estimate the full four dimensional fields of temperature and horizontal velocity over 35 days of the post convecticn period in the Gulf of Lion. The model used for relating the parameters and ensuring time evolution is simple geostrophy and persistence. In that respect, this analysis is a demonstration of the possibilities offered by data combination. A real assimilation procedure with a more complex model would extend in time and space the influence of the data and permit more detailed estimates of the different terms involved in the heat or vorticity equation.
The advantage of combining different types of data over multiplying a single type of data, relies on their complementarity. The ocean spectrum covers a wide range of scales, in time and space, and each dataset captures it through its own filter. So, unless one is interested only in this part of the spectrum captured by a single dataset, it is more efficient, for accessing the whole ocean spectrum, to merge data types with different filtering characteristics, rather than multiplying the number of sensors of the same type. In our experiment, CTD data have provided the initial temperature field. The time evolution of this field and access to the current was provided by the time series. Current meters give good estimates of barotropic and baroclinic modes but at a limited number of points which are difficult to relate, since most of the energy is distributed over spatial scales much smaller than the mooring spacing. Underwater floats can be merged with moored data, they illuminate the temperature and barotropic current field along their trajectory. Acoustic tomography, by controlling the mean temperature and current along sections, provides the link between these points, reduces the scintillation effect and allows to follow the displacement of eddies. Regarding the space spectrum, hydrography and Eulerian measurements provide a fair description of the baroclinic mode spectrum and tomography increases the estimated amplitudes at all horizontal scales. Float data bring information on the barotropic mode but the major contribution on this mode comes from the reciprocal tomography data, particularly at the largest scales.
Bibliography
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Gaillard, F., Y. Desaubies U. Send and F. Schott (1997). “A four-dimensional analysis of the thermal structure in the Gulf of Lion.” J. Geophys. Res. 102(C6) : 12515-12537.
