Variabilité, circulation et chlorophylle dans la région du Dôme d'Angola en février-mars 1971

Abstract

In February-March 1971 the hydrological conditions off Angola did not display the thermal dome mapped by Mazeika's averages (1967). Cold water cells observed are connected at the surface to a sinuous boundary between low-salinity coastal waters and high-salinity tropical oceanic waters. That boundary coincides rather regularly with an area where trades and SW winds alternate; photosynthesis growths rapidly in a thermoclinal layer that rises until 10 m of the surface but never outcrops. Below a poor and permanent homogeneous surface layer, chlorophyll concentrations show a distribution which is typical of divergence areas. Geostrophical and measured currents show off a transient process in horizontal and vertical movements, however the general curvature of the circulation is propitious to upwelling. Oxygen oversaturations of about 110%, suggest a moderate potential primary production which confirms slowness and alternation of movements. Also, the regular range of the various chemical and biological levels and moderate chlorophyll concentrations suggest an ecosystem where nutrients supply rapidly equilibrate phytoplankton consumption and not at all a 'phytoplankton bloom' area as that which exists in coastal upwelling. Values of Richardson's number show that instability becomes visible at the bottom of the euphotic layer. An evaluation of the vertical motion is inferred by the peculiar distribution and diurnal alternance of the winds shows that 'doming' structures may be sustained by local meteorological events.