• Planète et Univers/Océan, Atmosphère
  
• Sciences de l'ingénieur/Mécanique/Mécanique des fluides
  
• Physique/Mécanique/Mécanique des fluides
  

The Tropical South Pacific has been identified as a region with potentially decoupled biogeochemical processes involving nitrogen cycling, with likely consequences for the biological carbon pump. In order to test this hypothesis, biogeochemical field campaigns must necessarily choose a location from which to sample; additionally, due to the intense nature of biogeochemical rate measurements these sites must often be occupied for several days. Therefore, great care must be implemented in selecting areas that are both representative of surrounding water masses, as well as coherent for the duration of the measurements. We present the analysis undertaken for site selection and validation as implemented during the OUTPACE (Oligotrophy to UlTra-oligotrophy PACific Experiment, http://dx.doi.org/10.17600/15000900) cruise. The selection phase consists of remote sensing data (satellite altimetry, surface Chl-a, and SST; produced by CLS with support from CNES) analysis with indicators such as the O-W criterion and tracer gradients, along with more Lagrangian diagnostics such FSLE structures. Subsequently, post-cruise validation is undertaken with in situ data (including MVP, SADCP, and SVP floats) collected during OUTPACE. Perspectives are provided regarding the success of this methodology, the possible inclusion of other metrics, and general comments on how different types of flow would impact biogeochemical sampling in future campaigns.

This manuscript presents a combined experimental and numerical study of the impact of cross-reef fluxes on coral reef lagoon dynamics. The selected field site is the Ouano Lagoon (New Caledonia Island, France) in the South Western Pacific Ocean. Measurements of wave transformation above the reef and current profiles through passages and reef openings have been carried out during a 3-month survey. Data analysis reveals the preponderant roles played by both tides and waves on the lagoon dynamics. Based on field data, a simple parameterization of cross-reef fluxes is implemented in a coastal lagoon circulation model and a satisfactory agreement is found between parameterized model and field results. The model is thus used as a numerical experimental tool in order to analyse the cross-reef flows’ possible influence on a narrow lagoon dynamics. The results highlight the importance of cross-reef fluxes induced by wave breaking over the reef barrier on the whole lagoon circulation and water properties.

Mostly present in tropical regions, coral reefs both provide a unique habitat for a wide variety of living organisms and very efficient protection against ocean action on nearshore and lagoon areas. Recent tsunamis and cyclones events demonstrated that the weakening of coral barriers can have tremendous consequences on nearshore areas, including submersion, erosion and overwash deposit. The present field work aims to study the hydrodynamic processes over a reef barrier, focusing in particular of the offshore wave energy transmission toward the lagoon. The selected field site is the Ouano lagoon, New Caledonia, France. A three-months survey has been performed from september to november 2013 including wave measurements by pressure sensors at the outside reef, above the reef flat and inside the lagoon and velocity profiles measurements at selected places inside the lagoon. During the studied season, the reef is permanently exposed to long swell waves with significant wave height ranging from 0.4 to 2.7m. The wave energy over the reef flat is mostly controlled by the tidal elevation rather than by the incoming swell energy : the higher the mean level, the stronger the wave energy. Significant energy transfers toward low frequencies occur modulated by the tidal elevation. From mid to high tide, the energy transfer during wave breaking is dominant and the wave dynamics over the reef is focused in the infragravity band. From mid to low tide, the contribution of friction processes increases as the mean level decreases. Cross-reef current measurements show that the fluxes are overwhelmingly directed toward the lagoon. Few and short events of outflow are observed during the smallest swell periods either at high tide or at the beginning of rising tide. For small swells, the cross-reef current is modulated by the tidal elevation, while for large incoming waves the fluxes are much stronger (up to 40cm/s) and nearly constant.