296 miles and 1,244 hydrographic
stations later. During this period the U.S. and other nations collected
data using the sampling platforms mentioned above. These data constitute
an unprecedented set of observations of the Indian Ocean.
Contributions from U.S. WOCE are already producing new and exciting insights
into the nature of ocean circulation. These include multi-year measurements
of flow in major ocean current systems, global tracer data sets that provide
information on mixing rates and the ocean's capacity for absorbing excess
carbon dioxide, changes over decades in how the ocean transports heat, interactions
between the upper ocean and the atmosphere, and many more. Here are a few
examples of the types of observations being made:
- Current meter moorings were deployed to measure deep flow in the western
Pacific from its water source in the ACC, along the ridge system north of
New Zealand, through passages near Samoa, and into the North Pacific Ocean
east of Japan. [Note 7]
The data are still being analyzed, but they will provide information about
the amount of water from the southern hemisphere that enters the North Pacific
by this route. This is an important constraint for global circulation models.
- The throughflow from the Pacific Ocean to the Indian Ocean is not well
known. Recent cruises by American, [Note 8]
French, and Australian scientists are now providing
considerable information about seasonal variations in this flow. The data
should help us determine whether the throughflow of warm water is more important
than that of the cold water around Cape Horn in returning water from the
Pacific to the Atlantic to complete global circulation.
- The different forms of dissolved carbon dioxide have been measured on
almost all WOCE hydrographic lines. These data allow us to document directly
where carbon dioxide is taken up or released by the ocean and, by comparison
with earlier data, how fast the rate of uptake is changing relative to the
carbon dioxide buildup in the atmosphere.
Evidence for long-term variations and changes in the ocean has been discovered,
and its climatic importance is continually being assessed (IPCC, 1990; IPCC,
1992). However, the full impact and scope of knowledge gained from WOCE
is not likely to be realized before the early 2000s.
With the continued work of WOCE and similar research programs, we eventually
should be better able to make long-term climate forecasts and apply our
knowledge to predict the economic impacts of such changes.
Notes
- George Born, University of Colorado [Back
to text]
- Robert Molinari, Atlantic Oceanographic
and Meteorological Laboratory [Back to text]
- Russ Davis, Scripps Institution of Oceanography
[Back to text]
- Robert Key, Princeton University; Paul Quay,
University of Washington [Back to text]
- Tom Lee and Bill Johns, University of Miami
[Back to text]
- Albert Semtner, Naval Postgraduate School;
Rainer Bleck, University of Miami; Kirk Bryan, Geophysical Fluid Dynamics
Laboratory [Back to text]
- Worth Nowlin and Tom Whitworth, Texas A&M
University; Dan Rudnick, Scripps Institution of Oceanography; Zack Hallock,
Stennis Space Center; Breck Owens and Bruce Warren, Woods Hole Oceanographic
Institution; Dale Pillsbury, Oregon State University [Back
to text]
- Arnold Gordon, Lamont-Doherty Earth Observatory;
Nan Bray, Scripps Institution of Oceanography [Back
to text]
References
IPCC, 1990: Climate Change: The IPCC Scientific Assessment.
(J. T. Houghton, G. J. Jenkins and J. J. Ephraums, eds.) Cambridge University
Press, Cambridge, U.K., 365 pp.
IPCC, 1992: Climate Change: The Supplementary Report to the IPCC Scientific
Assessment. (J. T. Houghton, B. A. Callendar and S. K. Varney, eds.)
Cambridge University Press, Cambridge, U.K., 200 pp.
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Oceanography, Texas A&M University
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URL=http://oceanography.tamu.edu/Quarterdeck/QD4.1/Hill/hill_b.html
Updated May 27, 1996