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.
A. Carbon
system studies
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)
C. Deep and
shallow water mass and ventilation studies
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.
D.
Calibration of autonomous sensors
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.
E. Data for
Model 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.
An
integrated sampling strategy
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
Battle, M.,
M. Bender, P. Tans, J.W.C. White, J.T. Ellis, T. Conway, and R.J. Francey,
2000. Global carbon sinks and their variability inferred from atmospheric
O2 and d13C, Science, 287, 2467-2470.
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.
Gruber, N.,
J. L. Sarmiento and T. F. Stocker, 1996. An improved method for detecting
anthropogenic CO2 in the oceans. Global Biogeochem. Cycles, 10, 809-837.
Sarmiento,
J. L. and S. C. Wofsy, 1999. A U.S. Carbon Cycle Science Plan. U.S.
GCRP, Washington, D.C., 69 pp.
Bender , M.
et al., LSCOP, Large Scale Carbon Observation plan: oceans and atmosphere.
<http://www.ogp.noaa.gov/mpe/gcc/co2/observingplan/>
Orr, J. C,
E. Maier Reimer, et al. 2001 Estimates of anthropogenic carbon uptake from
four three-dimensional global ocean models.Global Biogeochem.Cycles, 15,
43-60.
Sarmiento,
J.L., T.M.C. Hughes, R.J. Stouffer, and S. Manabe, 1998. Simulated
response of the ocean carbon cycle to anthropogenic climate warming, Nature,
393, 245-249.
Smith, N.
and C. Koblinsky, 2000. Ocean Obs Conference Statement. Proceedings Ocean
Observation 1999 Conference, St. Raphael France.
Takahashi,
T., R.H. Wanninkhof, R.A. Feely, R.F. Weiss, D.W. Chipman, N. Bates, J.
Olafsson, C. Sabine, and S.C. Sutherland, Net sea-air CO2 flux over the
global oceans: An improved estimate based on the sea-air pCO2 difference,
in Proceedings of the 2nd International Symposium on CO2 in the Oceans,
edited by Y. Nojiri, pp. 9-15, Center for Global Environmental Research,
NIEST, Tsukuba, JAPAN, 1999.
Wong, A. P.
S., N. L. Bindoff and J. A. Church. 1999. Large-scale freshening of the
intermediate waters in the Pacific and Indian Oceans. Nature, 400,
440-443.
|