MAPPING MESOSCALE VARIABILITY OF THE AZORES CURRENT USING TOPEX POSEIDON AND ERS-1 ALTIMETRY, TOGETHER WITH HYDROGRAPHIC AND LAGRANGIAN MEASUREMENTS/

The SEMAPHORE mesoscale air/sea experiment was conducted in the Azores -Madeira region from July to November 1993. TOPEX/POSEIDON (T/P) and E RS 1 were flying simultaneously at that time. The main purposes of thi s paper are to evaluate the estimation of the oceanic mesoscale circul ation from the two different sets of altimetric data (T/P and ERS 1) a nd to compare the results with in situ measurements provided by the SE MAPHORE hydrographic surveys and surface drifters (three expendable ba thytermograph conductivity-temperature-depth surveys in a 500-km(2) bo x and a set of 47 Lagrangian surface drifters drogued at 150 m). Compa risons are carried out through the maps obtained by objective analysis from the four data sets. The mapping accuracy of T/P, ERS 1, T/P and ERS 1 combined, and in situ data is investigated, as well as the sensi tivity of the mapping to the correlation functions used. There is a go od qualitative agreement between altimetric maps and corresponding dri fter and hydrographic maps for the three hydrographic surveys. Correla tions are about 0.8, and the regression fit is about 0.6-0.7; the lowe r values are due to the smooth climatology used to reference the altim etric maps. The correlation for time differences is better, with regre ssion lines not significantly different from 1, especially when ERS 1 and T/P are combined. T/P mapping is almost as good as ERS 1 mapping, which was rather unexpected since the ERS 1 space-time sampling is bet ter suited for the mesoscale. This may reflect the fact that the signa l mapped by the hydrography and drifters does not contain the high fre quency/wavenumber components. T/P and ERS 1 combined provide better re sults, although the improvement is not as large as expected, probably for the same reason.