Repeat Hydrography and Carbon Inventory

Despite numerous technological advances over the last several decades, ship-based hydrography using research vessel remains the only method for obtaining high-quality, high spatial and vertical resolution measurements of a suite of physical, chemical, and biological parameters over the full water column (Hood et al. 2009). It is worth mentioning here that VOS and SOOP collect data while cruising, whereas research vessel stops at different locations and collect surface and subsurface data upto full vertical resolution. Ship-based hydrography is essential for documenting ocean changes throughout the water column, especially for the deep ocean below 2 km (52% of global ocean volume). Hydrographic measurements are needed to (a) reduce uncertainties in global freshwater, heat, and sea-level budgets, (b) determine the distributions and controls of natural and anthropogenic carbon (both organic and inorganic), (c) determine ocean ventilation and circulation pathways and rates using chemical tracers, (d) determine the variability and controls in water mass properties and ventilation, (e) determine the significance of a wide range of biogeochemi-cally and ecologically important properties in the ocean interior, and (f) augment the historical database of full water column observations necessary for the study of long-timescale changes.

Shipboard hydrographic data provide the quality standard against which the data from floats and other autonomous platforms and XBTs are compared, to assess their accuracy and for detection and correction of systematic errors. The high cost of shipboard hydrography is balanced against its broad and unique capability to measure many parameters that cannot be measured by other means, and to measure those that can with highest accuracy. Cost factors limit the global hydrographic survey to less than 103 profiles per year from the ocean surface to the bottom, while Argo floats deliver 105 thousand temperature/salinity profiles per year in the upper 2 km. The recommended hydrographic sections for the sustained decadal survey is shown in Fig. 3.4.

Due to the increasing amount of Argo profiling floats in the ocean and to their moving nature, Argo floats cannot be recovered and their sensors cannot be recalibrated at the end of their lifetime. The main problem concerns the conductivity sensor that may drift or show an offset due to biological fooling and other problems. To ensure the quality of data, salinity drift in the conductivity sensors are adjusted by comparison of Argo salinity to nearby high-quality salinity/temperature data (Wong and Owens 2009). In addition to salinity drift, systematic errors in float pressure measurements are also an ongoing concern (e.g. Willis et al. 2008). For both of these issues, the process of identifying and correcting systematic errors is dependent on, and its effectiveness is limited by, the volume and spatial distribution of recent shipboard CTD profiles. The requirements have not yet been established for high quality reference CTD data needed to validate and correct Argo. Similarly, shipboard CTD data are used to assess systematic changes over time in temperature versus depth errors from XBTs, for example to estimate and adjust the instrument's fall rate (Wijffels et al. 2008). Other parameters are required to be collected since

Fig. 3.4 Recommended hydrographic sections for the sustained decadal survey (solid lines) and high-frequency repeat lines (dashed lines)

Fig. 3.4 Recommended hydrographic sections for the sustained decadal survey (solid lines) and high-frequency repeat lines (dashed lines)

future Argo floats are likely to carry sensors for dissolved oxygen, chlorophyll-A, particulate organic carbon, and possibly others.

The CLIVAR and Carbon Hydrographic Data Office (CCHDO) is the repository and distribution center for global CTD, hydrographic, carbon, and tracer data of the highest quality. These data are a product of WOCE, CLIVAR, the International Ocean Carbon Coordination Project (IOCCP), and other oceanographic research programs—past, present and to come. Hydrographic data acquired by investigators are pooled, verified, assembled and disseminated to users in different formats. The CCHDO's primary window to the research community is via its web site (http:// cchdo.ucsd.edu).

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