Climate variability affects our daily lives. Economic and social impacts of climate anomalies such as the recent cold weather in the northeast United States or the flooding in Oregon can be enormous and far ranging. While climate variability occurs naturally, it also may be driven by human activities like greenhouse gas emissions, deforestation, and urban development.
Understanding and predicting climate changes is the goal of a broad range of scientists. The realization that human activity can and does impact global climate has led to renewed interest in studies of clouds, ocean circulation, land-surface processes, volcanic activity, and atmospheric chemistry. As part of the process, oceanographers worldwide have embraced the challenge of understanding the ocean's role in climate change.
How does the ocean affect climate?
The upper layer of the ocean contains as much heat as the whole atmosphere. Interplay between the two impacts us directly through changes in weather, sea level, and more. The ocean also absorbs trace gases implicated in global warming (particularly carbon dioxide), mitigating their immediate effects. More importantly, however, the ocean mixes and moves water away from the surface and redistributes it in deeper layers around the globe as part of large-scale ocean circulation. Thus, the ocean acts as a buffer to reduce some of the potential climatic shifts.
Unfortunately, we cannot be too sanguine. Oceanographers speculate, for example, that circulation in the Nordic Seas could change over as short a period as a few years. Resulting alterations in the North Atlantic may weaken and slow the Gulf Stream, which normally delivers warm water to the shores of northwest Europe. The ultimate result could be dramatically colder weather in this region. Therefore, we must learn more about the global ocean and its circulation to understand and predict its impact on Earth's climate.
Global research program has a home at Texas A&M
The World Ocean Circulation Experiment (WOCE) is a cooperative effort by scientists from more than 30 nations to study large-scale circulation of the ocean. The knowledge we gain during this unprecedented program will help unravel the role of ocean circulation in long-term climate change and help develop models for predicting such fluctuations.
Dr. Worth D. Nowlin, Jr., Distinguished Professor at Texas A&M University, leads the U.S. contribution to WOCE. He has been instrumental in planning and successfully implementing the U.S. WOCE program as well as establishing a U.S. WOCE program office located at Texas A&M.
The office coordinates diverse activities such as producing implementation plans, obtaining clearances for ships to work in the exclusive economic zones of various coastal states, arranging travel to planning meetings for the many U.S. scientists involved in WOCE, or working with scientists throughout the world to ensure WOCE data are collected and archived properly.
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.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.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).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.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.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.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.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,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.
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.
Worth D. Nowlin, Jr., U.S. WOCE Principal Investigator, observes as a current
meter is prepared for deployment in the Indian Ocean. (Photo
by Steve Rutz)
Oceanographers use an array of instruments deployed on the ocean floor,
in the water column, on the sea surface, and in space.
Researchers deploy a current meter on a WOCE hydrographic cruise in the
Pacific Ocean.
A map of the Indian Ocean shows cruises planned by WOCE offices.
The afterdeck of this oceanographic research vessel is cluttered with buoys
and weights that will stabilize an array of current-meter moorings awaiting
deployment.
