A coastal ocean model intercomparison study for a three-dimensional idealised test case

Several coastal ocean models have been used to compute the circulation on the Northwest European Continental Shelf. Five of them, developed within the European Union, are compared in the scope of an idealised three-dimensional test case, dealing with the geostrophic adjustment of a freshwater cylinder. As the central eddy adjusts, unstable baroclinic vortices start to grow. All the models are able to produce such unstable vortices. However, two of them produce an order-two instability, which is in accordance with a previous laboratory experiment, while the others exhibit an order- four instability. Using a simple scaling analysis, it is seen that the azimuthal wavenumber depends on the ratio of the kinetic energy to the available potential energy. It appears that the discrepancy in the azimuthal wavenumber is mainly due to the effect of the discretisation of the horizontal advection of momentum which could produce significant decrease of the total kinetic energy

Wave-induced flow over Mururoa atoll reef

A unique long-term timeseries of current over Mururoa atoll reef is presented here. Comparison with TOPEX/POSEIDON satellite altimeter data reveals that the daily-averaged cross-reef current is wave-driven and varies at the synoptic time scale. This current also exhibits variations at or near the tidal frequency. In order to understand this evolution, a previously published analytical model of the wave-induced flow over a reef flat is used (SYMONDS et al., 1995). Forced by the altimeter data and the tide, it gives results which agree well with field data. It appears that the model is able to reproduce the observed long-term variations of the inward current and also its variations at tidal frequency. Furthermore, the model gives an explanation of the observed six-hour period signal. It reveals that the response of the cross-reef current to the tidal forcing highly depends on the reef geometry and particularly on the water depth over the reef flat. This confirms that the response of the current over a reef flat at or near the tidal frequency is site-specific

A simple model of the tracer flux from the Mururoa lagoon to the Pacific

It is seen that a simple ordinary differential equation may be sufficient to predict the mass of tracers that may be present in the Mururoa lagoon, as well as the flux from the lagoon to the Pacific. This model depends on a key parameter, namely the lagoon turnover time, which is determined from the results of a complex, three-dimensional, hydrodynamic model

Sensitivity of the Antarctic sea ice to the thermal conductivity of snow

The sensitivity of a global, coarse-resolution ice-ocean model to a decrease in the thermal conductivity of the snow overlaying Antarctic sea ice (k(sant)) is investigated. This study was motivated by recent observations made in the eastern sector of the Southern Ocean and in the Bellingshausen, Amundsen, and Ross Seas, which suggest that the value of k(sant) usually used in large-scale sea ice models is about a factor two too high. When k(sant) is reduced by half in our model, the average thickness of the Antarctic ice pack decreases by 10 cm (10%) and the geographical distributions of snow and ice thicknesses become mon realistic. A strengthening of the Southern Ocean stratification and a concomitant weakening of the Antarctic Bottom Water meridional overturning are also noticed. This ultimately impacts on the sensible heat flux from the ocean to the ice and thus on the rate of thermodynamic ice growth

The water residence time in the Mururoa atoll lagoon: Sensitivity analysis of a three-dimensional model

The role of oceanic tide, wind stress, hoa inflow and stratification in the long-term circulation in Mururoa lagoon is investigated using a sensitivity analysis carried out by means of a three-dimensional model. Water renewal time scales are estimated. Wind stress is shown to be the dominant forcing. The hoa inflow slightly increases the turnover time, while stratification enhances the impact of motion in vertical planes parallel to the wind stress. The modelled turnover time is approximately 100 days, and becomes much larger than one year whenever the wind stress is disregarded