Background on the Repeat Hydrography Oversight Committee

The steering committees of US CLIVAR and the Carbon Cycle Science Program (CCSP) established a working group (see planning group members) to implement a joint program of repeat hydrography to fulfill the scientific objectives of these program that involve ocean surveys.  This idea met with approval at NSF-OCE (Eric Itsweire and Don Rice) and NOAA-OGP (Lisa Dilling and Mike Johnson).  The program has been funded for 6 years. The planning committee has now been replaced by an oversight committee (see oversight committee members).

RATIONALE FOR REPEAT HYDROGRAPHY SURVEYS IN SUPPORT OF CLIVAR AND CARBON CYCLE OBJECTIVES (2001).

(The following text predated the proposal for the initial six years of the U.S. repeat hydrography program. This proposal was funded. The proposal text itself is not included here.)

This paper summarizes the scientific rationale and scope of an integrated approach to a global observational program for carbon, hydrographic and tracer measurements. The program is driven by the need to monitor the changing patterns of carbon dioxide (CO₂) in the ocean and provide the necessary data to support continuing model development that will lead to improved forecasting skill for oceans and global climate. The WOCE/JGOFS survey during the 1990s has provided a full depth, baseline data set against which to measure future changes. By integrating the scientific needs in the following five areas, major synergies and cost savings will be achieved. These areas are of importance both for upcoming research programs, such as CLIVAR and the U.S. GCRP Carbon Cycle Science Program (CCSP), and for operational activities such as GOOS and GCOS. In this regard, consensus was reached at the First International Conference on Global Observations for Climate, held in St. Raphael, France in October 1999, that one component of a global observing system for the physical climate/CO2 system should include periodic observations of hydrographic variables, CO2 system parameters and other tracers (Smith and Koblinsky, 2000). The large scale observation component of the CCSP has also clearly defined a need for systematic observations of the invasion of anthropogenic carbon in the ocean superimposed on a variable natural background.

There is broad consensus based on a variety of atmospheric, oceanic and modeling constraints that the ocean that the ocean took up 2.0 +/- 0.6 Gt carbon annually during the last decade (Battle 2000, Takahashi, 1999; Orr et al, 2001). The data from the recent WOCE/JGOFS global carbon survey are providing the first comprehensive inventory of anthropogenic CO2 in the ocean. This survey provided a large data set on the total dissolved inorganic carbon (DIC) content of the ocean, at an unprecedented accuracy of 2 µmol/kg (or 0.1 % of the total concentration). This is equivalent to 1-2 year’s uptake of anthropogenic carbon in surface waters. The total anthropogenic inventory of DIC into the ocean can be determined using concurrent, hydrographic, alkalinity, oxygen nutrient and tracer measurements (Gruber et al., 1996). Utilizing transport estimates, the fluxes of carbon within and between oceans and ocean basins can be better constrained, particularly interhemispheric exchange of carbon through the ocean. Atmospheric interhemispheric exchange is an important diagnostic for models and pre-industrial oceanic carbon transport is a key parameter to estimate interhemispheric differences of carbon sources and sinks. The WOCE/JGOFS sections provide a valuable baseline to determine the possible large scale effects of global warming on the ocean’s biogeochemistry, whether due to changes in stratification, circulation, or perturbations such as a change in the dust deposition on the ocean's surface.

It is clearly important in terms of predicting long-term climate change and man’s effect on the rate of change that we continue to sample the ocean for dissolved carbon components. Further justification on the need for continued oceanic observations of the carbon system are given in the U.S. GCRP publication “A U.S. Carbon Cycle Science Plan” (Sarmiento and Wofsy, 1999) and detailed in the implementation plan (Bender et al., 2001). The repeat observational plan should provide sufficient coverage to determine basinwide changes in DIC and related biogeochemical parameters over a period of approximately a decade. It would serve as a backbone to assess changes in the ocean's biogeochemical cycle in response to natural and/or man induced activity. The proposed cruises can also be a venue for other relevant measurements such as the determination of the partial pressure of CO2 in the surface water which is a critical component to assess the air-sea CO2 flux, and which is a sensitive indicator of changes in the functioning of the biological pump in surface waters.

B. Heat and freshwater storage and flux studies

While we have a reasonably good understanding of the pathways of large-scale transport of heat and freshwater in the ocean, we have no real idea of how these pathways change over decadal time scales. One hypothesis is that systematic changes in temperature-salinity relations in the subtropical and subpolar regions are related to changes in the hydrological cycle (Wong et al., 1999). Both modeling and paleo-oceanographic studies suggest the ocean’s response to, for instance, changes in the forcing to be expected if atmospheric greenhouse gas concentrations continue to increase, can be rapid. Such changes might shut down the thermohaline circulation in the North Atlantic, for example, by capping the subpolar region with a layer of warmer, fresher water. Global warming-induced changes in the ocean’s transport of heat and salt that could affect the circulation in this way can only be followed through long-term measurements at particular sites. (The necessary heating is forecast to be of the order of 2-4 W/m² for a doubling of carbon dioxide; this is too small to measure with any confidence in the ocean.) This component is vital for CLIVAR and for the CCSP as changes in circulation can dramatically change carbon transport and sequestration estimates (Sarmiento et al., 1998)

While we know that water mass characteristics can change on short-term timescales (for example, the North Atlantic “great salinity anomaly” or the El Niño/La Niña system) and often in a non-linear fashion (Doney et al., 1998), we still do not understand how and on what time scales the full-depth water mass structure of the ocean responds to atmospheric variability. Chemical tracers such as chlorofluorocarbons CFCs, 3H/3He or 14C add a time dimension, which can vary between days or centuries. This time dimension can be used to: identify newly-ventilated water masses and their formation rates; determine pathways, time scales and rates of water mass spreading along with its anthropogenic CO2 imprint; determine rates of ventilation/subduction and mixing; monitor freshwater input into high latitudes; constrain rates of biogeochemical processes; and constrain model-based estimates of ocean mixing and circulation processes and parameterizations. There is, at present, no alternative to using shipboard sampling for these tracers, and it makes sense to combine such a sampling scheme with any planned sampling of the ocean carbon system. This is particularly true because estimates of anthropogenic CO2 inventories rely heavily on the tracer measurements. Thus this aspect is of importance to both CLIVAR and carbon research.

While the development of sensors for many parameters is ongoing, there is an immediate need for salinity calibration for the Argo program (www.argo.ucsd.edu). The release of some 3,000 PALACE-type floats in Argo is a major component of both the CLIVAR ocean program and the initial Global Ocean Observing System (GOOS). It is hoped that both temperature and salinity sensors will remain accurate to within about 0.01°C and 0.01 in salinity for the lifetime of each float (4-5 years). Temperature sensors seem to be stable (within specifications) for this length of time, but salinity sensors are not, being affected mainly by biofouling near the surface. Independent data are therefore necessary to check the salinities provided by these instruments, especially in regions such as the subpolar North Atlantic where deep T/S relationships are known to vary on decadal time scales. Other autonomous sensors, such as CO2, nutrient, and particle sensors, are presently being deployed. This new technology will need in situ validation and possibly calibration.

Data on the carbon dioxide system, hydrography and transient tracers provide key observational fields to validate process models, and for the calibration of (climate) models. To predict future atmospheric CO2 levels and global heat and freshwater balances, long-term model integrations must ensure water mass formation and transport occur at the correct rates. For example, large volumes of the ocean (e.g., the sub-thermocline Angola Basin or the deep North Pacific) are still free of either transient tracers. Thus, monitoring the penetration of tracers into these areas gives us a direct measure of the rate of uptake of greenhouse gases for comparison with model outputs. Similarly, regions of active ventilation, for instance, south of Iceland, or in the Labrador Sea, can be easily identified and provide a key diagnostic for ventilation rate estimates. Changes in carbon and heat inventory also provide strong constraints on models and their forcing functions.

The scientific and logistical interests of the ocean carbon, hydrographic, and tracer communities presently overlap, and considerable synergism (and cost reduction) will be achieved by occupying a series of full-depth hydrographic cruises at decadal intervals. A suggested minimum set of such lines is given in Table 1 (see strawman plan on sections). While this set has been selected for looking at long-term changes, not seasonal changes, some lines will monitored more frequently in companion efforts. The choice and sequencing of lines takes into consideration the overall objectives of the program, dates of last occupation during WOCE/WHP, international plans, providing global coverage, and anticipated resources.

Beyond the repeat hydrography program, a limited number of time-series stations is recommended but not proposed here. These can help determine whether observed changes are local, regional, or basin-wide, monitor temporal changes between survey cruises, and possibly even alert us to unexpected rapid changes associated with air-sea forcing such as the PDO or NAO that may need to be reassessed with survey cruises sooner than planned. Potential sites for such monitoring include the sites of the Ocean Weather Ships (e.g., Mike in the Norwegian Sea and Bravo in the Labrador Sea), as well as off Hawaii and Bermuda where observations have been taken throughout WOCE and JGOFS. Additional sites might take advantage of ongoing activities such as the TAO and PIRATA moorings to monitor the air-sea CO2 fluxes in the equatorial Pacific and Atlantic oceans. The necessary instrumentation to support such fixed stations either exists, or are in development, which will reduce the present heavy reliance on shipboard sampling. The large scale observational fields will also serve to put time series and process studies in proper spatial context.

As outlined in Table 1 the U.S. program likely will consist of one or two cruises per year on a 10-14 year rotation. For costing purposes , it is assumed that each cruise will last about 45 days. Using WOCE sampling rates of four full-depth stations per day, 30-mile station spacing, and a cruising speed of 10 kt, this gives a cruise track of about 5,500 miles/10,000 km. Obviously this will not suffice for a zonal section in the equatorial Pacific (>16,000 km), but it is overgenerous for almost all other lines. Costs, based on those of the U.S. WOCE Indian Ocean expedition of 1994-1996 adjusted for inflation and the higher costs of doing fewer lines per year, is estimated at $3,000 K. This estimate includes approximately $700 K for survey or basin specific ancillary measurements.

The integrated approach and multi-year proposal mechanism provides many scientific benefits as outlined above and also significant logistic advantages. Shiptime requirements can be planned well in advance and it provides continued support for groups of trained seagoing technicians for the analyses, together with the associated quality control and data archiving. It also facilitates investments in upgrades in quality control, data management and instruments necessary for the US to remain on the forefront of this type of research. Mechanisms must be put in place to ensure that data is rapidly disemminated to the community at large, and that opportunities are available to interpret the data and use the data in a meaningful fashion in modeling exercises. Without a commitment for long-term funding of such efforts, the full long-term potential of these measurements will not be realized.

References

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Doney, S.C., J.L. Bullister, and R. Wanninkhof, 1998. Climatic variability in ocean ventilation rates diagnosed using chlorofluorocarbons, Geophys. Res. Let., 25, 1399-1402.

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Sarmiento, J. L. and S. C. Wofsy, 1999. A U.S. Carbon Cycle Science Plan. U.S. GCRP, Washington, D.C., 69 pp.

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