Improved description of the mesoscale variability by combining four altimeter missions
C/Miquel Marquès, 21,
In this work we combine the data of four satellite altimeters [Jason-1, ERS-2/ENVISAT, Topex/Poseidon interleaved with Jason-1 and Geosat Follow-On] with the aim of improving the representation of the mesoscale variability in the global ocean. All the missions are intercalibrated and weekly gridded maps are produced. In areas of intense variability, the rms differences between a classical configuration of only two altimeters and the scenario merging four missions can reach up to 10 cm and 400 cm²/s², in SLA and EKE, respectively, which represents an important percentage of signal variance. In other areas of moderate activity, like the Mediterranean Sea, which is characterized by relatively small structures, the combination of Jason-1 + ERS-2/ENVISAT fails to reproduce some relevant mesoscale signals. On the contrary, when T/P and GFO are added, these features are well recovered and the EKE does not show significant discontinuities due to sampling effects. The merged Jason-1 + ERS-2/ENVISAT + T/P + GFO maps yield EKE levels 15% higher than Jason-1 + ERS-2/ENVISAT.
We carry out an external validation with surface drifters covering the global ocean, and it is shown that the four altimeter scenario resolves better some mesoscale structures that are not properly recovered with Jason-1 + ERS-2/ENVISAT. A comparison with 86 tide gauges reveals an improvement of about 25% in the estimation of sea level in coastal areas with 4 satellites compared to the errors obtained with 2 altimeters.
Unfortunately, the future of satellite altimetry is uncertain. At the moment (September 2005), only one altimeter mission (Jason-2) is planned and approved. There is an urgent requirement to fly a post ENVISAT altimeter mission but, as demonstrated in this paper, higher resolution will be ultimately needed.