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GCOS Ocean Sub-Surface ECV
Ocean Tracers

Introduction:  Systematic sampling of the global ocean is needed to fully characterise oceanic climate variability. Global implementation of upper-ocean measurements in ice-free regions is technically feasible, with proven techniques, but remains to be accomplished. This will be addressed initially through the implementation of the agreed upper-ocean network, the in situ component of which initially requires 3000 Argo profiling floats, about 40 repeat XBT lines, 30-40 surface reference moorings, and  about 120 tropical moorings, together with high-precision satellite altimetry. Similar to the surface, the Global Reference Mooring Network will provide essential reference-quality long-time records of sub-surface variables to identify climate trends and climate change.  They also provide critical platforms for the testing and pilot project use of technology for autonomous measurement of biogeochemical and other ecosystem variables.  The records from the Global Reference Mooring Network also will be important for testing climate models and their parameterizations. As new technologies are proven, as our understanding of the sampling requirements improves, and as our ocean analysis and reanalysis capabilities are exploited, the recommended global sub-surface observing system will evolve.
 
Indications of climate variability are present at all depths in the ocean. Argo can document change in temperature and salinity in the upper 2000 m of the ice-free ocean.  The only effective current approach to observing the full suite of ocean sub-surface ECVs involves reference-type repeat deep-ocean surveys. Accurate deep-ocean time series observations are essential for determining long-term trends.  Ocean water column surveys from research vessels are also our only present means for determining the large-scale decadal evolution of the anthropogenic CO2 inventory on a global and basin scale. Several overarching Actions are proposed that the international ocean community should take to ensure that a global sub-surface ocean observing system is implemented that will satisfy climate requirements.
 
OOPC, in conjunction with CLIVAR, JCOMM, the Partnership for Observations of the Global Oceans (POGO), the IOCCP, through the Global Ocean Ship-based Hydrographic Investigations Program (GO-SHIP) and through the national research institutions, will seek implementation of the agreed programme of global repeat full-depth water column sections (about 30 sections repeated on a roughly 10 year cycle, but more frequently where necessary because of known time scales of variability). It will reassess the sampling requirements after the first full repeat in order to account for the hitherto underappreciated interior ocean variability.
The Ship Observations Panel of JCOMM will coordinate the agreed basin-spanning Ship-of-Opportunity XBT/XCTD Repeat Section Programme (a combination of about 40 frequently-repeated sections and of high-density sections (about a 30% increase).  
 
The Argo Project through its Steering Team and in collaboration with the Observations Coordinating Group of JCOMM will seek to sustain the initial global network of about 3000 floats (3°x3° resolution) through long-term maintenance (estimated to require about 800 float deployments per year).

The Tropical Moored Buoy Implementation Panel of JCOMM, in cooperation with CLIVAR and the International Reference Time Series Mooring project, should seek to maintain the tropical Pacific array, develop plans for, and implement an Indian Ocean tropical moored array pilot project. The now established Prediction and Research Moored Array in the Atlantic (PIRATA) also needs maintaining.

WCRP will encourage the development of ocean climate reanalyses, including all appropriate historical data assimilated into ocean models, to create climate variability and trend analyses, and to support seasonal-interannual to decadal climate prediction. They will also encourage other efforts to develop analyses and reliable datasets and products of climate variability and trends.
 
For the biogeochemical and ecological variables, the extension of systematic observations from the fixed moored buoy reference network needs to occur through first the development of new technology, and then through the deployment of this technology. There is an overarching requirement for research and development and testing of new autonomous technologies and approaches for biogeochemical and ecological variables that cannot currently be measured in that manner.

Sub-surface Tracers: Ocean tracers are essential for identifying anthropogenic carbon uptake, storage, and transport in the ocean, as well as for understanding multi-year ocean ventilation, long-term mixing and ocean circulation and thereby, for providing essential validation information for climate change models.  Ocean tracers are not adequately sampled at present.  Present technology for most important tracers requires water samples and subsequent processing of these samples.  
 
The primary network contributing to sub-surface tracers is the Global Ocean Ship-based Hydrographic Investigations Program (GO-SHIP), complemented by research observations. With technology development, some tracers are expected to be observable from the time series reference moorings within the decade; pilot project use, as development proceeds, should be a priority.
 
To attain adequate sampling, the following technical challenges must be overcome:
 

  1. Improved technology for small water volume measurements needed.
  2. Technology development needed for autonomous sensors.

(Source: WMO/IOC Implementation Plan for the Global Observing System for Climate in Support of the UNFCCC (2010 Update) GCOS-138/GOOS-184/GTOS-76/WMO-TD/No. 1523)

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