Comparison of TOPEX/Poseidon and Jason Altimetry with ARAMIS In Situ Observations in the Tropical Atlantic Ocean

Abstract

Low frequency variability in the tropical Atlantic is complex and hard to witness due to the weakness of this signal compared to the dominant seasonal one. TOPEX/Poseidon and Jason provide a new tool to enlighten these topics by offering more than 10 years of continuous altimetric series. In the tropical regions, due to the vanishing of the Coriolis parameter, uncertainties of a few centimeters in sea level can result in large errors on geostrophic velocity which will propagate rapidly over the entire basin. Accuracy is then a crucial problem for these areas. The ARAMIS program (Altimétrie sur un Rail Atlantique et Mesures In Situ) has been developed by the French Institut de Recherche pour le Développement (IRD) and Centre National d'Etudes Spatiales (CNES) organizations in order to get a long-term survey of temperature, salinity and pCO2 structures in the tropical Atlantic along a merchant ship line. The first two ARAMIS cruises, in July 2002 and March 2003, were dedicated to Jason validation. The dynamical contrast between ARAMIS1 and ARAMIS2 is first analyzed here in agreement with seasonal variations of surface fluxes and wind forcing. Comparisons with TOPEX/Poseidon and JASON data are then presented in terms of sea level analysis. New geopotential models such as the Earth Gravitational Model 1996 (EGM96) that have become available with a resolution of undulations on the order of 50 km, are checked to get the absolute signal. Finally, the tropical Atlantic surface circulation characteristics are used to point out the agreements/discrepancies between all in situ/satellite products, as geostrophic current will emphasize the sea level results.

Keywords:

• altimetry
• XBT
• XBCT
• tropical Atlantic
• ARAMIS

Acknowledgments

This research was funded by the French IRD (Institut de Recherche pour le Développement) and CNES (Centre National d'Etudes Spatiales) institutions. Special thanks to Y. Ménard, P. Vincent and J. Dorandeu (CNES), and to M.H. Rio and J. Sudre (CLS, Argos) for helpful comments during the altimetric data processing. We are also grateful to: Mrs. Ronai and Mr. Huet, from Hamburg Sued company, for their assistance in preparing the ARAMIS cruises on-board the CGM/CMA Le Pasteur; Mr. Harms and Mr. Remp, and the CGM/CMA Le Pasteur crews, who made ARAMIS1 and ARAMIS2 a success; Our LODYC colleagues (in particular N. Sennechal) who helped us during various steps of the program. S. Arnault, D. Diverrès, S. Jacquin are supported by IRD, N. Chouaib and O. Coze by CNES.