Living Planet   



Dr Marie-Helene Rio(1), Dr Fabrice Hernandez(2) , and Dr Jean-Michel Lemoine(3)

(1) ISAC, Via del Fosso del Cavaliere, 100, 00133 ROMA, Italy
(2) CLS, Parc Technologique du canal - 8-10 rue Hermes, 31526 RAMONVILLE, France
(3) GRGS, 14 avenue Edouard Belin, 31400 Toulouse, France


New and fundamental insights are expected for oceanographic applications from the launch in 2006 of the Gravity Field and Steady- State Ocean Circulation Mission (GOCE) whose objective is to determine the geoid to 1 cm accuracy for spatial scales down to 100 km. The use of GOCE data together with altimetric measurements of the Sea Surface Height will finally allow retrieving with high accuracy the short wavelengths of the ocean dynamic topography whereas so far only its variable part can be deduced from altimetric heights. In order to reconstruct absolute sea level from altimetric sea level anomalies (SLA), the estimation of a realistic mean dynamic topography (MDT) consistent with altimetric SLA is a crucial issue. Also, estimating an accurate MDT will help the definition of future GOCE products for oceanographers and prepare for the calibration/validation of GOCE products. In that context, a method was developed to estimate globally the required MDT combining altimetric data, in-situ measurements and a geoid model. First, the geoid model EIGEN-2 is subtracted to the Mean Sea Surface Height CLS01 at spherical harmonics degree 30. The obtained field is used to improve the Levitus climatology at wavelengths longer than 660 km (major improvements are obtained at high latitudes) providing a first guess for the computation of the MDT. On the other hand, in situ measurements and altimetric data are combined using a synthetic method. This method consists in subtracting synoptically the altimetric sea level anomaly to in-situ measurements of the full dynamical signal so as to obtain local estimates of the mean field which are then used to improve the first guess using an inverse technique. The obtained combined MDT (CMDT) is compared to other existing MDT (like the Levitus climatology or mean fields issued from models). In particular, absolute dynamic topography values obtained referencing altimetric anomalies to the various solutions are compared to independent in situ measurements. RMS differences to the observations are significantly reduced when using the CMDT. In order to further understand the impact of future accurate geoid measurements, two particular points are then investigated. First, a focus is made on the Mediterranean Sea - whose mean circulation features short scales and is largely unknown- to better understand the future impact and limitation of GOCE data in areas where the mean circulation scales are expected to be inferior to GOCE resolution (i.e. 100 km). Second, the impact of using the new GRACE geoid - whose error is close to 1 cm at 360 km resolution C when estimating a CMDT is studied. GRACE information allows to significantly improve the MDT first guess, resulting in a more realistic CMDT in all areas where no in situ measurements were available.


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Keywords: ESA European Space Agency - Agence spatiale europeenne, observation de la terre, earth observation, satellite remote sensing, teledetection, geophysique, altimetrie, radar, chimique atmospherique, geophysics, altimetry, radar, atmospheric chemistry