The objectives of our investigation are to improve our knowledge of the response of the ocean at global and regional scales to tidal astronomical forcing and to short-term meteorological forcing (pressure and wind). New improvements are expected for the long-period tides (semi-monthly and monthly), and for non-linear tides over continental shelves. Better understanding of inverse barometer corrections is also expected. At a more basic level, we will address the problems of global ocean tidal dissipation and non-linear interactions between tides and shelf waves.
The objectives of the proposed program are to further investigate the response of the ocean at global and regional scales to tidal astronomical forcing and to short-term meteorological forcing (pressure and wind). New improvements for tidal and inverse barometer corrections are expected from these studies, in the context of the one-centimeter challenge of the Jason-1 mission. At a more basic level, this program will address the problem of tidal dissipation, which is still an open scientific question, and non-linear interactions between tides and continental shelf waves.
Major improvements of our knowledge of ocean tides have been gained recently through analysis of TOPEX/POSEIDON data combined with numerical hydrodynamic models and data assimilation techniques [Le Provost, 2000]. However, comparisons with in situ measurements seem to indicate that improvements in the accuracy of tidal predictions are still possible, especially over continental shelves and coastal areas [Lefèvre et al., 2000-a].
Hydrodynamic model improvements
The main directions of research are:
Improvement of long-period components
Recent results from the analysis of the T/P data set and hydrodynamic model results lead us to think that the ocean response to semi-monthly and monthly tidal forcing is quite different from static equilibrium. Preliminary results obtained with the CEFMO model combined with in situ data assimilation are already significant. These components will be further investigated.
Study of the limits of tide predictability
The harmonic and response tide prediction methods are based on the concept that the tidal constituents do not vary with time. This is certainly incorrect, at the level of accuracy we are now looking for. Non-linear interactions with meteorological forcing, or climate-related effects, also have to be considered: ice cover, interactions with western boundary currents, with storm surges, and so on. These questions will be investigated through analysis and correlation of both altimeter and in situ observations together with meteorological forcing.
Topographic wave trapping
Topographic Rossby waves are common features over continental shelves and seamounts. Altimeter data have not been used, up to now, to identify the existence of these waves. Such features are visible in the diurnal and long-period solutions produced with our CEFMO model. A systematic check of these features and their existence in the altimeter data will be performed, by combining T/P and ERS, and in future Jason and ENVISAT data (paying particular attention to the period where, hopefully, T/P and Jason will operate simultaneously, thus doubling spatial resolution).
Ocean response to pressure and wind forcing
The ocean does not strictly follow the inverse barometer response. This problem has been addressed by several authors within the T/P SWT. It is proposed here to further investigate the short-term response of the ocean to pressure and wind forcing, with the help of a time-stepping version of our finite element model. The flexibility of the finite element technique will allow us to increase the grid resolution over shelves and seamounts, and thus to better resolve the energy trapping and wave propagation processes over these topographic features. This part of the program will be pursued in collaboration with R. Ponte.
Le Provost C., 2000: Ocean tides, in Satellite Altimetry and Earth Sciences, L.L. Fu and A. Cazenave ed., Academic Press.
Lefèvre F., C. Le Provost, F. Lyard, 2000-a: How to improve a global ocean tide model at a regional scale? A test on the Yellow Sea and East China Sea. J. Geophys. Res., 105, C5, 8707-8727.
Ponchaut F., F. Lyard, C. Le Provost, 2000: Tidal analysis of the WOCE sea level data set. J.A.O.T., (in press).
Lefèvre F., C. Le Provost, F. Lyard, 2000-b: FES98: A New global ocean tide finite element solution independent of altimetry. Geophys. Res. Letters, Vol 27, 17, 2717-2720.
Lohmann K., U. Send, C. Le Provost, J. Fischer, F. Schott, 2001: Correction of ship-based transport measurements with a global tidal model: Examples from the Labrador Sea. Deep Sea Research (submitted).
Egbert G.D., R. Ray, 2000: Significant dissipation of tidal energy in the deep ocean inferred from satellite altimeter data. Nature, Vol. 405.
Ray R.D., G.T. Mitchum, 1997: Surface manifestation of internal tides in the deep ocean. Prog in Oceanog., Vol. 40, 135-162.
Ocean response to short-period atmospheric and tidal forcings