Mesoscale mapping capabilities of multisatellite altimeter missions: First results with real data in the Mediterranean Sea

Four altimeter missions [Jason-1, ERS-2, TOPEX/POSEIDON interleaved with Jason-1 (T/P) and Geosat Follow-On (GFO)] are intercalibrated and merged in an objective analysis scheme with the aim of improving the estimation of mesoscale surface ocean circulation in the Mediterranean Sea. A validation with independent altimetric data shows that, with the combination of three altimeters, in regions of large mesoscale variability, the sea level and velocity can be mapped with a relative accuracy of about 6% and 23%, respectively, which is a factor of 2.2 less than the results derived from Jason-1 alone, and a factor of about 1.5 less than the results obtained from Jason-1 + ERS-2. Mean eddy kinetic energy (EKE) is computed from the different altimeter configurations. It shows that the combination of Jason-1 + ERS-2 fails to reproduce some intense signals. On the contrary, when T/P is added, these features are well recovered and the EKE does not show significant discontinuities due to sampling effects. The impact of the fourth mission (GFO) is less critical but it also improves the representation of energetic structures. In average, the merged Jason-1 + ERS-2 + T/P + GFO maps yield EKE levels 15% higher than Jason-1 + ERS-2. Finally, we show that the consistency between altimetry and Sea Surface Temperature, drifting buoys and tide gauges, is significantly improved when four satellites are merged compared to the results derived from the two-satellite configuration. This study demonstrates that, at least three, but preferably four, altimeter missions are needed for monitoring the Mediterranean mesoscale circulation.