Rationale For Repeat Hydrography Surveys In Support Of Clivar And Carbon Cycle Objectives
The U.S. Global Ocean Carbon and Repeat Hydrography Program is part of the international GO-SHIP program, which carries out a systematic and global re-occupation of select hydrographic sections to quantify changes in storage and transport of heat, fresh water, carbon dioxide (CO2), and related parameters. By integrating the scientific needs of the carbon and hydrography/tracer communities, major synergies and cost savings are achieved. In addition to efficiency, the coordinated approach produces scientific advances that exceed those of individual programs. These advances contribute to the following overlapping scientific objectives:
- Data for Model Calibration and Validation
- Carbon System Studies
- Heat and Freshwater Storage and Flux Studies
- Deep and Shallow Water Mass and Ventilation Studies
- Calibration of Autonomous Sensors
The scientific objectives are important both for research programs, such as CLIVAR (Climate Variability and Prediction) and the carbon programs, and for operational activities such as GOOS and GCOS (Global Ocean/Carbon Observing Systems). The program addresses community needs 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 tracers throughout the water column.
The Global Ocean Ship-Based Hydrographic Investigations Program (GO-SHIP) brings together scientists with interests in physical oceanography, the carbon cycle, marine biogeochemistry and ecosystems, and other users and collectors of hydrographic data to develop a globally coordinated network of sustained hydrographic sections as part of the global ocean/climate observing system. GO-SHIP’s development of a globally coordinated network of sustained ship-based hydrographic sections has become an integral component of the ocean observing system.
The objective of the repeat hydrography program is to maintain decadal time-scale sampling of ocean transports and inventories of climatically significant parameters, such as carbon system components, nutrients, freshwater and heat over the entire water column. Earlier programs [e.g., WOCE and JGOFS during the 1990s] provided a full depth baseline data set that can be used, along with other data, for detection of future changes, and showed where atmospheric constituents are entering the oceans. The repeat hydrographic measurements reveal much about the stability of internal pathways and changing patterns in ocean properties. They continue to serve as a baseline to assess changes in the ocean’s biogeochemical cycle in response to natural and/or anthropogenic activity. Long-term measurements are used to follow global warming-induced changes in the ocean’s transport of heat and freshwater, which could affect the circulation by altering thermohaline overturning. While autonomous sampling programs such as Argo and the basin-scale volunteer observing ship programs can sample a portion of the climate-relevant fields, that is, parameters important for heat and freshwater in the upper part of the water column, they cannot at this time sample the entire water column nor can they sample important chemical tracer constituents. These semi-autonomous programs also cannot provide calibrated data; sensors presently in use are subject to drift and require occasional verification by in situ measurements. Broader societal impacts of the program include: broad and near immediate dissemination of data to enhance scientific and technological understanding; societal benefits of collection of a high quality data set, use of the data to assess climate change, and a resource for model calibration of the climate system; promotion of training and learning for graduate students, postdoctoral scientists, and new scientists.
The U.S. program measurements are presently divided into three levels in order of priority. Level 1 core measurements are mandatory on all cruises. The levels are also the suggested standard for international collaborators, and should be measured at the highest practical spatial resolution. The rationale for classifying a measurement as Level 1 is based on data required to directly quantify change in ocean carbon inventory, estimate anthropogenic CO2 empirically, characterize large-scale water mass ventilation rates, constrain horizontal heat, freshwater, carbon, nitrogen, and oxygen transports and/or net divergence, and provide an on-going basis for model evaluation. Level 2 measurements are highly desirable. They may be collected on coarser spacing and will be closely coordinated with the core effort. Level 3 ancillary measurements are done according to opportunity and space available. They should not significantly interfere with Level 1 or 2 efforts, and may be regional or specific to an individual cruise.
Within the U.S. a consortium of scientists typically leads each Level 1 and 2 observation type, working to integrate their measurement program within the framework of the Carbon/CLIVAR national and international program requirements.
Level 1 data:
- Dissolved inorganic carbon (DIC)
- Total Alkalinity (TAlk)
- CTD pressure, temperature, salinity (calculated)
- CTD oxygen (sensor)
- Bottle salinity
- Nutrients by standard auto analyzer (NO3/NO2, PO4, SiO3)
- Dissolved oxygen
- Chlorofluorocarbons (CFC-11, -12, -113) and SF6
- Dissolved organic carbon
- Dissolved organic nitrogen
- Surface underway system (T, S, pCO2)
- ADCP shipboard
- ADCP lowered
- Underway navigation and bathymetry
Level 2 data:
- Discrete pCO2
- 14C by AMS
- ∂13C of DIC
- Fe/trace metals
- CTD Transmissometer
- Surface underway system (nutrients, O2, Chl, skin temperature).
Level 3 data (examples):
- Primary production
- HPLC pigments
- Experimental continuous analyzers
- ∂18O of H2O
- Low level nutrients
- Total organic phosphorus
- Upper ocean optical
- isotopes of O2
- N2, Ar, O2
- Methyl halides
Cruise Organization, Planning, And Documentation
Each cruise will be planned by the chief scientist and principal investigators responsible for Level 1 and 2 measurements, taking into consideration any special requests by other principal investigators for Level 3 measurements. Priority for berths and ship laboratory space will be given to the observation programs in Levels 1 and 2. Reasonable requests for wire time, laboratory space and berths will be accommodated for Level 3 observations. Any disputes will be negotiated with assistance from the Oversight Committee. Investigators proposing add-on (Level 3) measurements should note that the issue of support for any required extra ship time, and the concomitant extra sea salaries for the shipboard scientific party, is a sensitive one for which no firm program or agency policy yet exists.
The sampling programs and investigator assignments are discussed by the Oversight Committee well in advance of each cruise, to allow time to cover any missing observation types. The committee works with funding agencies to ensure that all Level 1 measurements are covered adequately for each cruise, and that as many of the Level 2 measurements as possible are also included.
The final sampling program, including exact cruise track, nominal station locations, parameters to be sampled, anticipated precision and accuracy of each measurement, and list of measurement groups or personnel should be ready at least 3 months prior to the start of a cruise.
Full information regarding scientific operations at each station should be logged, including deck operations, intended sample levels, types of samples, any perceived problems with samples upon collection or initial analysis, personnel involved in analysis, problems or unusual features in the CTD profiles, etc. Sampling logs should be maintained, showing every water sample drawn from each rosette bottle along with their sequence or serial numbers, and with notations for mistrips, leaking bottles, and other problems.
Documentation of sampling and analytical protocols should be submitted with the data sets. Documentation with the initial data submission should include a brief description of shipboard sampling procedures and precision and accuracy of observations. Final documentation should also include detailed descriptions of sample preparations, analytical procedures, equipment calibrations, data reduction techniques, computation algorithms, citations and anything else deemed necessary to understand the suitability, accuracy and precision of the data values.
Data Quality Standards
U.S. measurement standards for the ocean carbon and repeat hydrography program adhere to or exceed those set by WOCE and JGOFS for CTD, hydrographic properties and carbon system components.
The U.S. teams encourage comparison studies of international scope.
To provide the opportunity for comparison with historical data, measurement techniques should be consistent with techniques used to collect the existing data unless there is significant scientific justification for change. When new techniques are adopted, methods for relating the new data to existing data should be developed. This requirement extends to regional comparisons as well.
As many measurements as possible should be made relative to a certified reference material standard. Such standards now exist for salinity, inorganic nutrients, dissolved inorganic carbon, total alkalinity and CFCs. Standards for other Level 1/2 measurements are urgently needed, based on experience in WOCE and JGOFS, for example, improved standardization of oxygen analysis procedures.
Cruise Data Management
The principal investigators are responsible for data collection, analysis, calibration, documentation, and submission to the data assembly centers.
The data assembly centers are responsible for data merging, verification and adjustment of data formats to community standards, online dissemination and documentation, any further quality control (QC), and archiving at NODC.
The program recognizes that at-sea data processing and documentation is by far the most effective, easily justifying its up-front expense. The oversight committee recommends that there should be at least one data management specialist on board each cruise, who is responsible for merging data, assembling documentation, and other matters. The consortium for each measurement type work together to help the lead PI for that measurement on a given cruise by providing tools, advice, standards, etc., but the PI leading the measurement for that cruise is responsible for the data processing and documentation. (Thus the PIs doing each cruise must include data processing funds in their proposals to do the sea work.) At a minimum, PI QC - applying quality flags as per a standard protocol - should be done, with a second, post-cruise level of QC carried out as needed for some parameters, e.g. carbon system.
The bottle S/O2/nutrient/CFC data must be quality controlled and merged with preliminary processed CTD data shipboard quickly. These should be made available to the carbon PIs within 5 weeks of the cruise in order for the carbon data to be submitted within six months.
A single system of QC flag assignments and record keeping should be used for all bottle parameters. The U.S. teams have agreed to use the WOCE Hydrographic Program quality flag scheme.
The data management groups will be responsible for making the data accessible to the community via worldwide web servers and online data reports. The data management groups also see that the data are provided to NODC.
Data Policy Overview
In unilateral recognition of the international importance of these globally distributed repeat hydrographic sections, and as an example to other nations, the U.S. has chosen to adopt an “early release” data policy which provides rapid access to all U.S. repeat hydrographic data to all investigators. Thus the following data policy for hydrographic observations has been adopted:
- Observations are made publicly available in preliminary form through the specified data assembly centers (the CLIVAR and Carbon Hydrographic Data Center - CCHDO for CTD and bottle data) as soon after collection as is practical (“early release”), with final calibrated data provided publicly when available. Collection is interpreted as the completion of the determination of the value of the particular parameter. Thus, for example, tritium/helium collection may not be complete for over a year after return to shore/laboratory. Timelines for each type of data are given below.
- All data collected as part of the repeat hydrography program will be submitted to a designated data management structure for quality control and dissemination for synthesis.
- Ultimately, all U.S. oceanographic data must be archived with the National Oceanographic Data Center, following timelines set by the funding agencies.
This data policy achieves at least two objectives:
- It allows data to be incorporated in a variety of analyses, including data assimilation, as soon as possible after collection, and carried out by numerous investigators, consonant with data policies for more rapidly sampled ocean observation programs, such as ARGO, VOS and satellite observations programs, as well as atmospheric observation programs.
- Reduction in the lengthy delays in data availability that occurred during previous work, such as WOCE, despite a stated maximum two-year proprietary period.
The “early release” policy for repeat hydrographic data recognizes that these data are a publicly funded asset of significant scientific and societal importance.
It is recognized that well-qualified, experienced and highly-motivated investigators are necessary for success of the observation programs. The U.S. program involves such investigators in acquisition of the data sets, for example, by supporting their participation at sea. Data analyses carried out through academic institutions are funded separately through competitive proposals.
International GO-SHIP does not require that participating nations subscribe to this policy of immediate data release. Most, if not all, other nations’ GO-SHIP cruises adhere to the WOCE requirements of a two-year release from time of analysis. Some nations or individual investigators are making at least some of their data available rapidly, similar to the U.S. policy.
Application Of The Data Policy - Timeline
The “early release” data policy applies to all measurements in the core program for repeat hydrography, including both Levels 1 and 2 (see core measurements).
Daily during cruise:
- Reduced temperature, salinity, depth data sets via GTS as TESAC messages
- Meteorological observations
Within 5 weeks of the cruise, released to the relevant data management structure:
- Preliminary CTD (pressure, temperature, salinity, oxygen if measured)
- A merged bottle data file including preliminary discrete salinity, oxygen, nutrients (and carbon system components)
- Preliminary CFC-11, CFC-12, CFC-113, SF6
- Underway data, including continuous (1-minute) navigation, bathymetry,
- shipboard meteorological measurements, temperature, salinity, pCO2 (if measured).
- Shipboard ADCP data
Within 6 months of the cruise, presuming the 5-week release of CTD and discrete salinity data:
- Final salinity, oxygen, nutrients, CFC, CTD data
- Final underway data
- Final shipboard ADCP data
- Final carbon system parameters (Total CO2 and Total Alkalinity required; pH, pCO2 if measured)
- CDOM if measured
- Lowered ADCP (if measured)
- Any other Level 2 measurements
Within 6 months of shore-based analysis:
- 14C and 13C
- DON if measured
Within 2 years of analysis (required NSF data release schedule):
- Any other (Level 3) observations. Those based on discrete bottle samples should be submitted to the hydrographic data management structure and merged with the other bottle data.
- Underway data should be submitted to the underway data management structure to be merged with the Level 1 and 2 underway data.
- Other discrete sampling programs that are likely to be carried out on many of the cruises, such as transmissometry and optics, should be submitted to the relevant data management groups (examples are a JGOFS SMP project for global transmissometry, and the NASA DAC for optics).
A subset of the temperature/salinity/depth and meteorological data should be submitted in near real-time during the cruise, through the GTS.
Data will be submitted following the cruise to the relevant data management structure within the required timelines specified above. Structures required for the repeat hydrography program (Level 1 and 2 data) should include centers for: CTD data, discrete bottle data, underway data, ADCP data and meteorological data. Most Level 3 data sets can be submitted to these centers.
U.S. Support For Observations On Non-U.S. Cruises
Long repeat hydrographic sections are being carried out by other nations. Some may not include all Level 1 and 2 observations. If U.S. groups with the capability to provide these measurements participate on non-U.S.-led repeat hydrography cruises, their data submission requirements as outlined here are waived, in recognition that the non-U.S.-led cruises will not be operating with the same submission deadlines. This also recognizes the importance of collecting these observations for the long-term record of ocean change.
Data collected by U.S. investigators on non-U.S. cruises should still be submitted to the designated data management structures at the time of public dissemination of the principal data sets or at two years, whichever is earlier. If the non-U.S. data sets have an embargo on publication that extends beyond 2 years, then the submitted U.S. data should remain proprietary, that is, without dissemination, until the non-U.S. release date.
The data management structures will make every effort to assemble the complete data sets through contact with the non-U.S. principal investigators.