Mapping the Oceanic Mesoscale Circulation: Validation of Satellite Altimetry Using Surface Drifters

Abstract

This study aims to show that Lagrangian surface drifters are a suitable means of validating the mapping of oceanic mesoscale circulation by satellite altimetry. Tests are done using Geosat data to simulate drifter trajectories in the Azores-Madeira area. Multivariate objective analysis is then done to estimate the dynamic topography and its associated formal error using the velocity measurements obtained along drifter trajectories. This dynamic-topography field is compared with the reference field as given by Geosat data. Sensitivity to drifter number and energy level is studied. It is shown that with 25 drifters in a 500-km ? 500-km area, the dynamic topography is obtained to within a formal accuracy of around 10%?20%. The difference between the estimated and reference fields is below 2 cm rms. These errors are smaller than the mapping errors induced by the space-time sampling of ERS-1 or TOPEX-POSEIDON satellites. According to these preliminary results, surface drifters are an efficient tool for validating mesoscale mapping by altimetry. More generally, the study shows that by comparing satellite altimetry with data from sufficient surface drifters, the differences between the signals can be estimated.