Wave-current interactions: from theory to practical applications

Fabrice Ardhuin


The French Navy Hydrographic Service (SHOM) has played a leading role in the development of the new "operational oceanography" (i.e. the forecasting of ocean currents). The quasi-geostrophic models of the 1990s is now being replaced by output of the primitive-equations OGCM ran in the French Mercator consortium, and developments are under way for the next generation of forecasting systems, now including the coastal zone, based on the HYCOM model. Major modifications to the "Miami HYCOM" have been introduced to allow a good rendering of internal tides (particularly strong in the Bay of Biscay) and tidal flats with wetting and drying (a necessity around Mont St Michel where the tidal range can reach 14 m). A model prototype of the Manche (English Channel) + Bay of Biscay + Iberian shelf and Bay of Cadiz at 1/60°resolution is under developments with validation sea cruises taking place every year since 2004. The model assimilates altimetry and SST, and is nested in either the 1/12 degree Mercator system or NRL/RSMAS' HYCOM. Applications of these systems are also changing focus, from underwater acoustics to other issues such as underwater visibility or surface drift. This is where the "new" operational oceanography meets the older art of wave forecasting. Works in that area at SHOM are based on a combination of WAVEWATCH III (WW3), and simple propagation models for the nearshore. Several source term parameterizations for wave generation and dissipation have been tested in WW3, and will be briefly discussed. Short of anything better so far, the best results in hindcasts have been obtained with the parameterizations now used at ECMWF (Bidlot etal. 2005).

The combination of wave and ocean models is now being investigated for upper ocean mixing and surface drift applications. Theoretical developments have lead to a modified form of the primitive equations based on the Generalized Lagrangian Mean of Andrews and McIntyre (1978), and recent parameterizations of wave-induced mixing at the ocean surface (e.g. Craig and Banner 1994). In horizontally-homogeneous conditions waves force the ocean currents through the Coriolis force induced by the wave momentum, mixing due to wave breaking, and advection by the Stokes drift. This cocktail provides a reasonable combination of strong mixing and large surface shears for the drift current (Rascle, Ardhuin & Terray, JGR, in press). This approach is now being extend to conditions with horizontal gradients, including the surf zone, with a first implementation in HYCOM and ROMS of some wave effects.