Oxygen, a tool for assessing ocean tracer transport models
Mouchet, A.; Campin, J.-M. (2011). Oxygen, a tool for assessing ocean tracer transport models, in: 43rd international Liège colloquium on ocean dynamics "Tracers of physical and biogeochemical processes, past changes and ongoing anthropogenic impacts" - May 2-6, 2011. pp. 1
In: (2011). 43rd international Liège colloquium on ocean dynamics "Tracers of physical and biogeochemical processes, past changes and ongoing anthropogenic impacts" - May 2-6, 2011. GHER, Université de Liège: Liège. 156 pp., meer
Various techniques are used by modellers in order to overcome difficulties inherent to the modelling of biogeochemical cycles over long time periods. Off-line modelling or asynchronous coupling are among the popular methods used for this purpose. In some other cases the numerical scheme used for the transport of tracers may differ from that used for the active variables (temperature T and salinity S) in order to guarantee positivity or conservation, or to get rid of spurious oscillations. These techniques imply that the different scalars in the model may be subject to slightly different numerical transport equations.It is highly desirable that the chosen method be assessed as it might strongly affect the ocean ventilation rate as actually experienced by the tracers. However the available techniques allowing to test for the consistency of the chosen method suffer from strong limitation. A salinity tracer essentially provides information about surface processes. Age tracers constitute interesting diagnosis tools but their use is impeded by the need of long-duration experiments.In this work we show how a simple tracer, the solubility oxygen, offers an easy way of testing the coherency of the numerical model. It is straightforward to implement and provides at once informations on surface as well as deep ocean processes without calling for additional experiments. Due to the nonlinearity of O2 solubility,mixing of different water masses may only result in a body of water slightly oversatured with respect to the final T and S. A solubility oxygen tracer, not affected by biological processes, is included in the transport model. This tracer obeys the same solubility equation as oxygen but its piston velocity is very large, so that surface waters in the model are at equilibrium with the atmosphere. By comparing the solubility oxygen distribution to the theoretical oxygen saturation obtained from the model temperature and salinity fields a clear diagnostic of the ability of the tracer model to correctly reproduce the ventilation scales of the OGCM may be drawn.This procedure is applied to transport models forced by the outputs of different OGCMs. Reasons for departures are investigated and discussed. We then present a method based on age tracers which allow to improve the consistency between the OGCM and the tracer model ocean ventilation rates.
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