AN IMPROVED MAPPING METHOD OF MULTISATELLITE ALTIMETER DATA

Objective analysis of altimetric data (sea level anomaly) usually assu mes that measurement errors are well represented by a white noise, tho ugh there are long-wavelength errors that are correlated over thousand s of kilometers along the satellite tracks. These errors are typically 3 cm rms for TOPEX/Poseidon (T/P). which is not negligible in low-ene rgy regions. Analyzing maps produced by conventional objective analysi s thus reveals residual long-wavelength errors in the form of tracks o n the maps. These errors induce sea level gradients perpendicular to t he track and, therefore, high geostrophic velocities that can obscure ocean features. To overcome this problem. an improved objective analys is method that takes into account along-track correlated errors is dev eloped. A specific data selection is used to allow an efficient correc tion of long-wavelength errors while estimating the oceanic signal. Th e influence of data selection is analyzed, and the method is first tes ted with simulated data. The method is then applied to real T/P and ER S-1 data in the Canary Basin (a region typical of low eddy energy regi ons), and the results are compared to those of a conventional objectiv e analysis method. The correction for the along-track long-wavelength error has a very significant effect. For T/P and ERS-1 separately. the mapping difference between the two methods is about 2 cm rms (20% of the signal variance). The variance of the difference in zonal and meri dional velocities is roughly 30% and 60%, respectively, of the velocit y signal variance. The effect is larger when TIP and ERS-1 are combine d. Correcting the long-wavelength error also considerably improves the consistency between the T/P and ERS-1 datasets. The variance of the d ifference (T/P-ERS-1) is reduced by a factor of 1.7 far the sea level, 1.6 for zonal velocities, and 2.3 for meridional velocities. The meth od is finally applied globally to T/P data. It is shown that it is tra ctable at the global scale and that it provides an improved mapping.