logo joomla

Embarquez avec des scientifiques, des ingénieurs et des marins pour une navigation-exploration des relations avec l'océan, le climat et les énergies marines dans la perspective du changement climatique 

      Témoignages       Fantaisie        Qu'avons-nous lu ?       Conférences

Voici l'argumentaire en faveur d'une expérience mondiale de prévision océanographique, début 1997, né d'échanges sur une "idée folle" de Michel Lefebvre et Neville Smith. Le programme GODAE est maintenant une réalité.

Many people have thought about the problem of establishing global real time ocean prediction systems, and some have even been so brave to try. The fundamental obstacle, compared with global weather prediction, is that the natural horizontal scales of the ocean are quite small (several tens of kilometres), so the information and computational requirements for global systems are correspondingly huge. 

The successes of various research programs and nascent operational ocean activities encouraged proposals for global ocean modelling and observing systems but they have thus far floundered, in part because a lack of faith that the global ocean is doable , and in part because the utility of the anticipated products does not appear to justify the investment required.

For many people, the immediate reaction to the idea of staging a global data assimilation experiment is scepticism, mainly because the reasons above. Too difficult and too grandiose, and not worth the effort.

However, encouraged by developments in science and technology and by the vision of what the community 10 years from now might demand in terms of systems and products, people connected with the planning and implementation of the climate components of a global ocean observing system began considering the implications of these developments and likely demands.

Looking toward the middle of the next decade, one can envisage a richness of satellite information though it might just be that alone if it is not supported by appropriate ground truthing and ocean data and models. The power to integrate eddy resolving models in real-time, with sophisticated assimilation, is also a reasonable expectation  but the models are likely to remain hampered by lack of working experience with integrated global data sets ; the models need global data to feed their developments and to prove their worth. Finally we can imagine a wide range of observational experiments and measurement systems, all extremely interesting and challenging in their own right, but mostly disconnected and of limited proven utility for operational systems.

Each of these elements constitutes a necessary part of a global ocean observing system but, because of lack of integration and demonstrated long term value, each would be pretty much in isolation.

For example, by the early part of the next decade, WOCE is likely to have successfully
synthesised the data from the WOCE observational period, but for many, it will be regarded as just another example of a good research experiment. The modelling and data assimilation will probably be extremely impressive but it will after all is said and done just be research.
And, after more than ten years of WOCE, people may reason we should
be putting resources elsewhere. WOCE alone will not provide an irresistible, persuasive case for making either the modelling or observational components permanent facilities.

It is also easy to imagine that the in situ community will still be grumbling at the relative
lack of support compared with, say, remote sensing, but as always many will be undertaking research of high quality anyway.

And what our customers likely to be demanding? Seasonal forecasts and climate change simulations and assessments for sure. The non-tropical folk might be wondering when it is their turn to benefit for predictability, but they may also be seeing some glimmers of hope from CLIVAR. 

The Coastal community will be doing operational forecasts but, like their regional counterparts in meteorology, bemoaning the lack of quality Information at the outer boundary; those using and exploiting the open sea, and those charged with protecting it will be demanding access to information on what the ocean is doing now as well as what it has done on average.

Scientists will still be demanding that at least part of the observing System be maintained routinely to allow new endeavours to be built Upon this baseline. 

Scientists may still demanding continuous altimetry and scatterometer missions (i.e. operational) but they will be competing with new technology whose supporters might argue that these instruments have had their turn to fly new experimental missions 
and so on....

The obvious question this raises is : Why would not at least part of theses systems be supported operationally? The answer would be the parts by themselves do not warrant it –the potential customers have seen the promo and done some “ tasting” but have not had the chance to test and add values to products from an integrated global system over a sustained period.

If one looks back at the development of meteorology : it is clear they went through rather similar phases from the 1950’s (the First Geophysical Year) through the late 1960’s and early 1970’s when the Global Atmospheric Research Program (GARP) came into being .GARP in turn mounted the First GARP Global Experiment (FGGE) in 1979.
Global weather prediction seemed feasible but demonstrations of feasibility where hard when you had only part of the required observing system in place at any one time and models and emerging data assimilation techniques could not be exercised and tested properly.
FGGE provided the opportunity to prove global numerical weather prediction was feasible and that a globally integrated remote and direct sampling system was both practical and of real benefit and impact.

Many are convinced that a global ocean data assimilation system fed by a suitable combination of remote and direct ocean measurements would be justified on the basis of what we know now of possible applications.

Certainly there is a tremendous potential, so it seems inevitable that such a facility will be created sometimes, the question is when, and how will it will happen through chance, the lead-time for developing observing networks (both direct and remote) is long and the chance of all the required systems being in the place at the same time is remote (witness the difficulties of WOCE synchronising altimetry and the hydrographic program or JGOFS waiting for SeaWifs and ocean colour). 

The inevitable conclusion is that te ocean community is going to have to make it happen, and now. We cannot wait until after all the components are tested and, in effect know how to do it. Such a conservative approach is likely to delay the implementation of the observing system a decade or longer, perhaps even indefinitely.

An examination of the satellite schedules that are firm suggests that from.
The aspect of the satellites alone 2002-2007 would be a good time to mount a demonstration of feasibility and practically, assuming satellites Agencies can be convinced to stay with the present schedules?

The modelling and data assimilation community still faces considerable challenges but, drawing a line from developments over the previous decade, and noting the concerted effort being mounted by WOCE as part of its AIMS phase, it is reasonable to assume that these components will be sufficiently advanced by this time. 
And it seems we have the potential at least for global direct sampling of the ocean, at least for the important large scales, if only enough ressources can be gathered and focussed on the target period.

The conclusion then is , despite all the obvious difficulties , doubts , political issues , national and international hesitations, and so on, that NOW is as good a time as we are likely to get to plan and begin the build up for such a demonstration.

The Global Ocean Data Assimilation Experiment was borne out of these ideas and thoughts. It is not a proposal in the conventional scientific sense, that is a (setof) scientific hypotheses or objectives which are going tested with a particular experimental approach . Rather it is a project for the demonstration of feasibility and practicality of operational oceanography, for the global oceans, and on a scale not yet practiced. Certainly for the build up of the experimental phase, and for much of the experimental period, testing of scientific hypotheses and experimentation will be essential. 
But ultimately the purpose is to demonstrate the feasibility, practicality and lasting value of implementing and maintaining a global ocean observing modelling assimilation and forecasting system permanently.

What are the consequences of not pursuing such objectives now? without such a demonstration there must remain considerable uncertainty with regard to the future of agencies supporting ocean observations. Satellite agencies have now put the day of fighting for instruments and justifying their survival on individual basis behind them, opting instead for support of integrated remote systems, and integrated remote and direct observing systems. Those involved with direct observational networks such as the Volunteer Observing Ship program, surface and subsurface floats and acoustic tomography also realise that strength and support will only come from unity and integration.

This experiment promises to provide that strength and support. It enables system development and applications for agencies that would otherwise be impractical. It will provide justification for operational global satellites (altimetry, scatterometry ,…) that is unlikely to come from a dis-integrated system. It will allow the smaller individual, but important contributors to gain strength through routine activities that are a part of a powerful global modelling, assimilation and production system. It will allow science to plan for the future safe in the knowledge that the fruits of past labours are not going to have to be sacrificed in order to move forward. Instead, those that have demonstrated effectiveness and impact, and a routinely feasible, will be integrated into an ongoing baseline system. It will provide a framework for scientists to develop , explore and test new ideas, much as the World Weather Watch and numerical weather prediction underpin much of the atmospheric sciences.

Choix utilisateur pour les Cookies
Nous utilisons des cookies afin de vous proposer les meilleurs services possibles. Si vous déclinez l'utilisation de ces cookies, le site web pourrait ne pas fonctionner correctement.
Tout accepter
Tout décliner
Outils utilisés pour analyser les données de navigation et mesurer l'efficacité du site internet afin de comprendre son fonctionnement.
Google Analytics