Imperative 3: Ocean observing system

Maintain over many decades a sustained ocean observing system capable of detecting and documenting global climate change

Prepared by: D. Roemmich, B. Sloyan, M. McPhaden, U. Send and Josh Willis

Rationale

As the Earth’s climate enters a new era where it is forced by human activities, it is critically important to maintain an observing system capable of detecting and documenting global climate change. Policy makers and the general public require climate observations to assess the present state of the ocean, cryosphere, atmosphere, and land, and place them in context with the past. To be of large-scale societal and scientific value, these observations must be sustained over many decades and remain of the highest quality. Climate observations are needed to initialise and evaluate climate models and to improve predictions of climate change. Such assessments are essential for guiding national and international policies that govern climate-related resources, and agreements aimed at mitigating long-term climate change.

Heat and water are the fundamental elements of the climate system and the ocean is the dominant reservoir for both. To understand the oceanic branch of the system, we must observe on a global basis the storage and transport of heat, freshwater, and carbon in the ocean, and their exchange across the air-sea interface.  In addition, the exchange of momentum across this interface drives much of the ocean’s circulation and must also be observed.

Scientific Background and Major Challenges

In the decade since OceanObs’99 autonomous technologies have brought about a revolution in observational capability that lets us view the subsurface oceans in ways that are comparable to global satellite measurements of the sea surface. The next decade will see equally important advances through expanded coverage and a multi-disciplinary approach. These are not expected to come from radically new technologies, but from the use and enhancement of technologies that are now maturing and from developments in existing systems.

The individual networks of the present Sustained Ocean Observing System for Climate (see the figure below) - including tropical moorings, XBTs, surface drifters, ship-based meteorology, tide gages, Argo floats, repeat hydrography, and satellite observations - have developed largely independently of one another. Progress will now come from integration across the networks since the next big observational challenges - including boundary currents, ice zones, the deep ocean, biological impacts of climate, and the global cycles of heat, freshwater, and carbon - demand multi-platform approaches and because exploiting the value of ocean observations is intrinsically an activity of integration and synthesis.

Schematic diagram of the Sustained Ocean Observing System for Climate. From Roemmich et al. 2010 (http://www.oar.noaa.gov/spotlite/archive/images/climateobs_map.jpg)

Progress over the last decade in the observing system (detailed OceanObs’09 plenary and community white papers) includes:

• Complete implementation of the core Argo mission with 3000 (out of a target of 3000) active floats delivering good data in the open and ice free oceans.
• Improvement in global distribution and number of surface drifters for SST.
• Continuous high precision satellite since 1992.
• Maintenance of Pacific Tropical moored array and extended coverage of the moored array into the Indian Ocean.
• The reinvigoration of the science of SST estimation based on the synthesis of the multiple satellite platform data streams and situ data; producing new and better SST products (with errors) via the Global High Resolution SST project (GHR-SST).
• The transition of the global XBT network from broad-scale monitoring (taken over by Argo) to circulation monitoring via frequently repeated (FRX) and high-resolution (HDX) lines with a global design.
• The success in internationally coordinated efforts to reoccupy a subset of hydrographic and tracer transects. This is the only data set delivering a global view of how the ocean is changing from the sea surface to the ocean floor, including its geochemical fields.

The status of the present ocean observing system and community recommendations for its enhancement were reviewed by the OceanObs’09 Conference and related activities. Participants reached a strong consensus that, above all, observations must be continuous in order to meet the requirements for climate. While revolutionary progress in satellite and autonomous in situ observations has provided the basis for a sustained ocean observing system, national commitments to maintain and further develop it are needed in the short term (5-10 years). Furthermore, enhancements are needed to adequately describe and model phenomena that are of relevance to society, such as anthropogenic climate change, the rates of sea level rise and its causes, and seasonal-to-decadal climate signals like the El Niño/Southern Oscillation, the Indian Ocean Dipole and monsoons. In many cases, present observing systems do not provide sufficient information for modelling or prediction of these phenomena. OceanObs’09 identified these gaps and recommended numerous enhancements that include observations of the deep and ice-covered oceans, boundary currents, mixing, sustained hydrography, and expanded biogeochemical measurements.

Observation Network: The top priority for the coming decade must be to sustain the present ocean observing system, while improving its coverage and data quality. The system also can be significantly enhanced by the following extensions of existing elements and by integration across elements:

• The sampling domain of autonomous platforms can become truly global through extensions to higher latitude, into marginal seas and the deep ocean, and through higher resolution observations in boundary current regions. Incremental technology developments and definition of new sampling requirements are needed for these extensions. A modest amount of dedicated (small) research vessel support is an essential adjunct for the autonomous networks.

• Multi-decadal ocean warming and acidification have impacts on marine ecosystems with severe socio-economic consequences. Given the value of ocean ecosystems to human health and welfare, it is important to understand the links between ocean and climate variability, marine chemical process and their impact on marine ecosystems. Thus, there is an urgent need to fully integrate biogeochemical and biological observations into the ocean observing system.

• The global network measuring the physical state of the oceans provides a platform for multi-disciplinary observations of biogeochemical and ecosystem impacts of climate change. Key requirements are further developments in low-power sensor accuracy and stability, and effective integration between autonomous and shipboard observational networks (e.g. definition of core variables, ensuring a sufficient quantity of reference-quality data for quality assurance of autonomous sensors).

• Improvements in the observation of the ocean surface layer and of air-sea exchanges require better utilisation of research vessels and commercial shipping, improvements to automated measurement systems, better coordination across networks, and a review of sampling requirements for marine meteorology and ocean surface velocity.

• Strong commitment to preserve and in some cases repair the continuity of satellite measurements of the air-sea momentum flux from scatterometers and variations in the ocean mass field from gravity satellites. The principal challenge remains to advocate, plan and finance and press for executing the transition of the critical satellite sensors to sustained status.

• A major effort is needed to ensure that data quality is maximised, that data access is simplified (including for data types extending across multiple observational networks), and that data products are useful and available.

Access to the oceans: Revolutionary new instruments will only fulfill their promise for global observation if there is an efficient means of getting them into and out of the ocean and an effective system for delivering their data and products to users. It is critical that the infrastructure of the observing system, including both physical and organizational elements, should evolve and be maintained in harmony with instrumental technologies and user requirements.

A major factor in the success of the observing system will be the effective utilisation of all available means of access to the oceans: research vessels (including both dedicated cruises and opportunistic use of transiting RVs); commercial ships; navy ships; Antarctic supply ships; and even aircraft. Improved information delivery, careful planning, and coordination are needed for this function at both national and international levels. The nascent JCOMM Observing Programme Support Centre is being developed in part for this need, but is under-resourced.

Data quality, delivery and products: The ocean observing system is heterogeneous, and data volumes are growing rapidly year on year. For maximum value, system interoperability is required in data formats, metadata protocols, and modes of data delivery. The synthesis and delivery of high-quality data and products are major undertakings that have historically been under-resourced. Each individual component of the observing system collects data and applies quality assurance, flagging, and data adjustments before archival of data and metadata that are required for documentation and to direct steps in processing. The component observing systems each strive to maximise data quantity and quality as well as to deliver datasets as quickly and efficiently as is practical for each data type. Many of these streams include both near real-time (operational) and delayed-mode (research-quality) versions. The availability of complementary observations from multiple observing systems is becoming increasingly important. Each data source has distinctive issues of quality and processing. There is a need for integrated datasets, unified access to distributed datasets, and archiving at world data centres to ensure long-term preservation.

There is also a need for data products, for example gridded datasets with uncertainty estimates, in addition to the observational datasets. The documentation and characterisation of products and datasets is essential along with guidance on suitability of datasets for a range of applications.

Strategic Plan

In the past 20 years, technology developments have transformed the vision of a global ocean observing system into an operating reality. Obstacles have been overcome but many remain that will need concerted analysis and planning if they are to be surmounted. First, a global climate observing system can only be deployed as a collaborative international effort with broad participation. Climate observations are made for the benefit of all nations and people. This must be recognised and supported through: i) free and open exchange of data and data-products; ii) international structures and agreements that enable measurements to be made without impediments; and iii) capacity-building in nations needing assistance to share the benefits. Great progress has been made on the first of these issues, with free and open exchange of data gaining wide acceptance as a basic principle of the observing system. However, with a third of the deep oceans lying inside Exclusive Economic Zones, and without an international agreement on the over-riding necessity to collect and freely exchange observations globally for the common good, the observing system remains vulnerable.

The OceanObs decadal reviews have been carried out under the guidance of the Ocean Observations Panel for Climate (OOPC) and the CLIVAR Global Synthesis and Observations Panel, both of which have identified functions similar to “help develop a process for on going evaluation and evolution of the observing system”. On a more frequent basis, conferences such as the International Association for the Physical Science of the Ocean (IAPSO) provide valuable forums for community-wide dialog. A recommendation is to regularise the decadal review process of OceanObs under OOPC, and to formalise more frequent and targeted evaluations of observing system adequacy utilising IAPSO or other appropriate forums.

Finally, for efficient evolution of the observing system, a strategy is needed for reviewing the status of all components on a regular basis and their adequacy in meeting user requirements. There are two parts to such a process. First, each observing system element has (or should have) an international implementation panel, and the terms of reference for these panels should include interactions with identified user communities. Observing system adequacy is application-specific, so the major users play critical roles in evaluating the spatial and temporal coverage of sampling and the data quality and timeliness in the context of their application. We believe that these interactions between implementation groups and users are occurring in most cases and should be further encouraged. Appropriate venues include regular meetings and workshops held by the implementation panels and by user groups such as GODAE OceanView and CLIVAR. The implementation panels and user groups have some cross-representation for this function.

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