Over the past 40 years, numerical modelling has developed rapidly in scope (from hydrodynamics to ecology) and resolution (from one-dimensional, 102 elements to three-dimensional, 108 elements) exploiting the contemporaneous development of computing power. Although we have made significant progress with the implementation of the global ocean observing system, concurrent development in observational capabilities has not been achieved yet in areas demanding high spatial resolution such as coastal domains (despite exciting advances in areas such as in remote sensing and sensor technologies).
Nowadays, diverse applications involving ocean forecasting systems range from short term prediction of the three-dimensional circulation and density fields, waves, tides and storm surges to coupled ocean-atmosphere-land scenario forecasting of the effects of global climate change on terrestrial, fluvial and ecology over millennia. The accuracy of model simulations depends on the availability and suitability (accuracy, resolution and duration) of both observational and linked meteorological, oceanic and hydrological model data to set-up, force, and assess calculations. Modelling is at a stage where major and sustained investments are required in infrastructure and organisation: e.g. access to supercomputers, software maintenance and data exchange (Shapiro et al. 2008).
Many research approaches developed under GODAE are just at their beginning and will require ongoing international research collaboration and coordination. There are still many challenges related to the development of services and links with end users (which are beyond the scope of this chapter). On the scientific side, many of the fundamental modelling issues that were evoked in the book edited by Chassignet and Verron (1998) are still unresolved. They represent new challenges and require step changes to our current efforts. An incomplete list of scientific challenges follows:
• Ocean modeling (for a more comprehensive list about ocean modelling issues see Griffies et al. 2010):
- Mesoscale eddying models can exhibit numerical diapycnal diffusion far larger than is observed. Spurious diapycnal mixing originating from numerical advection remains an issue, with consequences of variable and/or eddy-resolving resolutions and dynamical meshes largely unexplored. Reducing the level of spurious diapycnal mixing in models facilitates collaborative efforts to incorporate mixing theories into simulations, which in turn helps to focus observational efforts to measure mixing and determine its impact on ocean circulation. Progress has been made to rectify this problem through improve ments to tracer advection schemes, but further work is needed to quantify these advances.
- Largely unexplored areas of research involve the local scaling of viscosity and diffusivity coefficients. Lateral viscous friction remains the default approach for closing the momentum equation in ocean models. However, large levels of lateral viscous dissipation used by models do not mimic energy dissipation in the real ocean.
- The ocean floor should be represented continuously across finely resolve mesh regions to faithfully simulate topographically influenced flows. This property is routinely achieved with terrain following vertical coordinates, yet optimal strategies for unstructured mesh models remain under investigation.
- Large-scale ocean-waves-atmosphere coupling remains an area of active research. While wind-induced surface waves contribute primarily to mixing through generation of internal waves at the ocean surface, geostrophic motions may also sustain wave induced interior mixing. In addition, tidal waves can affect the whole water column.
- Submesoscale fronts and related instabilities are ubiquitous, and those active in the upper ocean provide a relatively rapid restratification mechanism that should be parameterized in ocean simulations, even those resolving the mesoscale.
- The coupling between physical, biogeochemical and ecosystem models in terms of consistency of scales, processes resolved and consistent parameteri-sations requires further research.
• Observing systems:
- The exploration of impact of new types of observations on forecasting systems (e.g. remotely sensed sea surface salinity, high resolution wide swath altimetry) requires dedicated efforts and resources.
- In collaboration with international programs such as IMBER and SOLAS research is under way on the implementation of real-time biogeochemical and ecosystem ocean observing systems, e.g. cost-effective sensor-technologies.
- An enhanced focus on observing system design and its analogue of adaptive sampling will allow assessments of individual components of the observing system and provide scientific guidance for improved design and implementation of the ocean observing system.
• Data assimilation:
- Development of data assimilation tools such as coupled atmosphere-ocean initialisation techniques that are fit-for-purpose for a wide range of applications, including short-range, seasonal-to-decadal and climate change prediction (in collaboration with WMO programs) is work-in-progress.
- Efficient data assimilation techniques for biogeochemical and ecosystem modules of ocean circulation models are being developed that are fit for operational purposes.
- Another research focus is the representation of model and data errors using ensemble methods based on various forecasting systems thus delivering more accurate background error estimates.
- Multi-scale data assimilation and joint estimation of interior and open boundary solutions in nested systems remain largely unresolved.
- Users increasingly demand an extension of the critical path of routinely-available global information (satellite and in-situ observations, nowcasts and forecasts) to coastal and littoral applications.
- A prerequisite for an enhanced user uptake of coastal ocean forecasts are enhancements to existing systems and development of new coastal ocean forecasting systems that downscale (and upscale, i.e. two-way coupling) the global basin-wide model estimates as part of the local data assimilation problem, resolving the rich scale interactions, tides and high frequencies, and experimenting novel approaches such as coupled modelling and unstructured grid modeling.
- These forecasting tools will need to be able to contribute to the objective design of observing systems for the coastal ocean, such as new satellite sensors, coastal observatories, etc.; use of such observations in the local forecasting system and upscaling of the information to the basin-scale systems.
Consequently, ocean forecasting in the twenty-first century still faces many challenges with time scales ranging from weather to climate. It is inherently an international issue, requiring broad collaboration to span the global oceans; it is beyond the capability of any one country. Over the past decade, GODAE through its International GODAE Steering Team (IGST) has coordinated and facilitated the development of global and regional ocean forecasting systems and has made excellent progress. GODAE as an experiment has ended in 2008.
The next decade will spawn new research activities in ocean forecasting under the auspices of the GODAE OceanView Science Team that will build on the success of GODAE. GODAE OceanView will promote the development of ocean modelling and assimilation in a consistent framework to optimize mutual progress and benefit. It will promote the associated utilization of improved ocean analyses and forecasts and will provide a means to assess the relative contributions of and requirements for observing systems, and their respective priorities. The GODAE OceanView programme will result in the long-term international collaboration and cooperation that is required for the next, sustained, phase of operational oceanography in the twenty-first century.
The grand vision and key research challenge is to develop coupled initialisation systems of numerical weather prediction and eddy-resolving ocean models. These systems will contribute to and benefit from recent progress in Earth systems modelling. With increasing computing resources the next decade is also likely to see an even stronger emphasis on "seamless" integrations across time and space scales, covering global, regional and coastal/near-shore ocean prediction systems and addressing an increasing number of user applications.
Acknowledgements This paper was written with inputs from the former members of the GODAE International Science Team, and, more recently, the members of the GODAE OceanView Science Team and their Patrons groups. The author would like to particularly thank Pierres-Yves Le Traon, Mike Bell, Eric Dombrowsky, Kirsten Wilmer-Becker, Pierre Brasseur, Pierre De Mey, Roger Proctor, Jacques Verron, Peter Oke and John Parslow for their contributions through many discussions on issues of relevance to this paper.
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