Wave-current interactions: from theory to
practical applications
Fabrice
Ardhuin
SHOM
Abstract:
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.