Jason CalVal experiments in the tropical Atlantic

Low frequency variability in the tropical Atlantic is complex and hard to witness due to the weakness of this signal. TOPEX/Poséídon (hereafter T/P) and Jason provide a new tool to enlighten these topics by offering more than 10 years of continuous altimetric series. However, due to the vanishing of the Coriolis parameter, uncertainties of a few centimetres 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. T/P and Jason GDR are processed over the tropical Atlantic Ocean with homogeneous corrections. During the commissioning phase (Feb - Aug 2002) a ∼ 15 cm bias and weak differences between T/P and Jason are observed and investigated over the 6 month overlap. This commissioning phase occurs during two main in-situ oceanographic experiments in that area which provide the opportunity of specific CalVal studies. The ARAMIS (Altimétrie sur un Rail Atlantique et Mesures In Situ) program has been developed in order to get a long term survey of temperature, salinity and pCO2 structures in the tropical Atlantic along a merchant ship line. The line crosses the major equatorial currents, the InterTropical Convergence Zone and the Atlantic regions of Maximum Salinity Water. It is superimposed to Jason track n61. The two first ARAMIS cruises are used for Jason CalVal topic. Comparisons with altimetric data are carried out in terms of sea level and geostrophic surface circulation analysis. New geopotential model EGM96 and mean sea surface CMDT RIO03 are checked to get the absolute signal. The PIRATA (PIlot Research Array moored in the Tropical Atlantic) project maintains a network of upper layer measurements in the tropical Atlantic in order to describe and understand ocean thermal structure and air-sea. A time series of velocity profile is gathered at 23W on the Equator in 2002, constituting for the first time a current measurement series simultaneous with altimetry. The altimetric equatorial sea surface slope anomaly is used as a proxy for the pressure gradient anomaly and compared to winds and velocities at 23W. Both experiments allow the Jason validation in terms of tropical Atlantic surface circulation characteristics. The good correlations obtained confirm the usefulness of Jason measurements for continuing the 10-year and highly accurate T/P time series.