High‐frequency variability of western boundary currents using ERS 1 three‐day repeat altimeter data

Abstract

ERS 1 three‐day repeat altimeter Ocean Products have been analyzed from September 3 to December 9, 1991. In the high mesoscale energy areas of the world ocean the obtained height variability is comparable to that derived from 2 years of Geosat altimeter data (17‐day repeat), while it is slightly lower elsewhere. The data reveal some high‐frequency variability of western boundary currents, but not the seasonal signals, nor the low‐frequency mesoscale signal which dominates elsewhere. Isocorrelation maps show that the e‐folding time is around 10 to 15 days in strong currents. The sea height variance, integrated over all wavelengths in the frequency‐wavenumber spectrum, corresponds to less than 5% of the total 3‐month variance for periods shorter than 20 days, up to 15% of this variance for periods shorter than 34 days, which are respectively the Nyquist frequencies of the TOPEX/POSEIDON and Geosat data. This signal covers a whole range of wavelengths that cannot be obtained from current meter data. In the Gulf Stream area the total dynamic height signal is estimated by adding a climatological dynamic topography to the mesoscale variability. Near 60°W, the height variations apparently correspond to meanders with periods of 30 to 40 days and latitudinal extension about 200 km. They are consistent with westward propagation at about 10 cm/s, and their surface transport varies by about 50% on this timescale, which is in agreement with in situ observations.