Flow variability near the Cape Verde frontal zone (subtropical Atlantic Ocean)

During the JGOFS-France/Eumeli programme, biogeochemical processes governing particle fluxes throughout the water column were studied in the Eastern Subtropical North Atlantic Ocean. From February 1991 to December 1992, three consecutive sets of sediment trap moorings were installed at two sites: the mesotrophic site: ~18° 30 N, 21° 00 W, and the oligotrophic site: ~21° 00 N, 31° 00 W. Currents were measured at each sediment trap level (250, 1000 and 2500 m) in order to understand better the dynamics in this area and its influence on the particle fluxes through the water column and to monitor the current speed conditions during the particle trapping. The results of these current measurements are presented here, including statistics, spectral analysis, horizontal and vertical correlations and the time variability which is studied by comparisons with previous studies and models. The main feature of the dynamics in this area is the presence of the Cape Verde frontal zone at the southeastern part of the North Atlantic subtropical gyre, which is an unstable boundary between the North Atlantic Central Water and the South Atlantic Central Water. At 250 m, highest mean current speeds (~8–14 cm s^–1) and highest variability are encountered at the mesotrophic site which is the closest to this frontal zone, whereas they are ~8 cm s^–1 at the oligotrophic site. At 1000 and 2500 m, they are of the same order for the two sites (~5 and 3 cm s^–1, respectively). In addition to the short time scale variability at the inertial periods (33.4 and 38 h) and semi-diurnal tidal period, long time scale variability at ~100 d is very high at 250 and 1000 m, particularly on the meridional component. Comparisons show that this result fits and expands previous eulerian and lagrangian observations and is consistent with Rossby waves predicted by models, in spite of small discrepancies.